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<title>Task parallelism</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Task parallelism</span></span>

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<p><b>Task parallelism</b> (also known as <b>function parallelism</b> and <b>control parallelism</b>) is a form of <a href="Parallelization" class="mw-redirect" title="Parallelization">parallelization</a> of <a href="Source_code" title="Source code">computer code</a> across multiple <a href="Central_processing_unit" title="Central processing unit">processors</a> in <a href="Parallel_computing" title="Parallel computing">parallel computing</a> environments. Task parallelism focuses on distributing <a href="Task_(computing)" title="Task (computing)">tasks</a>—concurrently performed by <a href="Process_(computing)" title="Process (computing)">processes</a> or <a href="Thread_(computing)" title="Thread (computing)">threads</a>—across different processors. In contrast to <a href="Data_parallelism" title="Data parallelism">data parallelism</a> which involves running the same task on different components of data, task parallelism is distinguished by running many different tasks at the same time on the same data.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> A common type of task parallelism is <a href="Pipeline_(computing)" title="Pipeline (computing)">pipelining</a>, which consists of moving a single set of data through a series of separate tasks where each task can execute independently of the others.

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<div class="mw-heading mw-heading2"><h2 id="Description">Description</h2></div>

<p>In a multiprocessor system, task parallelism is achieved when each processor executes a different thread (or process) on the same or different data. The threads may execute the same or different code. In the general case, different execution threads communicate with one another as they work, but this is not a requirement. Communication usually takes place by passing data from one thread to the next as part of a <a href="Workflow" title="Workflow">workflow</a>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

</p><p>As a simple example, if a system is running code on a 2-processor system (<a href="CPU" class="mw-redirect" title="CPU">CPUs</a> "a" & "b") in a <a href="https://en.wiktionary.org/wiki/parallel" class="extiw external" title="wikt:parallel">parallel</a> environment and we wish to do tasks "A" and "B", it is possible to tell CPU "a" to do task "A" and CPU "b" to do task "B" simultaneously, thereby reducing the <a href="Run_time_(program_lifecycle_phase)" class="mw-redirect" title="Run time (program lifecycle phase)">run time</a> of the execution. The tasks can be assigned using <a href="Conditional_(programming)" class="mw-redirect" title="Conditional (programming)">conditional statements</a> as described below.

</p><p>Task parallelism emphasizes the distributed (parallelized) nature of the processing (i.e. threads), as opposed to the data (<a href="Data_parallelism" title="Data parallelism">data parallelism</a>). Most real programs fall somewhere on a continuum between task parallelism and data parallelism.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p><b>Thread-level parallelism</b> (<b>TLP</b>) is the <a href="Parallel_computing" title="Parallel computing">parallelism</a> inherent in an application that runs multiple <a href="Thread_(computer_science)" class="mw-redirect" title="Thread (computer science)">threads</a> at once. This type of parallelism is found largely in applications written for commercial <a href="Server_(computing)" title="Server (computing)">servers</a> such as databases. By running many threads at once, these applications are able to tolerate the high amounts of I/O and memory system latency their workloads can incur - while one thread is delayed waiting for a memory or disk access, other threads can do useful work.

</p><p>The exploitation of thread-level parallelism has also begun to make inroads into the desktop market with the advent of <a href="Multi-core" class="mw-redirect" title="Multi-core">multi-core</a> microprocessors. This has occurred because, for various reasons, it has become increasingly impractical to increase either the clock speed or instructions per clock of a single core. If this trend continues, new applications will have to be designed to utilize multiple threads in order to benefit from the increase in potential computing power. This contrasts with previous microprocessor innovations in which existing code was automatically sped up by running it on a newer/faster computer.

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<div class="mw-heading mw-heading2"><h2 id="Example">Example</h2></div>

<p>The <a href="Pseudocode" title="Pseudocode">pseudocode</a> below illustrates task parallelism:

</p>

<pre>program:

...

if CPU = "a" then

do task "A"

else if CPU="b" then

do task "B"

end if

...

end program

</pre>

<p>The goal of the program is to do some net total task ("A+B"). If we write the code as above and launch it on a 2-processor system, then the runtime environment will execute it as follows.

</p>

<ul><li>In an <a href="SPMD" class="mw-redirect" title="SPMD">SPMD</a> (single program, multiple data) system, both <a href="CPU" class="mw-redirect" title="CPU">CPUs</a> will execute the code.</li>

<li>In a parallel environment, both will have access to the same data.</li>

<li>The "if" clause differentiates between the CPUs. CPU "a" will read true on the "if" and CPU "b" will read true on the "else if", thus having their own task.</li>

<li>Now, both CPU's execute separate code blocks simultaneously, performing different tasks simultaneously.</li></ul>

<p>Code executed by CPU "a":

</p>

<pre>program:

...

do task "A"

...

end program

</pre>

<p>Code executed by CPU "b":

</p>

<pre>program:

...

do task "B"

...

end program

</pre>

<p>This concept can now be generalized to any number of processors.

</p>

<div class="mw-heading mw-heading2"><h2 id="Language_support">Language support</h2></div>

<p>Task parallelism can be supported in general-purpose languages by either built-in facilities or libraries. Notable examples include:

</p>

<ul><li>Ada: Tasks (built-in)</li>

<li>C++ (Intel): <a href="Threading_Building_Blocks" title="Threading Building Blocks">Threading Building Blocks</a></li>

<li>C++ (Intel): <a href="Cilk_Plus" class="mw-redirect" title="Cilk Plus">Cilk Plus</a></li>

<li>C++ (Open Source/Apache 2.0): <a href="RaftLib" title="RaftLib">RaftLib</a></li>

<li>C, C++, Objective-C, Swift (Apple): <a href="Grand_Central_Dispatch" title="Grand Central Dispatch">Grand Central Dispatch</a></li>

<li>D: <a href="Task_(computing)" title="Task (computing)">tasks</a> and <a href="Fiber_(computer_science)" title="Fiber (computer science)">fibers</a></li>

<li>Delphi (System.Threading.TParallel)</li>

<li>Go: <a href="Goroutine" class="mw-redirect" title="Goroutine">goroutines</a></li>

<li>Java: <a href="Java_concurrency" title="Java concurrency">Java concurrency</a></li>

<li>.NET: <a href="Task_Parallel_Library" class="mw-redirect" title="Task Parallel Library">Task Parallel Library</a></li></ul>

<p>Examples of fine-grained task-parallel languages can be found in the realm of <a href="Hardware_Description_Language" class="mw-redirect" title="Hardware Description Language">Hardware Description Languages</a> like <a href="Verilog" title="Verilog">Verilog</a> and <a href="VHDL" title="VHDL">VHDL</a>.

</p>

<ul><li><a href="Algorithmic_skeleton" title="Algorithmic skeleton">Algorithmic skeleton</a></li>

<li><a href="Data_parallelism" title="Data parallelism">Data parallelism</a></li>

<li><a href="Fork%E2%80%93join_model" title="Fork–join model">Fork–join model</a></li>

<li><a href="Parallel_programming_model" title="Parallel programming model">Parallel programming model</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFReinders2007" class="citation news cs1">Reinders, James (10 September 2007). <a rel="nofollow" class="external text" href="https://www.zdnet.com/article/understanding-task-and-data-parallelism/">"Understanding task and data parallelism"</a>. <i>ZDNet</i><span class="reference-accessdate">. Retrieved <span class="nowrap">8 May</span> 2017</span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFQuinn2007" class="citation book cs1">Quinn, Michael J. (2007). <i>Parallel programming in C with MPI and openMP</i> (Tata McGraw-Hill ed.). New Delhi: Tata McGraw-Hill Pub. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0070582019</bdi>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFHicks" class="citation web cs1">Hicks, Michael. <a rel="nofollow" class="external text" href="http://www.cs.umd.edu/class/fall2013/cmsc433/lectures/concurrency-basics.pdf">"Concurrency Basics"</a> <span class="cs1-format">(PDF)</span>. <i>University of Maryland: Department of Computer Science</i><span class="reference-accessdate">. Retrieved <span class="nowrap">8 May</span> 2017</span>.</cite></span>

</li>

</ol></div></div>

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</style><div id="Parallel_computing346" style="font-size:114%;margin:0 4em"><a href="Parallel_computing" title="Parallel computing">Parallel computing</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">General</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Distributed_computing" title="Distributed computing">Distributed computing</a></li>

<li><a href="Parallel_computing" title="Parallel computing">Parallel computing</a></li>

<li><a href="Parallel_algorithm" title="Parallel algorithm">Parallel algorithm</a></li>

<li><a href="Massively_parallel" title="Massively parallel">Massively parallel</a></li>

<li><a href="Cloud_computing" title="Cloud computing">Cloud computing</a></li>

<li><a href="High-performance_computing" title="High-performance computing">High-performance computing</a></li>

<li><a href="Multiprocessing" title="Multiprocessing">Multiprocessing</a></li>

<li><a href="Manycore_processor" title="Manycore processor">Manycore processor</a></li>

<li><a href="General-purpose_computing_on_graphics_processing_units" title="General-purpose computing on graphics processing units">GPGPU</a></li>

<li><a href="Computer_network" title="Computer network">Computer network</a></li>

<li><a href="Systolic_array" title="Systolic array">Systolic array</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Levels</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Instruction-level_parallelism" title="Instruction-level parallelism">Instruction</a></li>

<li><a href="Memory-level_parallelism" title="Memory-level parallelism">Memory</a></li>

<li><a href="Pipeline_(computing)" title="Pipeline (computing)">Pipeline</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Multithreading_(computer_architecture)" title="Multithreading (computer architecture)">Multithreading</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Temporal_multithreading" title="Temporal multithreading">Temporal</a></li>

<li><a href="Simultaneous_multithreading" title="Simultaneous multithreading">Simultaneous</a> (SMT)</li>

<li><a href="Simultaneous_and_heterogeneous_multithreading" title="Simultaneous and heterogeneous multithreading">Simultaneous and heterogenous</a></li>

<li><a href="Speculative_multithreading" title="Speculative multithreading">Speculative</a> (SpMT)</li>

<li><a href="Preemption_(computing)" title="Preemption (computing)">Preemptive</a></li>

<li><a href="Computer_multitasking#Cooperative_multitasking" title="Computer multitasking">Cooperative</a></li>

<li><a href="Bulldozer_(microarchitecture)#Bulldozer_core" title="Bulldozer (microarchitecture)">Clustered multi-thread</a> (CMT)</li>

<li><a href="Hardware_scout" title="Hardware scout">Hardware scout</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Theory</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Parallel_RAM" title="Parallel RAM">PRAM model</a></li>

<li><a href="Parallel_external_memory" title="Parallel external memory">PEM model</a></li>

<li><a href="Analysis_of_parallel_algorithms" title="Analysis of parallel algorithms">Analysis of parallel algorithms</a></li>

<li><a href="Amdahl's_law" title="Amdahl's law">Amdahl's law</a></li>

<li><a href="Gustafson's_law" title="Gustafson's law">Gustafson's law</a></li>

<li><a href="Cost_efficiency" title="Cost efficiency">Cost efficiency</a></li>

<li><a href="Karp%E2%80%93Flatt_metric" title="Karp–Flatt metric">Karp–Flatt metric</a></li>

<li><a href="Parallel_slowdown" title="Parallel slowdown">Slowdown</a></li>

<li><a href="Speedup" title="Speedup">Speedup</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Elements</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Process_(computing)" title="Process (computing)">Process</a></li>

<li><a href="Thread_(computing)" title="Thread (computing)">Thread</a></li>

<li><a href="Fiber_(computer_science)" title="Fiber (computer science)">Fiber</a></li>

<li><a href="Instruction_window" title="Instruction window">Instruction window</a></li>

<li><a href="Array_(data_structure)" title="Array (data structure)">Array</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Coordination</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Multiprocessing" title="Multiprocessing">Multiprocessing</a></li>

<li><a href="Memory_coherence" title="Memory coherence">Memory coherence</a></li>

<li><a href="Cache_coherence" title="Cache coherence">Cache coherence</a></li>

<li><a href="Cache_invalidation" title="Cache invalidation">Cache invalidation</a></li>

<li><a href="Barrier_(computer_science)" title="Barrier (computer science)">Barrier</a></li>

<li><a href="Synchronization_(computer_science)" title="Synchronization (computer science)">Synchronization</a></li>

<li><a href="Application_checkpointing" title="Application checkpointing">Application checkpointing</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Computer_programming" title="Computer programming">Programming</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Stream_processing" title="Stream processing">Stream processing</a></li>

<li><a href="Dataflow_programming" title="Dataflow programming">Dataflow programming</a></li>

<li><a href="Parallel_programming_model" title="Parallel programming model">Models</a>

<ul><li><a href="Implicit_parallelism" title="Implicit parallelism">Implicit parallelism</a></li>

<li><a href="Explicit_parallelism" title="Explicit parallelism">Explicit parallelism</a></li>

<li><a href="Concurrency_(computer_science)" title="Concurrency (computer science)">Concurrency</a></li></ul></li>

<li><a href="Non-blocking_algorithm" title="Non-blocking algorithm">Non-blocking algorithm</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Computer_hardware" title="Computer hardware">Hardware</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Flynn's_taxonomy" title="Flynn's taxonomy">Flynn's taxonomy</a>

<ul><li><a href="Single_instruction%2C_multiple_threads" title="Single instruction, multiple threads">Array processing</a> (SIMT)</li>

<li><a href="Flynn's_taxonomy#Pipelined_processor" title="Flynn's taxonomy">Pipelined processing</a></li>

<li><a href="Flynn's_taxonomy#Associative_processor" title="Flynn's taxonomy">Associative processing</a></li></ul></li>

<li><a href="Dataflow_architecture" title="Dataflow architecture">Dataflow architecture</a></li>

<li><a href="Instruction_pipelining" title="Instruction pipelining">Pipelined processor</a></li>

<li><a href="Superscalar_processor" title="Superscalar processor">Superscalar processor</a></li>

<li><a href="Vector_processor" title="Vector processor">Vector processor</a></li>

<li><a href="Multiprocessing" title="Multiprocessing">Multiprocessor</a>

<ul><li><a href="Symmetric_multiprocessing" title="Symmetric multiprocessing">symmetric</a></li>

<li><a href="Asymmetric_multiprocessing" title="Asymmetric multiprocessing">asymmetric</a></li></ul></li>

<li><a href="Semiconductor_memory" title="Semiconductor memory">Memory</a>

<ul><li><a href="Shared_memory" title="Shared memory">shared</a></li>

<li><a href="Distributed_memory" title="Distributed memory">distributed</a></li>

<li><a href="Distributed_shared_memory" title="Distributed shared memory">distributed shared</a></li>

<li><a href="Massively_parallel" title="Massively parallel">Massively parallel</a> computer</li>

<li><a href="Computer_cluster" title="Computer cluster">Computer cluster</a>

<ul><li><a href="Beowulf_cluster" title="Beowulf cluster">Beowulf cluster</a></li></ul></li>

<li><a href="Grid_computing" title="Grid computing">Grid computer</a></li>

<li><a href="Hardware_acceleration" title="Hardware acceleration">Hardware acceleration</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="API" title="API">APIs</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Ateji_PX" title="Ateji PX">Ateji PX</a></li>

<li><a href="Boost_(C%2B%2B_libraries)" title="Boost (C++ libraries)">Boost</a></li>

<li><a href="Chapel_(programming_language)" title="Chapel (programming language)">Chapel</a></li>

<li><a href="Charm%2B%2B" title="Charm++">Charm++</a></li>

<li><a href="Coarray_Fortran" title="Coarray Fortran">Coarray Fortran</a></li>

<li><a href="Dryad_(programming)" title="Dryad (programming)">Dryad</a></li>

<li><a href="Global_Arrays" title="Global Arrays">Global Arrays</a></li>

<li><a href="GPUOpen" title="GPUOpen">GPUOpen</a></li>

<li><a href="OpenMP" title="OpenMP">OpenMP</a></li>

<li><a href="OpenCL" title="OpenCL">OpenCL</a></li>

<li><a href="OpenHMPP" title="OpenHMPP">OpenHMPP</a></li>

<li><a href="OpenACC" title="OpenACC">OpenACC</a></li>

<li><a href="Parallel_Extensions" title="Parallel Extensions">Parallel Extensions</a></li>

<li><a href="Pthreads" title="Pthreads">pthreads</a></li>

<li><a href="RaftLib" title="RaftLib">RaftLib</a></li>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Problems</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Automatic_parallelization" title="Automatic parallelization">Automatic parallelization</a></li>

<li><a href="Deadlock_(computer_science)" title="Deadlock (computer science)">Deadlock</a></li>

<li><a href="Deterministic_algorithm" title="Deterministic algorithm">Deterministic algorithm</a></li>

<li><a href="Embarrassingly_parallel" title="Embarrassingly parallel">Embarrassingly parallel</a></li>

<li><a href="Parallel_slowdown" title="Parallel slowdown">Parallel slowdown</a></li>

<li><a href="Race_condition" title="Race condition">Race condition</a></li>

<li><a href="Software_lockout" title="Software lockout">Software lockout</a></li>

<li><a href="Scalability" title="Scalability">Scalability</a></li>

<li><a href="Starvation_(computer_science)" title="Starvation (computer science)">Starvation</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div>

<ul><li><span class="noviewer" typeof="mw:File"><span title="Category"></span></span> Category: Parallel computing</li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2024-07-31" href="https://en.wikipedia.org/wiki/?title=Task_parallelism&oldid=1237880513">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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