High Performance Computing and Communications GlossaryKen Hawick, email@example.com
A significant part of the material of this glossary was adapted from material originally written by Gregory V. Wilson which appeared as "A Glossary of Parallel Computing Terminology" (IEEE Parallel & Distributed Technology, February 1993), and is being re-printed in the same author's "Practical Parallel Programming" (MIT Press, 1995). Several people have contributed additions to this glossary, especially Jack Dongarra, Geoffrey Fox and many of my colleagues at Edinburgh and Syracuse.
A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z.
acyclic graph (n.) a graph without any cycles.
adaptive (adj.) Taking available information into account. For example, an adaptive mesh-generating algorithm generates a finer resolution mesh near to discontinuities such as boundaries and corners. An adaptive routing algorithm may send identical messages in different directions at different times depending on the local density information it has on message traffic. See also oblivious
address generation (n.) a cycle during execution of an instruction in which an effective address is calculated by means of indexing or indirect addressing.
address mask (n.) in internet communications, a 32 bit long mask used to select an IP address for subnet addressing. The mask selects the network portion of the IP address and one or more bits of the local LAN address.
address space (n.) A region of a computer's total memory within which addresses are contiguous and may refer to one another directly. A shared memory computer has only one user-visible address space; a disjoint memory computer can have several.
address translation (n.) in networking, the process of converting external addresses into standardized network addresses and vice versa. This facilitates the interconnection of multiple networks which each have their own address plan.
adjacency matrix (n.) a Boolean matrix indicating for each pair of vertices I and J whether there is an edge from I to J.
all-pairs shortest-path problem (n.) given a weighted graph, find the shortest path between every pair of vertices.
Amdahl's Law (n.) A rule first formalised by Gene Amdahl in 1967, which states that if F is the fraction of a calculation that is serial and 1-F the fraction that can be parallelized, then the speedup that can be achieved using P processors is: 1/( F + (1-F)/P) which has a limiting value of 1/F for an infinite number of processors. This no matter how many processors are employed, if a calculation has a 10% serial component, the maximum speedup obtainable is 10.
AND tree (n.) a search tree whose nonterminal nodes are all AND nodes.
AND-parallelism (n.) A form of parallelism exploited by some implementations of parallel logic programming languages, in which the terms in conjunctions are evaluated simultaneously, and parent computations are allowed to proceed only once their immediate children have completed. See also OR-parallelism .
AND/OR tree (n.) a search tree with both AND and OR nonterminal nodes.
ANSI (n.) American National Standards Institute, a United States based organization which develops standards and defines interfaces for telecommunications amongst other things.
AppleTalk (n.) Apple Computer's proprietary network suite of protocols.
applicative language a language that performs all computations by applying functions to values.
architecture (n.) The basic plan along which a computer has been constructed. Popular parallel architectures include processor arrays, bus-based multiprocessors (with caches of various sizes and structures) and disjoint memory multicomputers. See also Flynn's taxonomy.
ARP (n.) Address resolution protocol under TCP/IP used to dynamically bind a high level IP address to a low-level physical hardware address. ARP is limited to a single physical network that supports hardware broadcasting.
ARPAnet (n.) The US Advanced Research Project Agency network, which was the first large multi-site computer network.
array constant (n.) an array reference within an iterative loop, all of whose subscripts are invariant.
array processor (n.) Any computer designed primarily to perform data parallel calculations on arrays or matrices. The two principle architectures used are the processor array and the vector processor.
artificial intelligence (n.) A class of problems such as pattern recognition, decision making and learning in which humans are clearly very proficient compared with current computers.
associative memory (n.)Memory that can be accessed by content rather than by address; content addressable is often used synonymously. An associative memory permits its user to specify part of a pattern or key and retrieve the values associated with that pattern. The tuple space used to implement the generative communication model is an associative memory.
asynchronous (n.) A method of transmission which does not require a common clock, but separates fields of data by stop and start bits.
asynchronous transfer mode (n.) A broadband cell relay protocol that cuts subscriber data into fixed size packets for transfer across the wide area network.
atomic(adj.) Not interruptible. An atomic operation is one that always appears to have been executed as a unit.
attached vector-processor (n.) A specialised processor for vector computations, designed to be connected to a general purpose host processor. The host processor supplies I/O functions, a file system and other aspects of a computing system environment.
automatic vectorization (n.) A compiler that takes code written in a serial language (often Fortran or C) and translates it into vector instructions. The vector instructions may be machine specific or in a source form such as array extensions or as subroutine calls to a vector library. See also compiler optimization.
auxiliary memory (n.) Memory that is usually large, in capacity, but slow and inexpensive, often a rotating magnetic or optical memory, whose main function is to store large volumes of data and programs not being actively used by the processors.
AVL tree (n.) binary tree having the property that for any node in the tree, the difference in height between the left and right subtrees of that node is no more than 1.
B8ZS (n.) Bipolar with eight zero substitution, a line code used for T1 which converts any string of 8 zeros of a DS-1 signal into a code which at the far end is converted back to eight zeros. The coding actually inserts BPVs that are realized at the next multiplexer point and that taken out of the signal.
banded matrix (n.) a matrix in which the non-zero elements are clustered around the main diagonal. If all the non-zero elements in the matrix are within m columns of the main diagonal, then the bandwidth of the matrix is 2m+1.
bandwidth (n.) The communications capacity (measured in bits per second) of a transmission line or of a specific path through the network. Contiguous bandwidth is a synonym for consecutive grouped channels in mux, switch or DACS; i.e. 256kbps (4 64kbps channels).
bank conflict (n.) A bank "busy-wait" situation. Memory chip speeds are relatively slow when required to deliver a single word, so supercomputer memories are placed in a large number of independent banks (usually a power of 2). A vector of data laid out contiguously in memory with one component per successive bank, can be accessed at one word per cycle (despite the intrinsic slowness of the chips) through the use of pipelined delivery of vector-component words at high bandwidth. When the number of banks is a power of two, then vectors requiring strides of a power of 2 can run into a bank conflict.
bank cycle time (n.) The time, measured in clock cycles, taken by a memory bank between the honoring of one request to fetch or store a data item and accepting another such request. On most supercomputers this value is either four or eight clock cycles. See also prefetch.
barrier synchronization(n.) An event in which two or more processes belonging to some implicit or explicit group block until all members of the group have blocked. They may then all proceed. No member of the group may pass a barrier until all processes in the group have reached it. See also fuzzy barrier.
batch search (n.) a concurrent search for a number of names.
benchmark (n.) a quantitative measure of performance for a computing system. The measure may be in terms of computational performance, which is often rated in FLOPS, or in terms of memory speed, or communications bandwidth or some more application-oriented measure such as LIPS or OLTPS. A collection of benchmark results for many computer systems is available on line from the National Software Exchange.
Bernstein's Condition (n.) A sufficient condition for the independence of two sections of a program as stated by Bernstein in 1966. If Ri(Wi) is the set of variables read(written) by a section of code i, then Bernstein's Condition states that sections i and j may be executed in an arbitrary order, or concurrently if: there is no true dependence, output dependence or antidependence among the statements in the sections.
BGP (n.) border gateway protocol.
BiCMOS (n.) Bi-polar CMOS. BiCMOS is a merger of ECL and CMOS wafer processes allowing both types of circuit to exist on the same chip. This gives the advantage of the small feature size and large scale integration of CMOS with ECL's high power, fast driver circuits.
BISDN (n.) Broadband Integrated Services Digital Network is a packet switching technique which uses packets of fixed length, resulting in lower processing and higher speeds. See also ATM or cell relay.
bisection bandwidth(n.) The rate at which communication can take place between one half of a computer and the other. A low bisection bandwidth, or a large disparity between the maximum and minimum bisection bandwidths achieved by cutting the computers elements in different ways is a warning that communications bottlenecks may arise in some calculations.
bit-addressable (adj.) Allowing direct access to individual bits, rather than requiring bits to be selected by applying arithmetic or other operations to whole words. The local memory of each processing element in many processor arrays is bit addressable.
bitonic sequence (n.) a sequence of numbers A0,A1,...An-1 with the property that (1) there exists an index i, 0<=i<=n-1, such that A0 through Ai is monotonically increasing and Ai through An-1 is monotonically decreasing, or else (2) there exists a cyclic shift of indices so that condition 1 is satisfied.
BLAS (n.) Basic linear algebra software: a suite of very basic linear algebra routines, out of which almost any other matrix calculation can be built. See the National Software Exchange for the BLAS source and documentation.
block (v.) To suspend one's own operation, or the operation of another process. A process may block itself, or be blocked by the system, until some event occurs. If all processes are simultaneously blocked, and no external event can cause any of them to become unblocked, then deadlock has occurred. The term block is also often used to refer to a chunk of data or program. See also basic block.
blocking (adj.) An operation that causes the process executing it to block. Usually applied to communications operations, where it implies that the communicating process cannot perform any other operations until the communication has completed. See also asynchronous, non-blocking and synchronous.
boundary value swapping(n.) A technique used when performing calculations on a mesh on which geometric decomposition has been used. During a boundary value swap, each process exchanges values on the edges of its tile or tiles for the complementary values of its neighbours. See also indirect method.
broadcast (n.) To send a message to all possible recipients. Broadcast can be implemented as a repeated send, but is more efficiently implemented by using spanning trees and having each node in the tree propagate the message to its descendants. See also multicast and process group.
buffer (n.) A temporary storage area in memory. Many methods for routing messages between processors use buffers at the source and destination, or at intermediate processors. See also packet switching, virtual cut-through and wormhole routing.
bus (n.) A single physical communications medium shared by two or more devices. The network shared by processors in many distributed computers is a bus, as is the shared data path in many multiprocessors. See also link.
busy-waiting (n.) a situation whereby processor cycles are used to test a variable until it assumes a desired value.
butterfly(n.) A topology in which nodes are organised into levels, and there is a unique path from any node in the first level to any node in the last. A butterfly is a recursive topology . See also hypercube and shuffle exchange network.
cache (n.) A high-speed memory, local to a single processor , whose data transfers are carried out automatically in hardware. Items are brought into a cache when they are referenced, while any changes to values in a cache are automatically written when they are no longer needed, when the cache becomes full, or when some other process attempts to access them. Also (v.) To bring something into a cache.
cache hit (n.) a cache access that successfully finds the requested data.
cache line (n.) The unit in which data is fetched from memory to cache.
cache miss (n.) A cache access that fails to find the requested data. The cache must then be filled from main memory at the expense of time.
CAM (n.) Computer-Aided Manufacturing.
CCITT (n.) Consultative Committee International Telephone and Telegraph is an international organization which develops standards and defines interfaces for telecommunications.
cellular automata (n.) A system made up of many discrete cells, each of which may be in one of a finite number of states. A cell or automaton may change state only at fixed, regular intervals, and only in accordance with fixed rules that depend on cells own values and the values of neighbours within a certain proximity.
cellular computer (n.) A term sometimes used to describe fine grain systems such as neural networks, systolic array, and SIMD systems. Such systems tend to be well suited for the implementation of cellular automata .
CEPT (n.) Conference on European Post and Telegraph is a European organization which develops standards and defines interfaces for telecommunications.
CFD (n.) Computational fluid dynamics; the simulation or prediction of fluid flow using computers, a field which has generally required twice the computing power available at any given time.
chaining (n.) the ability to take the results of one vector operation and use them directly as input operands to a second vector instruction, without the need to store to memory or registers the results of the first vector operation. Chaining (or linking as it is sometimes called) can significantly speed up a calculation.
channel (n.) A point-to-point connection between two processes through which messages can be sent. Programming systems that rely on channels are sometimes called connection-oriented, to distinguish them from the more widespread connectionless systems in which messages are sent to named destinations rather than through named channels. See also CSP, channel mask.
channel mask (n.) A non-negative integer specifying a set of communication channels. If the numbers of the channels to be specified are I0,I1,...In-1, then the channel mask is the integer for which these bit numbers are set to 1 and all other bits are 0. For example, if the channels to be specified are numbered 0, 1 and 4, the corresponding channel mask is the binary number 10011, or 19, in which the bits numbered 0, 1 and 4 are set to 1.
chime (n.) Chained vector time, approximately equal to the vector length in a DO-loop. The number of chimes required for a loop dominates the time required for execution. A new chime begins each time a resource such as a functional unit, vector register or memory path, must be reused.
chunksize (n.) The number of iterations of a parallel DO-loop grouped together as a single task in order to increase the granularity of the task.
circuit switching (n.) A switching method where a dedicated path is set up between the transmitter and receiver. The connection is transparent, meaning that the switches do not try to interpret the data.
CISC (adj.) Complicated instruction set computer; a computer that provides many powerful but complicated instructions. This term is also applied to software designs that give users a large number of of complex basic operations. See also RISC.
clausal logic (n.) a form of logic in which all propositions are expressed in terms of AND, OR and NOT. A six-stage process transforms any predicate calculus formula into clausal form. See also clause.
clock cycle (n.) The fundamental period of time in a computer. Current technology will typically have this measured in nanoseconds.
clock time(n.) Physical or elapsed time, as seen by an external observer. Nonrelativistic time. In small computer systems where all components can be synchronized, clock time and logical time may be the same everywhere, but in large systems it may be difficult for a processor to correlate the events it sees with the clock time an external observer would see. The clock times of events define a complete order on those events.
Cobegin/Coend (n.) a structured way of indicating a set of statements that can be executed in parallel.
combining (v.) Joining messages together as they traverse a network. Combining may be done to reduce the total traffic in the network, to reduce the number of times the start-up penalty of messaging is incurred, or to reduce the number of messages reaching a particular destination.
combining switch (n.) an element of an interconnection network that can combine certain types of requests into one request and produce a response that mimics serial execution of the requests.
common subexpression (n.) a combination of operations and operands that is repeated, especially in a loop. A good compiler will not recompute the common subexpressions but will save them in a register for reuse.
communication width (n.) the size of shared memory in an SIMD model assuming a global memory.
compiler directives (n.) special keywords often specified as comments in the source code, but recognised by the compiler as providing additional information from the user for use in optimization.
compiler optimization (n.) Rearranging or eliminating sections of a program during compilation to achieve higher performance. Compiler optimization is usually applied only within basic blocks and must account for the possible dependence of one section of a program on another.
complexity (n.) a measure of time or space used by an algorithm. Without adjective this refers to time complexity.
compress/index (n.) a vector operation used to deal with the nonzeroes of a large vector with relatively few nonzeroes. The location of the nonzeroes is indicated by an index vector (usually a bit vector of the same length in bits as the full vector in words). The compress operation uses the index vector to gather the nonzeroes into a dense vector where they are operated on with a vector instruction. See also gather/scatter.
computation-to-communication ratio (n.) The ratio of the number of calculations a process does to the total size of the messages it sends. A process that performs a few calculations and then sends a single short message may have the same computation-to-communication ratio as a process that performs millions of calculations and then sends many large messages. The ratio may also be measured by the ratio of the time spent calculating to the time spent communicating, in which case the ratio's value depends on the relative speeds of the processor and communications medium, and on the startup cost and latency of communication. See also granularity.
concurrent processing (adj.) simultaneous execution of instructions by two or more processors within a computer.
condition synchronization (n.) process of delaying the continued execution of a process until some data object it shares with another process is in an appropriate state.
configuration (n.) A particular selection of the types of processes that could make up a parallel program. Configuration is trivial in the SPMD model, in which every processor runs a single identical process, but can be complicated in the general MIMD case, particularly if user-level processes rely on libraries that may themselves require extra processes. See also mapping.
conjugate gradient method (n.) A technique for solving systems of linear algebraic equations, which proceeds by minimizing a quadratic residual error function. The method is iterative but quite powerful: in the absence of roundoff error, it will converge exactly in M steps, where M is the order of the system in question.
contention (n.) Conflict that arises when two or more requests are made concurrently for a resource that cannot be shared. Processes running on a single processor may contend for CPU time, or a network may suffer from contention if several messages attempt to traverse the same link at the same time.
context switching (n.) Saving the state of one process and replacing it with that of another that is time sharing the same processor. If little time is required to switch contexts, processor overloading can be an effective way to hide latency in a message passing system.
control process (n.) A process which controls the execution of a program on a concurrent computer. The major tasks performed by the control process are to initiate execution of the necessary code on each node and to provide I/O and other service facilities for the nodes.
cost (n.) complexity of an algorithm multiplied by the number of processors used.
critical node (n.) when a node is inserted into an AVL tree, the critical node is the root of the subtree about which a rebalancing is going to take place.
critical section (n.) a section of program that can be executed by at most one process at a time.
CSP (n.) Communicating sequential processes; an approach to parallelism in which anonymous processes communicate by sending messages through named point-to-point channels. CSP was coined by Hoare in 1985. All communication is synchronous in that the process that reaches the communication operation first is blocked until a complementary process reaches the same operation. See also guard.
cycle a cycle of the computer clock is an electronic signal that counts a single unit of time within a computer.
cyclic reduction (n.) a parallel algorithm to solve general first order linear recurrence relations.
DACS (n.) Digital Access Cross-connect System is a switch that enables test access and switching of digital signals in a T system.
data cache (n.) a cache that holds data but does not hold instructions.
data flow analysis (n.) process of finding dependencies among instructions.
data flow graph (n.) (1) machine language for a data flow computer; (2) result of data flow analysis.
data parallelism (n.) A model of parallel computing in which a single operation can be applied to all elements of a data structure simultaneously. Typically, these data structures are arrays, and the operations are arithmetic and act independently on every array element, or reduction operations. See also array processor, processor array, SIMD and vector processor.
data-driven (n.) a data flow architecture in which execution of instructions depends on availability of operands.
dataflow (n.) A model of parallel computing in which programs are represented as dependence graphs and each operation is automatically blocked until the values on which it depends are available. The parallel functional and parallel logic programming models are very similar to the dataflow model.
dead code(n.) A portion of a program that does not have to be executed (because the values it calculates will never be used) or that will never be entered. Compiler optimization usually removes sections of dead code. See also dependence.
deadlock (n.) A situation in which each possible activity is blocked, waiting on some other activity that is also blocked. If a directed graph represents how activities depend on others, then deadlock arises if and only if there is a cycle in this graph. See also dependence graph.
decision problem (n.) a problem whose solution, if any, is found by satisfying a set of constraints.
declustered (adj.) A file system that distributes blocks of individual files between several disks. This contrasts with a traditional file system, in which all blocks of a single file are placed on the same disk. See also striped.
decomposition (n.) A division of a data structure into substructures that can be distributed separately, or a technique for dividing a computation into subcomputations that can be executed separately. The most common decomposition strategies in parallel computing are: functional decomposition; geometric decomposition and iterative decomposition.
dedicated throughput (n.) the number of results returned for a single job per time unit.
dependence (n.) The relationship of a calculation B to a calculation A if changes to A, or to the ordering of A and B, could affect B. If A and B are calculations in a program, for example, then B is dependent on A if B uses values calculated by A. There are four types of dependence: true dependence, where B uses values calculated by A; antidependence, where A uses values overwritten by B; output dependence, where A and B both write to the same variables; control dependence, where B's execution is controlled by values set in A. Dependence is also used in message routing to mean that some activity X cannot proceed until another activity Y has completed. For example, if X and Y are messages attempting to pass through a region with limited buffer space, and Y currently holds some or all of the buffer, X may depend on Y releasing some buffer space before proceeding.
dependence graph (n.) A directed graph whose nodes represent calculations and whose edges represent dependencies among those calculations. If the calculation represented by node k depends on the calculations represented by nodes i and j, then the dependence graph contains the edges i-k and j-k. See also compiler optimization, dataflow, dependence.
dependence analysis (n.) an analysis by compiler or precompiler that reveals which portions of a program depend on the prior completion of other portions of the program. Dependency analysis usually relates statements in an iterative code construct.
depth (n.) parallel time complexity.
deque (n.) a double ended queue; that is a list of elements on which insertions and deletions can be performed at both the front and rear.
deterministic model (n.) a task model in which precedence relations between tasks and the execution time needed by each task are fixed and known before the schedule is devised.
diameter (n.) The distance across a graph, measured by the number of links traversed. Diameter is usually taken to mean maximum diameter (ie the greatest internode distance in the graph, but it can also mean the average of all internode distances. Diameter is sometimes used as a measure of the goodness of a topology.
direct mapping (n.) a cache that has a set associativity of one so that each item has a unique place in the cache at which it can be stored.
direct method (n.) Any technique for solving a system of equations that relies on linear algebra directly. LU decomposition with back substitution is an example of a direct method. See also indirect method.
directed graph (n.) a graph in which the edges have an orientation, denoted by arrowheads.
disjoint memory (n.) Memory that appears to the user to be divided amongst many separate address spaces. In a multicomputer, each processor typically has its own private memory and manages requests to it from processes running on other processors. Disjoint memory is more commonly called distributed memory, but the memory of many shared memory> computers is physically distributed.
disk striping (n.) technique of interleaving a disk file across two or more disk drives to enhance input/output performance. The performance gain is a function of the number of drives and channels used.
distributed computer (n.) A computer made up of many smaller and potentially independent computers, such as a network of workstations. This architecture is increasingly studied because of its cost effectiveness and flexibility. Distributed computers are often heterogeneous. See also multi-processor, multicomputer.
distributed memory (n.) Memory that is physically distributed amongst several modules. A distributed memory architecture may appear to users to have a single address space and a single shared memory or may appear as disjoint memory made up of many separate address spaces.
divide and conquer (n.) a problem solving methodology that involves partitioning a problem into subproblems, solving the subproblems, and then combining the solutions to the subproblems into a solution for the original problem.
DS0 (n.) .Digital service hierachy level 0 with a maximum channel capacity of 64kbps.
DS1 (n.) Digital service hierachy level 1 with a maximum channel capacity of 1.544Mbps. This term is used interchangeably with T1. 24 DS-0 channels per DS1.
dusty deck (n.) A term applied to old programs (usually Fortran or Cobol). The term is derived from the image of a deck of punched cards grown dusty over the years.
e-cube routing(n.) A message routing algorithm used on binary hypercubes. If <|Sn-1...S0|> and <|Dn-1...D0|> are the binary co-ordinates of the source and destination of the message, and <|Xn-1...X0>| their difference, then the e-cube algorithm routes the message along link parallel to the axes corresponding to 1's in <|Xn-1...X0|>, in order from highest to lowest. See also Metropolis routing, randomized routing.
eager evaluation (n.) A scheduling policy under which each computation begins as soon as its inputs are ready or its necessary preconditions have been satisfied. This is the scheduling policy used in most programs, but contrasts with the lazy evaluation policy often used in functional and logic programming. See also dataflow, dependence dependence graph.
EGP (n.) External Gateway protocol is used by gateways in different autonomous systems. EGP allows gateways to share routing information through advertisements.
enumeration sort (n.) a sort that finds the position of each name by determining the number of names smaller than it.
EPROM (n.) Electronically programmable ROM; a memory whose contents can be changed using special hardware. This usually involves removing the chips from their environment in order to "burn" a new pattern into them.
ES-IS (n.) End System to Intermediate System is an OSI protocol that allows end systems to announce themselves as intermediate systems. End Systems are equivalent to the internet concept of a host that uses all layers in the communications model. Intermediate Systems are relay communications nodes or forwarders between End Systems and are effectively a subset of End Systems.
ethernet (n.) A LAN protocol that supports high speed communications in the local area. Usually rates are at 10Mbps.
expand(n.) a vector computer instruction that stores the elements of a vector according to the values in a corresponding masking vector.
expected space complexity (n.) the average amount of space used by an algorithm over all possible inputs.
expected time complexity (n.) the average amount of time used by an algorithm over all possible inputs.
fairness (n.) A property of a concurrent system. If a system is fair, then in the long run all processes are served equally. No process has preferential access to semaphores and in particular no process can livelock. See also deadlock.
fast packet (n.) Fast packet is a general term for various streamlined packet technologies including frame relay, BISDN and ATM. Compared to X.25 packet switching, fast packet contains a much reduced functionality, but with the lower overhead, fast packet systems can operate at higher rates at the same processing cost.
FastPacketTM (n.) StrataCom Corporation's trademarked term for their proprietary switching technique, which uses 192 bit packets.
fetch-and-add (n.) a computer synchronization instruction that updates a word in memory, returns the value before the update, and produces a set of results as if the processors executed in some arbitrary order. See also prefetch.
FFT (n.) The fast Fourier transform is a technique for the rapid calculation of discrete Fourier transform of a function specified discretely at regular intervals. The technique makes use of a butterfly data structure.
FIFO (n.) First in, first out, a queue.
file server (n.) A process running on a computer that provides access to files for remote user systems.
file system (n.) The hardware used for nonvolatile data storage; the system software that controls this hardware; the architecture of this hardware and software. A parallel file system is one that can be read or written to by many processors simultaneously. See also RAID.
finite difference method (n.) A direct method for the approximate solution of partial differential equations on a discrete grid, by approximating derivatives of the unknown quantities on the grid by linear differences. Array processors lend themselves very well to the efficient implementation to this sort of application. See also finite element method.
finite element method (n.) An approximate method for solving partial differential equations by replacing continuous functions by piecewise approximations defined on polygons, which are referred to as elements. Usually polynomial approximations are used. The finite element method reduces the problem of finding the solution at the vertices of the polygons to that of solving a set of linear equations. This task may then be accomplished by a number of methods, including Gaussian elimination, the conjugate gradient method and the multigrid method. See also finite difference method.
Flynn's Taxonomy (n.) A classification system for architectures that has two axes: the number of instructions streams executing concurrently, and the number of data sets to which those instructions are being applied. The scheme was proposed by Flynn in 1966. See also SPMD, MIMD, SIMD, SISD.
forall (n.) A programming construct that specifies a set of loop iterations and further specifies that these iterations can be done in any order. data parallel and shared variable programs are often expressed using forall loops.
frame slip (n.) Also called just slip, is any shift of the timing on a circuit.
FT-1 (n.) Fractional digital service hierachy level 1 with service in multiples of 56/64Kbps 2 channels (112/128Kbps) or above, and up to 23 channels. 256/512/768/1024Kbps are common rates for this type of service. Also called fractional T1.
functional unit (n.) functionally independent part of the ALU each of which performs a specific function, for example: address calculation; floating-point add or floating-point multiply.
futures (n.) A programming construct specifying that the value of some computation will be needed at some later point, but allowing the system to schedule that computation to run at any arbitrary time. Futures are one way to present lazy evaluation in shared variable programming. See also eager evaluation.
fuzzy barrier(n.) A pair of points in a program such that no process may pass the second point before all processes have reached the first. Fuzzy barriers are often used inside loop iterations to ensure that no changes are made during an iteration before all values needed in the previous iteration have been read. See also barrier, barrier synchronization, dependence.
GaAs (n.) Gallium arsenide; an relatively new semiconductor material, still not yet fully mastered by chip manufacturers. GaAs components can run much faster than silicon-based components. See also CMOS.
game tree (n.) the state space tree representation of a game position.
Gantt chart (n.) a diagram used to illustrate a deterministic schedule.
gather/scatter (n.) the operations related to large sparse data structures. A full vector with relatively few nonzeroes is transformed into a vector with only those nonzeroes by using a gather operation. The full vector, or one with the same structure, is built from the inverse operation or scatter. The process is accomplished with an index vector, which is usually the length of the number of nonzeroes, with each component being the relative location in the full vector. See also compress/index.
Gauss-Seidel method (n.) An iterative method for solving partial differential equations on a grid. When updating a grid point the new value depends on the current values at the neighbouring grid points, some of which are from the previous iteration and some, which have already been updated, are from the current iteration. So updates are performed by sweeping through the grid in some user-chosen fashion. The key feature of this algorithm is that it makes use of new information (updated grid point values) as soon as they become available.
Gaussian elimination (n.) A method for solving sets of simultaneous linear equations by eliminating variables from the successive equations. The original equation in the form Ax=b (A is a matrix, b the vector of known values, and x the unknown solution vector) is reduced to Ux=c, where U is an upper triangular matrix. The solution vector x can then be found by back substitution. This method is usually formulated as LU decomposition.
generative communication(n.) A model of parallel computing in which processes that have been spawned dynamically turn into data upon completion, and data may be stored in tuples in one or more shared tuple spaces. A process may add tuples to a tuple space, or remove then by matching against their contents. See also associative memory, shared variables, virtual shared memory.
GGP (n.) gateway to gateway protocol is the protocol used by core gateways to exchange routing information.
GKS (n.) the Graphics Kernel Standard; a graphics standard developed for pen plotters and now supported on a wide variety of pixel based devices.
global addressing (n.) A frame relay addressing scheme which uses the DLCI to identify a specific end across device somewhere else in the frame relay network. In a global addressing scheme, the DLCI is a unique identifier for each IPX port in the network.
granularity (n.) The size of operations done by a process between communications events. A fine grained process may perform only a few arithmetic operations between processing one message and the next, whereas a coarse grained process may perform millions. See also computation-to-communication ratio.
graph (n.) an entity consisting of a set of vertices and a set of edges between pairs of vertices.
Gray code (n.) A mapping which labels the lattice points on an n-dimensional grid with the integers 0,1,2,...2^d-1, so that the labels at adjacent grid points differ in precisely one bit in their integer representation.
Grosch's Law (n.) an empirical rule that the cost of computer systems increases as the square root of the computational power of the systems.
guard ring (n.) Parallel algorithms operating on a two dimensional grid generally store grid values in a two dimensional array, geometrically decomposed across the nodes of the parallel computer. The guard ring is the additional storage allocated around the edges of the array to hold the values at the grid points lying along the adjacent boundaries of neighbouring nodes. These values are obtained by communication between nodes.
Gustafson's Law(n.) A rule stating that if the size of most problems is scaled up sufficiently, then any required efficiency can be achieved on any number of processors, as stated by Gustafson in 1988. See also Amdahl's Law, efficiency.
halo (n.) The region around a point in a mesh, whose values are used when updating the value of the central point. In a cellular automata, the halo comprises those neighbouring cells whose values are used by the automaton's update rule.
Hamiltonian(n.) A closed path through a graph which passes through each node in the graph exactly once.
Hawick (n.) a Scots word meaning a "town surrounded by a hedge"; also an actual town on the border between Scotland and England; also my surname. This is not relevant to HPCC except that this is a usful way of ensuring my email address (firstname.lastname@example.org) does not get lost from this file so you can always seek out the latest version of this glossary.
HDB3 (n.) High density bipolar three, a line interface standard for E1 which is similar to B8ZS, which eliminates data streams with 8 or more consecutive zeros. Allows for 64Kbps clear channel capacity and still assure a minimum ones density required by E1 lines.
HDLC (n.) high level link control is an ISO link level protocol standard. CCITT uses HDLC for its link access protocol with X.25 networks. HDLC was used in the ARPANET to transfer frames between hosts and packet switched networks.
height (n.) in graph theory, height is the length of the longest path from the root of a tree to one of its leaves.
heterogeneous(adj.) Containing components of more than one kind. A heterogeneous architecture may be one in which some components are processors, and others memories, or it may be one that uses different types of processor together. See also distributed computer, homogeneous.
high-order interleaving (n.) memory interleaving strategy based on high-order bits of an address.
hit ratio (n.) the ratio of the number of times data requested from a cache is found (or hit) to the number of times it is not found (or missed).
homogeneous(adj.) Made up of identical components. A homogeneous architecture is one in which each element is of the same type; processor arrays and multicomputers are usually homogeneous. See also heterogeneous.
horizontal processing (n.) act of processing a two dimensional array row by row.
Horn clause (n.) a clause that contains at most one conclusion.
hot-spot contention (n.) an interference phenomenon observed in multiprocessors caused by memory access statistics being slightly skewed from a uniform distribution to favour a specific memory module.
HPCC (n.) an acronymn for High Performance Computing and Communications, which is the field of information addressed by this glossary. A USA National Coordination Office for HPCC also exists, and other information on HPCC can be found from the Northeast Parallel Architectures Center, the Center for Research in Parallel Computing the National Software Exchange or the Edinburgh Parallel Computing Centre depending upon your geography.
hypercube (n.) A topology of which each node is the vertex of a d-Dimensional cube. In a binary hypercube, each node is connected to n others, and its co-ordinates are one of the 2^n different n-bit sequences of binary digits. Most early American multicomputers used hypercubic topologies, and so the term hypercube is sometimes used as a synonym for multicomputers. See also butterfly, e-cube routing, shuffle exchange network.
I/O (n.) refers to the hardware and software mechanisms connecting a computer with its environment. This includes connections between the computer and its disk and bulk storage system and also connections to user terminals, graphics systems, and networks to other computer systems or devices. Standard I/O is a particular software package developed under UNIX for the C programming language.
ICMP (n.) is the internet control message protocol and is used to handle errors and control messges at the internet protocol layer. ICMP is considered to be part of IP and is used to test whether a destination is reachable and responding.
IGP (n.) interior gateway protocol is used to exchange routing information between routers in the internet. It is generally used for routers within an autonomous system. The RIP and OSPF are examples of IGP.
ILMI (n.) Interim Local Management Interface that is the de facto standard until a standard based committee designates a standardized protocol suite. In the last few years ILMI has been popular due to the frequency of changes in the tele-communications industry.
implicitly parallel (n.) language semantics that do not allow the user to explicitly describe which computations are independent and can be performed in parallel. For an implicitly parallel language, the compiler must deduce or prove independence in order to make use of parallel hardware. The comparative difficulty of the deduction separates implicitly parallel languages from explicitly parallel ones.
indirect method (n.) Any technique for solving a system of equations that does not rely on linear algebra directly. Successive over-relaxation is an example of an indirect method. See also direct method.
information (n.) a collection of related data objects.
instruction buffering (n.) process of prefetching instructions with the goal of never making the processor wait for an instruction to be fetched. This is sometimes also known as instruction look-ahead.
instruction cache (n.) a cache memory that holds only instructions but not data.
instruction scheduling (n.) a strategy of a compiler to analyse the outcome of the operations specified in a program and to issue instructions in an optimal manner. That is, the instructions are not necessarily issued in the order specified by the programmer, but in an order that optimally uses the registers, functional units and memory paths of the computer, while at the same time guaranteeing correct results for the computation.
instruction set (n.) the set of low level instructions that a computer is capable of executing. Programs expressed in a high level language must ultimately be reduced to these.
instruction stream (n.) sequence of instructions performed by a computer.
interconnection network (n.) the system of logic and conductors that connects the processors in a parallel computer system. Some examples are bus, mesh, hypercube and Omega networks.
interleaved memory (n.) memory divide into a number of modules or banks that can be accessed simultaneously.
internal sort (n.) sorting a table of names contained entirely in primary memory.
interprocessor communication (n.) the passing of data and information among the processors of a parallel computer during the execution of a parallel program.
interprocessor contention (n.) conflicts caused when multiple CPU's compete for shared system resources. For example, memory bank conflicts for a user's code in global memory architectures are caused by other processors running independent applications.
interrupt-driven system (n.) A system whose processes communicate by message passing in such a way that when a message is delivered to its destination process it interrupts execution of that process and initiates execution of an interrupt handler process, which stores the message for subsequent retrieval. On completion of the interrupt-handler process (which sets some flag or sends some signal to denote an available message), the original process resumes execution.
interval routing (n.) A routing algorithm that assigns an integer identifier to each possible destination and then labels the outgoing links of each node with a single contiguous interval or window so that a message can be routed simply by sending it out the link in whose interval its destination identifier falls.
invariant (n.) a variable, especially in a DO-loop, that appears only on the right side of an equals sign. The variable is read only, it is never assigned a new value.
IP (n.) The internet protocol that defines the unit of information passed between systems that provides a basis packet devlivery service. It handles best effort connectionless delivery service and includes ICMP. See also IP address.
IP address (n.) The Internet protocol address which is a 32-bit address assigned to a host. The IP address has a host component and a network component.
IPX/link (n.) This application for NetWare connects a PC Novell NetWare LAN through a network interface device.
ISO (n.) International Standards Organization, which, among other things, sets standards for programming languages.
isoefficiency(n.) A way to quantify the effect of scaling problem size on an algorithm's efficiency. For a given efficiency, the isoefficiency function for an algorithm specifies what size of problem must be solved on a given number of processors to achieve that efficiency.
iterative decomposition (n.) a decomposition involving breaking down a calculation in which one or more operations are repeatedly applied to one or more data values by treating each data subset / operation subset as a separate task and distributing tasks amongst available processors, either deterministically or randomly. In a deterministic decomposition, the data to be processed are fixed and the same operations are applied to each. In a speculative decomposition, different operations are applied simultaneously to the same input until at least one has completed. See also functional decomposition and geometric decomposition.
iterative deepening (n.) use of a D-ply search to prepare for a (D+1)-ply search.
Jacobi method (n.) A stationary, iterative method for solving a partial differential equation on a discrete grid. The update of each grid point depends only on the values at neighbouring grid points from the previous iteration. See also Gauss-Seidel method.
kernel (n.) A process providing basic services. A service kernel can run on every processor to support minimal operating system services, while a routing kernel can handle or forward incoming messages.
key (n.) unique object of a search.
knowledge (n.) information plus semantic meaning.
LAN (n.) Local area network, a network of multiple interconnected data terminals or devices within a local area to facilitate data transfer. Most notable of LAN topologies is ethernet, token ring, FDDI, etc.
LAP (n.) link access procedure is a modified form of HDLC that CCITT specified for X.25 networks. LAP-B is link access procedures- balanced and is the X.25 implementation of SDLC and similarly, LAP-D is the ISDN and frame relay implementation of SDLC.
latency (n.) The time taken to service a request or deliver a message which is independent of the size or nature of the operation. The latency of a message passing system is the minimum time to deliver a message, even one of zero length that does not have to leave the source processor; the latency of a file system is the time required to decode and execute a null operation. See also startup cost.
lazy evaluation (n.) A Scheduling policy under which no calculation is begun until it is certain that its result is needed. This policy contrasts with the eager evaluation used in most programs, but is often used in functional and logic programming. See also dataflow, dependence, dependence graph, futures.
LD-1 (n.) An integrated T1 with possibly voice, data and frame relay circuits. Fractional digital service hierachy level 1 with service much the same as FT-1 except the service is integrated with voice, data, video and frame relay.
light-weight process (n.) A process which executes concurrently with other processes, in the same address space and in an unprotected fashion. Light-weight processes are used by systems such as MACH to reduce the overhead of process start-up.
linear speedup (n.) Speedup that is directly proportional to the number of processors used. According to Amdahl's Law, linear speedup is not possible for a problem that contains any sequential portion, no matter how small. Gustafson's Law however, states that linear speedup can be achieved if the problem size is increased as well as the number of processor employed. See also superlinear speedup.
linear vector scan (n.) a table lookup algorithm for pipelined vector processors that in a single operation compares a large number of contiguous elements of the table against the key.
link loading (n.) The amount of communication traffic carried by a link, or by the most heavily loaded link in the system. As link loading increases, both latency and contention are likely to increase. See also bisection bandwidth.
linked array (n.) a data structure designed to allow the joining of various sized lists so that the inserting and deleting of the list elements can be done in parallel without contention.
linked triadic operation (n.) act of executing a pair of vector operations, such as V+S*V as if they were a single, longer operation by taking the output of the first pipelined functional unit and directing it to the second pipelined functional unit, avoiding a trip to main memory and back.
LINPACK (n.) A linear algebra software package, which has been mounted on a wide range of platforms. It has now been superceded by LAPACK. (n.) also a set of widely quoted performance benchmarks based on linear algebra and available from the National Software Exchange.
LIPS (n.) logical inferences per second; procedure calls per second in a logic programming system.
live variable(n.) A variable visible to a process and whose value can be changed by actions taken outside that process. For example, if a variable is shared by two processes, one of which can write to it at some point, then that variable is live within the other process. See also race condition, shared variables.
LIW(n.) Long Instruction Words: the use of long (64 or more bits) instruction words in a processor to improve its ability to pipeline calculations.
LMI (n.) Local Management Interface, a protocol, with 4 different versions, used to control the local interface from a routing device to the IPX Switch. Also used for configuration, flow control and maintenance of the local connection.
load balance(n.) The degree to which work is evenly distributed among available processors. A program executes most quickly when it is perfectly load balanced, that is when every processor has a share of the total amount of work to perform so that all processors complete their assigned tasks at the same time. One measure of load imbalance is the ratio of the difference between the finishing times of the first and last processors to complete their portion of the calculation to the time taken by the last processor.
locality (n.) The degree to which the computations done by a processor depend only on data values held in memory that is close to that processor, or the degree to which computations done on a point in some data structure depend only on values near that point. Locality can be measured by the ratio of local to nonlocal data accesses, or by the distribution of distances of, or times taken by, nonlocal accesses. See also halo, stencil.
locality of reference (n.) the observation that references to memory tend to cluster. Temporal locality refers to the observation that a particular datum or instruction, once referenced, is often referenced again in the near future. Spatial locality refers to the observation that once a particular location is referenced, a nearby location is often referenced in the near future.
lock (n.) Any device or algorithm whose use guarantees that only one process can perform some action or use some resource at a time.
logarithmic cost criterion (n.) cost criterion that assumes the cost of performing an instruction is proportional to the length of the operands. The integer N requires at least log N + 1 bits of memory, hence the name.
logic (n.) the branch of mathematics that investigates the relationships between premises and conclusions of arguments.
logical time (n.) Elapsed time as seen from within processes. This may differ from clock time, because processes can block or be suspended during multitasking and because they can run at different speeds. The logical times of events only define a partial order on those events.
loop unrolling (n.) A compiler optimization technique in which the body of a loop is replicated L times, and the number of iterations of that loop reduced by a corresponding factor L. By lengthening the basic block inside the loop, this can increase the scope for vectorization and other optimizations.
loopback test (n.) A circuit test at any device which will tie the transmit data to the receive data in order to apply a signal and receive the data back for interpretation.
loose synchronization (adj.) A program running on a concurrent computer is said to be running in loose synchronization if the nodes are constrained to intermittently synchronize with each other via some communication. Frequently, some global computational parameter such as a time or iteration count provides a natural synchronization reference. This parameter divides the running program into compute and communication cycles. See also synchronous.
low-order interleaving (n.) a memory interleaving based on the low-order bits of the address.
lower triangular matrix (n.) a matrix with no nonzero elements above the main diagonal.
LU decomposition (n.) a technique where a matrix A is represented as the product of a lower triangular matrix, L, and an upper triangular matrix U. This decomposition can be made unique either by stipulating that the diagonal elements of L be unity, or that the diagonal elements of L and U be correspondingly identical.
M-section (n.) a table lookup algorithm for pipelined vector processors that combines features of bisection and linear vector scan.
MACH (n.) an operating system based on Berkely UNIX developed by Carnegie Mellon University.
macrotasking (n.) technique of dividing a computation into two or more large tasks to be executed in parallel. Typically the tasks are subroutine calls executed in parallel.
mailbox (n.) an address used as a source or destination designator in a message.
marshall(v.) To compact the values of several variables, arrays, or structures into a single contiguous block of memory; copying values out of a block of memory is called unmarshalling. In most message passing systems, data must be marshalled to be sent in a single message.
memory bank conflict (n.) a condition that occurs when a memory unit receives a request to fetch or store a data item prior to completion of its bank cycle time since its last such request.
memory protection (n.) Any system that prevents one process from accessing a region of memory being used by another. Memory protection is supported both in hardware and by the operating system of most serial computers, and by the hardware kernel and service kernel of the processors in most parallel computers.
mesh(n.) A topology in which nodes form a regular acyclic d-dimensional grid, and each edge is parallel to a grid axis and joins two nodes that are adjacent along that axis. The architecture of many multicomputers is a two or three dimensional mesh; meshes are also the basis of many scientific calculations, in which each node represents a point in space, and the edges define the neighbours of a node. See also hypercube, torus.
message passing (n.) A style of interprocess communication in which processes send discrete messages to one another. Some computer architectures are called message passing architectures because they support this model in hardware, although message passing has often been used to construct operating systems and network software for uniprocessors and distributed computers. See also routing.
message typing (n.) The association of information with a message that identifies the nature of its contents. Most message passing systems automatically transfer information about a message's sender to its receiver. Many also require the sender to specify a type for the message, and let the receiver select which types of messages it is willing to receive.
Metropolis routing(n.) A routing algorithm for meshes, in which an ordering is imposed on axes, and messages are sent as far along the most significant axis as they need to go, then as far along the next most significant axis, and so on until they reach their destination. See also e-cube routing, randomized routing.
microtasking (n.) technique of employing parallelism at the DO-loop level. Different iterations of a loop are executed in parallel on different processors.
MIMD (n.) Multiple Instruction, Multiple Data; a category of Flynn's taxonomy in which many instruction streams are concurrently applied to multiple data sets. A MIMD architecture is one in which heterogeneous processes may execute at different rates.
minimax (n.)algorithm used to determine the value of a game tree.
minimum spanning tree (n.) a spanning tree with the smallest possible weight among all spanning trees for a given graph.
MIR (n.) Minimum Information Rate or Committed Information Rate (CIR), is the minimum transmit and receive data rate for a connection.
MISD (n.) Multiple Instruction, Single Data. A member of Flynn's taxonomy almost never used. This category has an ambiguous meaning. It refers to a computer which applies several instructions to each datum. The closest real implementation to this category is a vector computer with an instruction pipeline.
module (n.) a memory bank, often used in the context of interleaved memory.
monitor (n.) a structure consisting of variables representing the state of some resource, procedures to implement operations on that resource, and initialization code.
Monte Carlo (adj.) Making use of randomness. A simulation in which many independent trials are run independently to gather statistics is a Monte Carlo simulation. A search algorithm that uses randomness to try to speed up convergence is a Monte Carlo algorithm.
motherboard (n.) A printed circuit board or card on which other boards or cards can be mounted. Motherboards will generally have a number of slots for other boards, by which means the computer system may be expanded. When all the slots are used up however, it is usually difficult to expand further, and this is the manufacturer's way of telling you to buy a bigger system.
multicomputer(n.) A computer in which processors can execute separate instruction streams, have their own private memories and cannot directly access one another's memories. Most multicomputers are disjoint memory machines, constructed by joining nodes (each containing a microprocessor and some memory) via links. See also architecture, distributed computer, multiprocessor, processor array.
multigrid method (n.) A method for solving partial differential equations in which an approximate solution on a coarse resolution grid is used to obtain an improved solution on a finer resolution grid. The method reduces long wavelength components of the error or residual by iterating between a hierarchy of coarse and fine resolution grids.
multiprocessor(n.) A computer in which processors can execute separate instruction streams, but have access to a single address space. Most multiprocessors are shared memory machines, constructed by connecting several processors to one or more memory banks through a bus or switch. See also architecture, distributed computer, multicomputer, processor array.
multitasking(n.) Executing many processes on a single processor. This is usually done by time-slicing the execution of individual processes and performing a context switch each time a process is swapped in or out, but is supported by special-purpose hardware in some computers. Most operating systems support multitasking, but it can be costly if the need to switch large caches or execution pipelines makes context switching expensive in time.
mutual exclusion(n.) A situation in which at most one process can be engaged in a specified activity at any time. Semaphores are often used to implement this. See also contention, deadlock, critical sections.
n-1/2 (n.) (pronounced as n-one-half, usually written with a subscript.) The minimum vector length on which a pipelined architecture delivers one half of its theoretical peak performance. The larger n-1/2 is, the longer calculations must be to amortize the startup cost of the pipeline. This measure was coined by Hockney and Jesshope in 1988. See also r-inf.
necklace (n.) the nodes a data item travels through in response to a sequence of shuffles.
need-predictable (adj.) Used to describe a concurrent algorithm in which the need for but not the nature of point-to-point communication between nodes is known prior to program execution. Need predictable problems are loosely synchronous.
NeTBIOS (n.) Network Basic Input/Output System that provides a Session layer interface between network applications running on a PC and the underlying protocol software of the Transport and Network layers on the OSI model. Normally a LAN protocol.
neural network (n.) Man made devices using interconnects and processing capabilities suggested by models of the cortex are termed neural networks. These systems are widely used for optimization problems including content addressable memories and pattern recognition.
NFS (n.) network file system ia a protocol developed to use IP and allow a set of computers to access each other's file systems as if they were on the local host.
node (n.) generic term used to refer to an entity that accesses a network.
non-blocking(adj.) An operation that does not block the execution of the process using it. Usually applied to communications operations, where it implies that the communicating process may perform other operations before the communication has completed. See also blocking.
non-deterministic model (n.) a task model in which the execution time of each task is represented by a random variable.
NP complete (adj.) A term used to describe a problem if it can only be solved in polynomial time by non-deterministic methods. In practice such problems are "hard" and are only solved by a number of heuristic methods that only give approximate answers near to the optimum solution.
NUMA (adj.) Nonuniform memory access; not supporting constant-time read and write operations. In most NUMA architectures, memory is organised hierarchically, so that some portions can be read and written more quickly by some processors than by others. See also UMA
oblivious(adj.) Working in the same fashion regardless of surroundings or history. An oblivious scheduling strategy always schedules processes in the same way, no matter how much work they have done in the past; an oblivious routing algorithm always routes messages in the same direction, regardless of local load. See also adaptive.
OEM (n.) Original Equipment Manufacturer; a company which adds components to someone else's computers and sells the result as a complete product.
OLTP (n.) On-line transaction processing; handling transactions (such as deposits and withdrawals) as they occur. An application area of great importance to banks and insurance companies.
Omega network (n.) a composition of shuffle-exchange networks with programmable switches.
OOF (n.) out of frame condition counter that increments every change in the framing status of a circuit or device.
operation oriented language (n.) a programming language using remote procedure calls as the principle means for interprocess communication and synchronization.
optimization block (n.) a block of code (rarely a whole subprogram, but often a single DO-loop) in which a compiler optimizes the generated code. A few compilers attempt to optimize across such blocks; many just work on each block separately.
optimization problem (n.) a problem whose solution involves satisfying a set of constraints and minimising (or maximising) and objective function.
OR-parallelism (n.) A form of parallelism exploited by some implementations of parallel logic programming languages, in which the terms in disjunctions are evaluated simultaneously, and the parent computation allowed to proceed as soon as any of its children have completed. See also AND-parallelism.
OSF (n.) Open Software Foundation; an organization established by a number of the major computer manufacturers to set software standards.
packet switching(n.) A routing technique in which intermediate nodes wait until they have received the whole of a message before forwarding any of it. Packet switching often requires a large amount of buffer space, and contention for access to this space can lead to deadlock. See also virtual cut-through, wormhole routing.
PAD (n.) packet assembler/disassembler, a device that converts a serial data stream into discrete packets in the transmit direction and converts the received packets back into a serial data stream. Adds header information in the transmit packet to allow it to be routed to the proper destination.
page (n.) The smallest unit of a virtual memory system. The system maintains separate virtual-to-physical translation information for each page.
parallel balance point (n.) The point at which using more processors to do a calculation increases the time taken to complete that calculation. See also Amdahl's Law, efficiency, Gustafson's Law, isoefficiency, speedup.
parallel computation thesis (n.) time on an unbounded parallel model of computation is (polynomially) equivalent to space on a sequential model of computation. (Unproved)
parallel computer (n.) A computer system made up of many identifiable processing units working together in parallel. The term is often used synonymously with concurrent computer to include both multiprocessor and multicomputer. The term concurrent generally dominates in usage in the USA, whereas the term parallel is the more widely used in Europe.
parallel prefix (n.) An operation applying an associative binary operator o to an n-vector V to produce: <(V0)(V0oV1)(V0oV1oV2)...(V0oV1o...oVn)>. Variations of this operation, which is also sometimes called a scan, may leave the operations identity element in the first element of the vector, apply the operation downward from the last element of the vector, and so on. See also reduction, scan-vector model, segmented parallel prefix operation.
parallel slackness (n.) Hiding the latency of communication by giving each processor many different task, and having them work on the tasks that are ready while other tasks are blocked (waiting on communication or other operations).
parallel unification(n.) finding the set of possible unifications for a goal in parallel.
parallelization (n.) Turning a serial computation into a parallel one. Also sometimes turning a vector computation into a parallel one. This may be done automatically by a parallelising compiler or (more usually) by rewriting (parts of) the program so that it uses some parallel paradigm. See also dataflow, data parallelism, futures, generative communication, message passing, shared variables.
parsing (n.) process whereby a compiler analyzes the syntax of a program to establish the relationship among operators, operands, and other tokens of a program. Parsing does not involve any semantic analysis.
partial cascade sum (n.)parallel algorithms to compute partial sums in logarithmic time.
partial sum (n.) the result from a summation of some subsequence of the elements in a sequence of operands.
partitioning (n.) process of restructuring a program or algorithm into independent computational segments. The goal is to have multiple processors simultaneously work on these independent computational segments.
PDE (n.) partial differential equation.
percentage parallelization (n.) the percentage of processor expenditure processed in parallel on a single job. It is not usually possible to achieve 100 percent of an application's processing time to be shared equally on all processors. See Amdahl's Law.
percentage vectorization (n.) The percentage of an application executing in vector mode. This percentage may be calculated as a percentage of CPU time or as the percentage of lines of code (usually Fortran) in vector instructions. The two approaches are not consistent and may give very different ratings. The first calculation method leads to performance improvement as measured by CPU time, while the second method measures the success rate of the compiler in converting scalar code to vector code. The former is the more meaningful hardware performance measure. See also vectorize.
ping (n.) The packet internet groper is a program useful in testing and debugging LAN/WAN troubles. It sends out an echo and expects a specified host to respond back in a specified time frame.
pipelining (n.) Overlapping the execution of two or more operations. Pipelining is used within processors by prefetching instructions on the assumption that no branches are going to preempt their execution; in vector processors, in which application of a single operation to the elements of a vector or vectors may be pipelined to decrease the time needed to complete the aggregate operation; and in multiprocessors and multicomputers, in which a process may send a request for values before it reaches the computation that requires them. See also architecture.
pivot (n.) A particular element of a matrix during some algorithm. Many matrix algorithms need carefully chosen pivots to improve their numerical accuracy and stability. Pivoting involves arranging that the pivotal element has an appropriate numerical value by reordering and/or swapping rows and columns in the matrix.
point of presence (pop) (n.) The physical access location interface between a local exchange carrier and the main network. The point to which the telephone company terminates a subscriber's circuit for long distance service or leased line communications.
polling (adj.) of a communication system. Polling involves a node inspecting the communication hardware - typically a flag bit - to see if information has arrived or departed. Polling is an alternative to an interrupt driven system. The natural synchronization of the nodes imposed by polling is used in the implementation of blocking communications primitives.
ports (n.) a variant of mailboxes allowing multiple client processes but only a single server process.
POSIX (n.) A standard of definition of the interface to the UNIX operating system.
PRAM (n.) Parallel random access machine; a theoretical model of parallel computation in which an arbitrary but finite number of processors can access any value in an arbitrarily large shared memory in a single time step. Processors may execute different instruction streams, but work synchronously. The three most important variations of the PRAM are: EREW - Exclusive read, exclusive write; any memory location may only be accessed once in any one step. CREW - Concurrent read, exclusive write; any memory location may be read any number of times during a single step, but only written to once, with the write taking place after the reads. CRCW - Concurrent read, concurrent write; any memory location may be written to or read from any number of times during a single step. A CRCW PRAM model must define some rule for resolving multiple writes, such as giving priority to the lowest-numbered processor or choosing amongst processors randomly. The PRAM is popular because it is theoretically tractable and because it gives algorithm designers a common target. However, PRAMs cannot be emulated optimally on all architectures. See also NC.
preconditioning (n.) a technique for improving the convergence of iterative matrix inversion algorithms such as the conjugate gradient method, by transforming the matrix of equation coefficients so that the eigenvalues are redistributed.
prefetch (v.) to fetch or load a data entity or program instruction from memory in advance of actually starting to process it. Processors that have prefetch instructions can avoid some of the bottlenecks that arise from a memory system that is slower than processing speed.
private line (n.) A full duplex dedicated channel between two specified points.
private memory (n.) Memory that appears to the user to be divided between many address spaces, each of which can be accessed by only one process. Most operating systems rely on some memory protection mechanism to prevent one process from accessing the private memory of another; in disjoint memory machines, the problem is usually finding a way to emulate shared memory using a set of private memories. See also virtual shared memory.
procedure oriented language (n.) a programming language in which process communication and synchronization are accomplished through the use of of shared variables.
process (n.) the fundamental entity of the software implementation on a computer system. A process is a sequentially executing piece of code that runs on one processing unit of the system.
process creation (n.) The act of forking or spawning a new process. If a system permits only static process creation, then all processes are created at the same logical time, and no process may interact with any other until all have been created. If a system permits dynamic process creation, then one process can create another at any time. Most first and second generation multicomputers only supported static process creation, while most multiprocessors, and most operating systems on uniprocessors, support dynamic process creation. See also configuration, generative communication.
process group (n.) A set of processes that can be treated as a single entity for some purposes, such as synchronization and broadcast or multicast operations. In some parallel programming systems there is only one process group, which implicitly contains all processes; in others, programmers can assign processes to groups statically when configuring their program, or dynamically by having processes create, join and leave groups during execution.
process migration (n.) Changing the processor responsible for executing a process during the lifetime of that process. Process migration is sometimes used to dynamically load balance a program or system.
processor array (n.) A computer that consists of a regular mesh of simple processing elements, under the direction of a single control processor. Processor arrays are usually SIMD machines, and are primarily used to support data parallel computations. See also array processor, vector processor.
Prolog (n.) a language for logic programming.
pseudovector (n.) a scalar temporary variable.
QCD (n.) Quantum Chromodynamics; a model of the behaviour of matter on sub-nuclear scales, the simulation of which is very hungry of computing power.
QIR (n.) quiescent information rate, is the transmit and receive connection information rate which is the initial transmit rate after a period of channel inactivity. Used with Foresight connections only. A value between the MIR and the PIR, normally equals the QIR.
r-inf (n.) (pronounced r-inf, often written with a subscript) The performance a pipelined architecture would deliver on an infinitely long vector; that is, the performance of such an architecture when startup costs are not considered. This parameter was coined by Hockney and Jesshope in 1988. See also n-1/2.
race condition(n.) A situation in which the final result of operations being executed by two or more processes depends on the order in which those processes execute. For example, if two processes A and B are to write different values VA and VB to the same variable, then the final value of the variable is determined by the order in which A and B are scheduled.
RAID (n.) Redundant array of inexpensive disks; a file system containing many disks, some of which are used to hold redundant copies of data or error correction codes to increase reliability. RAIDS are often used as parallel access file systems, where the sheer size of storage capacity required precludes using more conventional (but more expensive) disk technology.
random uniform game tree (n.) a game tree whose terminal node values are randomly chosen from some uniform distribution.
randomized routing (n.) A routing technique in which each message is sent to a randomly chosen node, which then then forwards it to its final destination. Theory and practice show that this can greatly reduce the amount of contention for access to links in a multicomputer.
RDMS (n.) Relational Database Management System; software to manage a database in which data are stored by attribute.
reasonable (adj.) a parallel model is said to be reasonable if the number of processors each processor can communicate with directly is bounded by a constant.
recurrence (n.) a relationship in a DO-loop whereby a computation in one iteration of the loop depends upon completion of a previous iteration of the loop. Such dependencies inhibit vectorization.
redundant computation (n.) Calculations that are carried out more than once or by more than one processor. Computations may be done redundantly because it is cheaper to have every processor calculate a value for itself than to have one processor calculate the value and then broadcast it, or because processes may not have enough memory to store all the values they calculate and may need to overwrite some during execution.
relaxation method (n.) A type of indirect method in which the values making up a trial solution to an equation are repeatedly updated according to some rule until convergence criteria are met. See also direct method
remote procedure call (n.) a structured implementation of a client-server interaction.
RGB (adj.) Red-Green-Blue; the most common form of colour display hardware.
RISC (adj.) Reduced instruction set computer; a computer that provides only a few simple instructions but executes them extremely quickly. RISC machines typically rely on instruction prefetching and caching to achieve higher performance than CISC machines. The term is also applied to software designs that give users a small number of simple but efficient operations.
ROM (n.) Read Only Memory; a computer memory which cannot be written to during normal operation.
routing (n.) The act of moving a message from its source to its destination. A routing technique is a way of handling the message as it passes through individual nodes. See also e-cube routing, interval routing, Metropolis routing, packet switching, randomized routing, virtual cut-through, wormhole routing.
RPC (n.) remote procedure call is a popular model for implementing distributed client-server computing environments. It is an alternative to inter-process communication (IPC) which allows remote systems to execute a set of procedures to share information.
satisfiable (adj.) true under the rules of logic.
SAXPY (n.) elemental BLAS operation involving "constant (a) times vector (x) plus vector (y)". The S refers to Fortran Single precision. SAXPY and related BLAS operations are often implemented by the hardware manufacturer as part of the system software and the execution time for such operations has been used a a primitive benchmark of a high performance computing system.
scalable (adj.) Capable of being increased in size, or more accurately, capable of delivering an increase in performance proportional to an increase in size. A scalable architecture is one that can be used as a design for arbitrarily large machines, or one whose increase in performance is linear in the amount of hardware invested. The term is also applied to programming systems, although its meaning is less clear in these cases. It is generally used to imply that the methods and algorithms employed in such systems are in principle capable of performing correctly equally well on large and small hardware systems. See also Gustafson's Law
ScaLAPACK (n.) A linear algebra software package, which has been mounted on a wide range of platforms. This is a version of LAPACK suitable for distributed memory computer systems. The software is available from the National Software Exchange. See also LAPACK.
scalar temporary (n.) a compiler created scalar variable that holds the value of a vectorizable expression on each iteration of a DO-loop.
scan-vector model (n.) A theoretical model of parallel computing in which a scalar processor and a vector processor have access, respectively to a memory holding scalar values and a memory holding vectors of arbitrary length. Vector operations take either a single time step or a time proportional to the logarithm of the number of elements. See parallel prefix, data parallelism, reduction operation.
scheduling (n.) Deciding the order in which the calculations in a program are to be executed, and by which processes. Allocating processes to processors is usually called mapping. See also load balance.
scoreboard (n.) A hardware device that maintains the state of machine resources to enable instructions to execute without conflict at the earliest opportunity.
SCSI (n.) Small Computer Systems Interface; a hardware standard for interfacing to devices such as disks.
segmented parallel prefix operation (n.) A parallel prefix operation in which the vector being operated on is divided into segments, and the operator is applied to each segment as if it were a separate vector. This is usually implemented by supplying a separate vector of segment lengths.
self-scheduling (adj.) Automatically allocating work to processes. If T tasks are to be done by P processors, and P < T, then they may be self-scheduled by keeping them in a central pool from which each processor claims a new job when it finishes executing its old one. See also task farming.
semaphore (n.) A data type for controlling concurrency. A semaphore can be initialised to any non negative integer value. After that, only two operations may be applied to it: "signal" which increments the semaphore's value by one, and "wait" which blocks its caller until the semaphore's value is greater than zero, then decrements the semaphore. The value of a semaphore typically represents the amount of some resource that is currently available, while waiting on a semaphore forces processes to block until some of that resource can be claimed. A binary semaphore is one that can only take on the values 0 and 1.
sequential computer (n.) synonymous with a Von Neumann architecture computer and is a "conventional" computer in which only one processing element works on a problem at a given time. See also uniprocessor.
serialize (v.) To put potentially concurrent operations in a strictly sequential order. If concurrent processes must claim a lock before doing some operation, for example, then their operations will be serialized.
set associative (n.) A cache structure in which all tags in a particular set are compared with an access key in order to access an item in cache. The set may have as few as one element or as many elements as there are lines in the full cache.
shared memory (n.) Memory that appears to the user to be contained in a single address space and that can be accessed by any process. In a uniprocessor or multiprocessor there is typically a single memory unit, or several memory units interleaved to give the appearance of a single memory unit. See also disjoint memory, distributed memory.
shared variables (n.) Variables to which two or more processes have access, or the model of parallel computing in which interprocess communication and synchronization are managed through such variables. See also data parallelism, futures, generative communication, live variable, message passing.
shortstopping (v.) using the output of a functional unit before it is routed back to memory.
shuffle exchange network (n.) A topology containing N = 2^L nodes, each of which is labeled by a unique L-bit integer. If two nodes have labels <|IL...I0|> and <|JL...J0|>, then I and J are connected if Ik=Jk for 1<=k<(L-1) and I0<>J0, or if J is a left or right cyclic shift of I. See also butterfly, hypercube.
SIMD (adj.) Single instruction, multiple data; a category of Flynn's taxonomy in which a single instruction stream is concurrently applied to multiple data sets. A SIMD architecture is one in which homogeneous processes synchronously execute the same instructions on their own data, or one in which an operation can be executed on vectors of fixed or varying size. See also array processor, processor array, vector processor.
SNMP (n.) simple network management protocol, a network management tool used in TCP/IP based networks that is used to manage the network equipment and processes. Usually graphic on an X-window display.
simulated annealing (n.) An optimization technique introduced by Kirkpatrick in 1983 which uses statistical physics methods to find an approximate optimal solution to a problem. Typically a thermodynamic analogy is used for the model system under study and the task of finding an optimal solution is mapped to that of finding the ground state of the thermodynamic system.
single-source shortest-path problem (n.) problem of finding the shortest path from a single designated vertex (the source) to all the other vertices in a weighted, directed graph.
SOR (n.) Successive over-relaxation is a technique for accelerating the convergence of relaxation methods for solving sets of simultaneous linear equation, Ax=b. It typically involves adding an appropriate multiple of the unit matrix to the matrix of coefficients A.
space complexity (n.) space used up by an algorithm as a function of problem size.
spanning tree (n.) A tree containing a subset of the links in a graph which reaches every node in that graph. A spanning tree can always be constructed so that its depth (the greatest distance between its root and any leaf) is no greater than the diameter of the graph. Spanning trees are frequently used to implement broadcast operations.
SPARC (n.) Scalable Processor ARChitecture; a family of chips which can be manufactured using a variety of technologies, but still be compatible in some ways.
speedup (n.) The ratio of two program execution times, particularly when times are from execution on 1 and P nodes of the same computer. Speedup is usually discussed as a function of the number of processors, but is also a function (implicitly) of the problem size. See also Amdahl's Law, Gustafson's Law, isoefficiency, optimal.
spin lock (n.) an implementation of the lock primitive that causes a processor to retest a semaphore until it changes value. Busy-waits will spin until the lock is free.
SPMD (adj.) Single program, multiple data; a category sometimes added to Flynn's taxonomy to describe programs made up of many instances of a single type of process, each executing the same code independently. SPMD can be viewed either as an extension of SIMD, or as a restriction of MIMD. See also process group, SISD.
SQL (n.) Standard Query Language; a standard for adding data to, or recovering data from, databases.
SRAM (n.) Static RAM; memory which stores data in such a way that it requires no memory refresh cycle and hence have low power consumption. Generally this type of RAM is faster but more expensive than DRAM.
startup cost (n.) The time taken to initiate any transaction with some entity. The startup cost of a message passing system, for example, is the time needed to send a message of zero length to nowhere. See also latency.
static decomposition (n.) task allocation policy that assumes tasks and their precedence relations are known before execution.
stencil (n.) A pattern of data accesses used when updating the values in a mesh. A stencil is usually represented as a grid around a central point, which indicates the location of the value being updated.
stream parallelism (n.) a pipelined variant of AND parallelism.
stream unit (n.) transmits vectors into the vector arithmetic section on some vector CPUs.
strength reduction (n.) a process whereby a compiler attempts to replace instructions with less time-costly instructions that produce identical results. For example in Fortran, X**2 might be automatically replaced by X*X.
stride (n.) a term derived from the concept of walking or striding through data from one location to the next. The term is often used in relation to vector storage. A mathematical vector is represented in a computer as an array of numbers. Most computers use contiguous storage of vectors, locating them by the address of the first word and by the vector length. Many applications in linear algebra however, require load and store of vectors with components that do not reside contiguously in memory, such as the rows of a matrix stored in column order. The row vectors are spaced in memory by a distance or stride of the leading dimension of the array representing the matrix. Some vector computers allow vector fetching and storing to occur with randomly stored vectors. An index vector locates successive components. This is often useful in storing the nonzero elements of a sparse vector.
stripmining (n.) a process used by a compiler on a register-to-register vector processor whereby a DO-loop of long or variable iteration count is executed in strips which are the length of a vector register, except for a remainder strip whose length is less.
strong search (n.) an algorithm that searches for a given key, locks the node associated with that key, and returns the node.
strongly ordered game tree (n.) game tree with the following properties: (1) 70 percent of the time the first move chosen from any nonterminal node is the best move, and (2) 90 percent of the time the best move from any nonterminal node is one of the first 25 percent of the moves searched.
subcube (n.) term for a subset of nodes of a hypercube hypercube. The hypercube architecture has a natural recursive definition so that a cube of dimension d1 includes within it 2^(d1-d2) lower dimensional sets of nodes, each of which is itself a hypercube of dimensionality d2. These subsets of nodes are known as subcubes.
subgraph (n.)given a graph, a subgraph is another graph whose vertices and edges are in the original graph.
superlinear speedup (n.) Speedup that is greater than an amount proportional to the number of processors used. While super-linear speedup is theoretically impossible, in practice it may occur because distributing a problem among many processors may increase the effective total size of the cache being used, or because distribution may change the order in which nondeterministic operations are carried out, which can lead to earlier termination of the program.
superword (n.) a term used on some computers to describe a group of eight 64-bit words, or alternatively, sixteen 32-bit half-words. The memory units on such machines, generally fetch and store data in superwords (also called swords), regardless of the size of the data item referenced by the user program.
switch (n.) A physical communication medium containing nodes that perform only communications functions. Examples include crossbar switches, in which N+M buses cross orthogonally at NM switching points to connect N objects of one type to M objects of another, and multistage switches in which several layers of switching nodes connect N objects of one type to N objects of another type. See also butterfly, combining, network, shuffle exchange network.
synchronization (n.) The act of bringing two or more processes to known points in their execution at the same clock time. Explicit synchronization is not needed in SIMD programs (in which every processor either executes the same operation as every other or does nothing), but is often necessary in SPMD and MIMD programs. The time wasted by processes waiting for other processes to synchronize with them can be a major source of inefficiency in parallel programs. See also asynchronous, barrier synchronization, synchronous.
synchronous (adj.) Occurring at the same clock time. For example, if a communication event is synchronous, then there is some moment at which both the sender and the receiver are engaged in the operation. See also asynchronous.
task farming (n.) A technique for implementing self-scheduling calculations. In a task farm, a source process generates a pool of jobs, while a sink process consumes results. In between, one or more worker processes repeatedly claim jobs from the source, turn them into results, dispatch those results to the sink, and claim their next jobs. If the number of jobs is much greater than the number of workers, task farming can be an effective way to load balance a computation.
telnet (n.) An application that provides virtual terminal services for a wide variety of remote systems. It allows a user at one site to interact with applications at other sites as if the user's terminal is local.
thrashing (n.) a phenomenon of virtual memory systems that occurs when the program, by the manner in which it is referencing its data and instructions, regularly causes the next memory locations referenced to be overwritten by recent or current instructions. The result is that referenced items are rarely in the machines physical memory and almost always must be fetched from secondary storage, usually a disk. Cache thrashing involves a similar situation between cache and physical memory.
thread (n.) a lightweight or small granularity process.
throughput(n.) number of results produced per unit time.
time-processor product (n.) The product of the time taken to execute a program and the number of processors used to achieve that time, often used as a measure of goodness for parallel algorithms. See also Amdahl's Law, efficiency, Gustafson's Law, speedup.
TLB (n.) translation look-aside buffer; the memory cache of the most recently used page table entries within the memory management unit.
token ring (n.) Token ring is an IBM based LAN protocol that uses a ring shaped network topology. Token Ring has speeds at 4Mbps and 16Mbps. A distinguishing packet is transferred from machine to machine and only the machine that is in control of the token is able to transmit.
topology (n.) A family of graphs created using the same general rule or that share certain properties. The processors in a multicomputer, and the circuits in a switch, are usually laid out using one of several topologies, including the mesh, the hypercube, the butterfly, the torus and the shuffle exchange network. See also bisection bandwidth, diameter.
torus (n.) A topology in which nodes form a regular cyclic d-dimensional grid, and each edge is parallel to a grid axis and joins two nodes that are adjacent along that axis. The architecture of some multicomputers is a two or three dimensional torus. See also hypercube, mesh.
trace scheduling(n.) A compiler optimization technique that vectorizes the most likely path through a program as if it were a single basic block, includes extra instructions at each branch to undo any ill effects of having made a wrong guess, vectorizes the next most likely branches and so on.
transposition table (n.) a hash table that stores previously evaluated game positions.
transputer (n.) A single integrated circuit which contains a CPU, communications links, memory and some cache memory. The name transputer refers to a proprietary series of chips manufactured by Inmos, although other node chips have had similar characteristics.
tree (n.) a connected, undirected, acyclic graph. The most commonly encountered tree in computer science is the regular binary tree, in which a root node has two children but no parent, each interior node has a single parent and two children, and leaf nodes have a single parent but no children.
true ratio (n.) the frequency with which the "true" branch of a Fortran IF-test occurs. If the true ratio is know at compile time, some compilers can take advantage of this knowledge. However the true ratio is often data dependent and cannot effectively be dealt with automatically. See also interactive vectorizer.
tuple (n.) An ordered sequence of fixed length of values of arbitrary types. Tuples are used for both data storage and interprocess communication in the generative communication paradigm. See also tuple space.
UART (n.) Universal Asynchronous Receive-Transmit; a standard protocol for device drivers.
UDP (n.) user datagram protocol is a transport layer protocol in the internet protocol suite. UDP uses IP for packet delivery, and is unreliable, connectionless, and stateless. However, UDP does not use handshaking before exchanging data and therefore acknowledgements and guaranteed delivery are not available. UDP relies on higher protocol layers to provide end to end data delivery and integrity.
undirected graph (n.)a graph whose edges have no orientation.
unification (v.) instantiation of a variable with a value.
uniform cost criterion (n.) the assumption that every instruction takes one unit of time and every register requires one unit of space.
unneeded store (n.) situation resulting when two or more stores into the same memory location without intermediate reads occur in an optimization block, especially within a DO-loop, such that only the last store need actually be performed.
utilization (n.) percentage of time a processor spends executing useful tasks during execution of a program.
uucp (n.) Unix to Unix copy program is a protocol used for communicating between Unix hosts using serial interfaces.
V.35 (n.) A data communications interface standard adopted for use with circuits 56Kbps and above.
valence (n.) The number of edges connected to a vertex in a graph; for example, every node in a regular square mesh has a valence of 4. Confusingly, valence also means the number of branches below a tree node, which is one fewer than the number of edges incident to that node - every node in a binary tree has a valence of 2. The term arity is sometimes also used in this sense.
vector (n.) an ordered list of items in a computer's memory. A simple vector is defined as having a starting address, a length, and a stride. An indirect address vector is defined as having a relative base address and a vector of values to be applied as indices to the base.
vector processor (n.) A computer designed to apply arithmetic operations to long vectors or arrays. Most vector processors rely heavily on pipelining to achieve high performance. See also array processor.
vector register (n.) a storage device that acts as an intermediate memory between a computer's functional units and main memory.
vertex (n.) component of a graph, also sometimes called a node.
vertical processing (n.) processing a two dimensional array column by column.
virtual concurrent computer (n.) A computer system that is programmed as a concurrent computer of some number of nodes P, but which is implemented on either a real concurrent computer of some number of nodes less than P or on a uniprocessor running software to emulate the environment of a concurrent machine. Such an emulation system is said to provide virtual nodes to the user.
virtual cut-through (n.) A technique for routing messages in which the head and tail of the message both proceed as rapidly as they can. If the head is blocked because a link it wants to cross is being used by some other message, the tail continues to advance, and the message's contents are put into buffers on intermediate nodes. See also packet switching, wormhole routing.
virtual memory (n.) A system that stores portions of an address space that are not being actively used in in some medium other than main high-speed memory, such as a disk or slower auxiliary memory medium. When a reference is made to a value not presently in main memory, the virtual memory manager must swap some values in main memory for the values required. Virtual memory is used by almost all uniprocessors and multiprocessors, but is not available on some array processors and multicomputers, which still employ real memory storage only on each node.
virtual shared memory (n.) Memory that appears to users to constitute a single address space, but which is actually physically disjoint. Virtual shared memory is often implemented using some combination of hashing and local caching.
VLIW (n.) Very Long Instruction Word; the use of extremely long instructions (256 bits or more) in a computer to improve its ability to chain operations together.
VLSI (adj.) Very Large Scale Integration; applied to technologies capable of putting hundreds of thousands or more components on a single chip, or sometimes applied to those chips so manufactured.
VMS (n.) Virtual Machine System; an operating system developed by DEC and widely used on VAX machines. Popularity of this OS is probably waning in favour of UNIX like systems.
Von Neumann architecture (n.) Used to describe any computer which does not employ concurrency or parallelism. Named after John Von Neumann (1903-1957) who is credited with the invention of the basic architecture of current sequential computers.
WARPS (n.) Words accessed randomly per second; a measure of memory access performance, equal to the rate of uniformly random accesses across the whole of the address space visible to a process that a machine supports. See also FLOPS.
weak search (n.) a search algorithm that searches for a key and returns the node that contained the key at the time it was examined. Weak searches are not guaranteed to provide an up-to-date result. See also strong search.
weight matrix (n.) a matrix indicating, for each pair of vertices I and J, the weight of the edge from vertex I to vertex J.
weighted graph (n.) a graph with a real number assigned to each edge.
working set (n.) Those values from shared memory that a process has copied into its private memory, or those pages of virtual memory being used by a process. Changes a process makes to the values in its working set are not automatically seen by other processes.
wormhole routing (n.) A technique for routing messages in which the head of the message establishes a path, which is reserved for the message until the tail has passed through it. Unlike virtual cut-through, the tail proceeds at a rate dictated by the progress of the head, which reduces the demand for intermediate buffering. See also packet switching.
worst-case space complexity (n.) greatest amount of space used by an algorithm, over all possible inputs of a given size.
wrap-around scalar (n.) a scalar variable whose value is set in one iteration of a DO-loop and referenced in a subsequent iteration and is consequently recursive. All common reduction-function scalars are wrap-around scalars and usually do not prevent vectorization. All other wrap-around scalars usually do prevent vectorization of the loop in which they appear.
X.25 (n.) is the CCITT standard protocol for transport-level network services and was originally created for connecting terminals to computers. It provides reliable stream-oriented transmission services and is widely used in Europe. TCP/IP can be implemented as a layer above X.25.
XDR (n.) eXternal Data Representation is a standard for machine independent data structures.
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