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<a class="nav-link dropdown-toggle jmu-gold rounded" href="Extended6Primes.html#" id="navbarDropdownChapters" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Contents</a>
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<a class="dropdown-item" tabindex="-1" href="Extended6Primes.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="Themes.html"> 1.3. Themes and Guiding Principles</a>
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<a class="dropdown-item" href="Architectures.html"> 1.4. System Architectures</a>
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<a class="dropdown-item" href="StateModels.html"> 1.5. State Models in UML</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="KernelMechanics.html"> 2.3. Kernel Mechanics</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="ProcessCycle.html"> 2.5. Process Life Cycle</a>
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<a class="dropdown-item" href="UnixFile.html"> 2.6. The UNIX File Abstraction</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 3</b></a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="MMap.html"> 3.4. Shared Memory With Memory-mapped Files</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="ShMem.html"> 3.7. Shared Memory</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="TCPSockets.html"> 4.5. TCP Socket Programming: HTTP</a>
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<a class="dropdown-item" href="UDPSockets.html"> 4.6. UDP Socket Programming: DNS</a>
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<a class="dropdown-item" href="AppBroadcast.html"> 4.7. Application-Layer Broadcasting: DHCP</a>
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<a class="dropdown-item" href="Extended4CGI.html"> 4.8. Extended Example: CGI Web Server</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 5</b></a>
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<a class="dropdown-item" href="InternetOverview.html"> 5.1. The Internet and Connectivity</a>
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<a class="dropdown-item" href="AppLayer.html"> 5.2. Application Layer: Overlay Networks</a>
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<a class="dropdown-item" href="TransLayer.html"> 5.3. Transport Layer</a>
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<a class="dropdown-item" href="NetSec.html"> 5.4. Network Security Fundamentals</a>
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<a class="dropdown-item" href="NetLayer.html"> 5.5. Network Layer: IP</a>
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<a class="dropdown-item" href="LinkLayer.html"> 5.6. Link Layer</a>
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<a class="dropdown-item" href="Wireless.html"> 5.7. Wireless Connectivity: Wi-Fi, Bluetooth, and Zigbee</a>
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<a class="dropdown-item" href="Extended5DNS.html"> 5.8. Extended Example: DNS client</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 6</b></a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="ProcVThreads.html"> 6.2. Processes vs. Threads</a>
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<a class="dropdown-item" href="RaceConditions.html"> 6.3. Race Conditions and Critical Sections</a>
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<a class="dropdown-item" href="POSIXThreads.html"> 6.4. POSIX Thread Library</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 7</b></a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="Semaphores.html"> 7.4. Semaphores</a>
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<a class="dropdown-item" href="Barriers.html"> 7.5. Barriers</a>
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<a class="dropdown-item" href="Condvars.html"> 7.6. Condition Variables</a>
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<a class="dropdown-item" href="Deadlock.html"> 7.7. Deadlock</a>
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<a class="dropdown-item" href="Extended7Events.html"> 7.8. Extended Example: Event Log File</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 8</b></a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 9</b></a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="ParallelDesign.html"> 9.3. Parallel Design Patterns</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistTiming.html"> 9.5. Timing in Distributed Environments</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Appendix A</b></a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<a class="dropdown-item" href="Functions.html"> A.5. Functions and Scope</a>
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<a class="dropdown-item" href="Pointers.html"> A.6. Pointers and Dynamic Allocation</a>
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<a class="dropdown-item" href="Strings.html"> A.7. Strings</a>
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<a class="dropdown-item" href="FunctionPointers.html"> A.8. Function Pointers</a>
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<a class="dropdown-item" href="Files.html"> A.9. Files</a>
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</div>
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</li>
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</div>
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<ul class="navbar-nav flex-row ml-md-auto d-none d-md-flex">
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<li class="nav-item"><a class="nav-link jmu-gold" href="https://w3.cs.jmu.edu/kirkpams/OpenCSF/Books/csf/source/Extended6Primes.rst"
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target="_blank" rel="nofollow">Show Source</a></li>
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<div class="container center">
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«  <a id="prevmod" href="Extended6Input.html">6.7. Extended Example: Keyboard Input Listener</a>
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  ::  
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<a class="uplink" href="index.html">Contents</a>
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  ::  
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<a id="nextmod" href="SynchOverview.html">7.1. Synchronization Primitives</a>  »
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "Extended6Primes";ODSA.SETTINGS.MODULE_LONG_NAME = "Extended Example: Concurrent Prime Number Search";ODSA.SETTINGS.MODULE_CHAPTER = "Concurrency with Multithreading"; ODSA.SETTINGS.BUILD_DATE = "2021-06-01 15:31:50"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="extended-example-concurrent-prime-number-search">
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<h1>6.8. Extended Example: Concurrent Prime Number Search<a class="headerlink" href="Extended6Primes.html#extended-example-concurrent-prime-number-search" title="Permalink to this headline">¶</a></h1>
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<p>This second example shows a different style of multithreaded programming.
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Instead of assigning a unique task to a particular thread, this example creates
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multiple threads that all execute the same code, but on different input
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parameters. For systems with <a class="reference internal" href="Glossary.html#term-multiprocessing"><span class="xref std std-term">multiprocessing</span></a> capabilities, this style of
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programming forms the foundation of parallel execution. Since each thread’s
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calculations are independent of all the others’ calculations, they can run in
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parallel and compute the same answers.</p>
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<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0"> 1
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|
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|
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96</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><stdbool.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><pthread.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><stdlib.h></span><span class="cp"></span>
|
|||
|
|
<span class="cp">#include</span> <span class="cpf"><assert.h></span><span class="cp"></span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Specify the number of threads to run in concurrently */</span>
|
|||
|
|
<span class="cp">#define NUM_THREADS 4</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Each thread gets a start and end number and returns the number</span>
|
|||
|
|
<span class="cm"> Of primes in that range */</span>
|
|||
|
|
<span class="k">struct</span> <span class="n">range</span> <span class="p">{</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">start</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">end</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">count</span><span class="p">;</span>
|
|||
|
|
<span class="p">};</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Thread function for counting primes */</span>
|
|||
|
|
<span class="kt">void</span> <span class="o">*</span>
|
|||
|
|
<span class="nf">prime_check</span> <span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="n">_args</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="cm">/* Cast the args to a usable struct type */</span>
|
|||
|
|
<span class="k">struct</span> <span class="n">range</span> <span class="o">*</span><span class="n">args</span> <span class="o">=</span> <span class="p">(</span><span class="k">struct</span> <span class="n">range</span><span class="o">*</span><span class="p">)</span> <span class="n">_args</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">iter</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">value</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Skip over any numbers < 2, which is the smallest prime */</span>
|
|||
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">args</span><span class="o">-></span><span class="n">start</span> <span class="o"><</span> <span class="mi">2</span><span class="p">)</span> <span class="n">args</span><span class="o">-></span><span class="n">start</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Loop from this thread's start to this thread's end */</span>
|
|||
|
|
<span class="n">args</span><span class="o">-></span><span class="n">count</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
<span class="k">for</span> <span class="p">(</span><span class="n">value</span> <span class="o">=</span> <span class="n">args</span><span class="o">-></span><span class="n">start</span><span class="p">;</span> <span class="n">value</span> <span class="o"><=</span> <span class="n">args</span><span class="o">-></span><span class="n">end</span><span class="p">;</span> <span class="n">value</span><span class="o">++</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="cm">/* Trivial and intentionally slow algorithm:</span>
|
|||
|
|
<span class="cm"> Start with iter = 2; see if iter divides the number evenly.</span>
|
|||
|
|
<span class="cm"> If it does, it's not prime.</span>
|
|||
|
|
<span class="cm"> Stop when iter exceeds the square root of value */</span>
|
|||
|
|
<span class="kt">bool</span> <span class="n">is_prime</span> <span class="o">=</span> <span class="nb">true</span><span class="p">;</span>
|
|||
|
|
<span class="k">for</span> <span class="p">(</span><span class="n">iter</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span> <span class="n">iter</span> <span class="o">*</span> <span class="n">iter</span> <span class="o"><=</span> <span class="n">value</span> <span class="o">&&</span> <span class="n">is_prime</span><span class="p">;</span> <span class="n">iter</span><span class="o">++</span><span class="p">)</span>
|
|||
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">value</span> <span class="o">%</span> <span class="n">iter</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="n">is_prime</span> <span class="o">=</span> <span class="nb">false</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">is_prime</span><span class="p">)</span> <span class="n">args</span><span class="o">-></span><span class="n">count</span><span class="o">++</span><span class="p">;</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* All values in the range have been counted, so exit */</span>
|
|||
|
|
<span class="n">pthread_exit</span> <span class="p">(</span><span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="kt">int</span>
|
|||
|
|
<span class="nf">main</span> <span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">pthread_t</span> <span class="n">threads</span><span class="p">[</span><span class="n">NUM_THREADS</span><span class="p">];</span>
|
|||
|
|
<span class="k">struct</span> <span class="n">range</span> <span class="o">*</span><span class="n">args</span><span class="p">[</span><span class="n">NUM_THREADS</span><span class="p">];</span>
|
|||
|
|
<span class="kt">int</span> <span class="kr">thread</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Count the number of primes smaller than this value: */</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">number_count</span> <span class="o">=</span> <span class="mi">100000000L</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Specify start and end values, then split based on number of</span>
|
|||
|
|
<span class="cm"> threads */</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">start</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">end</span> <span class="o">=</span> <span class="n">start</span> <span class="o">+</span> <span class="n">number_count</span><span class="p">;</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">number_per_thread</span> <span class="o">=</span> <span class="n">number_count</span> <span class="o">/</span> <span class="n">NUM_THREADS</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Simplification: make range divide evenly among the threads */</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">number_count</span> <span class="o">%</span> <span class="n">NUM_THREADS</span> <span class="o">==</span> <span class="mi">0</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Assign a start/end value for each thread, then create it */</span>
|
|||
|
|
<span class="k">for</span> <span class="p">(</span><span class="kr">thread</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="kr">thread</span> <span class="o"><</span> <span class="n">NUM_THREADS</span><span class="p">;</span> <span class="kr">thread</span><span class="o">++</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span> <span class="o">=</span> <span class="n">calloc</span> <span class="p">(</span><span class="k">sizeof</span> <span class="p">(</span><span class="k">struct</span> <span class="n">range</span><span class="p">),</span> <span class="mi">1</span><span class="p">);</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">start</span> <span class="o">=</span> <span class="n">start</span> <span class="o">+</span> <span class="p">(</span><span class="kr">thread</span> <span class="o">*</span> <span class="n">number_per_thread</span><span class="p">);</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">end</span> <span class="o">=</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">start</span> <span class="o">+</span> <span class="n">number_per_thread</span> <span class="o">-</span> <span class="mi">1</span><span class="p">;</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">pthread_create</span> <span class="p">(</span><span class="o">&</span><span class="n">threads</span><span class="p">[</span><span class="kr">thread</span><span class="p">],</span> <span class="nb">NULL</span><span class="p">,</span> <span class="n">prime_check</span><span class="p">,</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">])</span> <span class="o">==</span> <span class="mi">0</span><span class="p">);</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* All threads are running. Join all to collect the results. */</span>
|
|||
|
|
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">total_number</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
<span class="k">for</span> <span class="p">(</span><span class="kr">thread</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="kr">thread</span> <span class="o"><</span> <span class="n">NUM_THREADS</span><span class="p">;</span> <span class="kr">thread</span><span class="o">++</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">pthread_join</span> <span class="p">(</span><span class="n">threads</span><span class="p">[</span><span class="kr">thread</span><span class="p">],</span> <span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
<span class="n">printf</span> <span class="p">(</span><span class="s">"From %ld to %ld: %ld</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">start</span><span class="p">,</span>
|
|||
|
|
<span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">end</span><span class="p">,</span> <span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">count</span><span class="p">);</span>
|
|||
|
|
<span class="n">total_number</span> <span class="o">+=</span> <span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]</span><span class="o">-></span><span class="n">count</span><span class="p">;</span>
|
|||
|
|
<span class="n">free</span> <span class="p">(</span><span class="n">args</span><span class="p">[</span><span class="kr">thread</span><span class="p">]);</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Display the total number of primes in the specified range. */</span>
|
|||
|
|
<span class="n">printf</span> <span class="p">(</span><span class="s">"===============================================</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
|||
|
|
<span class="n">printf</span> <span class="p">(</span><span class="s">"Total number of primes less than %ld: %ld</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">end</span><span class="p">,</span>
|
|||
|
|
<span class="n">total_number</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
<p>The time command-line utility can be used to measure the amount of time it takes
|
|||
|
|
to run a program. However, time gives you multiple pieces of information to
|
|||
|
|
interpret. The <em>real</em> time is how much total time elapsed on the system clock
|
|||
|
|
from the start to finish. The <em>user</em> time is how much total CPU time was spent
|
|||
|
|
on all of the processors combined. For instance, if a program runs in perfect
|
|||
|
|
parallelism on 2 cores for 15 minutes, the real time would be 15 minutes, but
|
|||
|
|
the user time would be 30 minutes.</p>
|
|||
|
|
<p>The following output shows the results for running the prime number program
|
|||
|
|
above twice. The first set of results (approximately 7.5 minutes for both real
|
|||
|
|
and user time) used only a single thread. The second set used 4 threads (best
|
|||
|
|
choice for the quad-core system used).</p>
|
|||
|
|
<div class="highlight-none border border-dark rounded-lg bg-light px-2 mb-3 notranslate"><div class="highlight bg-light"><pre class="mb-0"><span></span>$ time ./prime
|
|||
|
|
From 2 to 99999999: 5761455
|
|||
|
|
===============================================
|
|||
|
|
Total number of primes less than 100000000: 5761455
|
|||
|
|
|
|||
|
|
real 7m39.361s
|
|||
|
|
user 7m38.650s
|
|||
|
|
sys 0m0.325s
|
|||
|
|
|
|||
|
|
[... recompile to use 4 threads ...]
|
|||
|
|
|
|||
|
|
$ time ./prime
|
|||
|
|
From 2 to 24999999: 1565927
|
|||
|
|
From 25000000 to 49999999: 1435207
|
|||
|
|
From 50000000 to 74999999: 1393170
|
|||
|
|
From 75000000 to 99999999: 1367151
|
|||
|
|
===============================================
|
|||
|
|
Total number of primes less than 100000000: 5761455
|
|||
|
|
|
|||
|
|
real 3m24.030s
|
|||
|
|
user 9m49.405s
|
|||
|
|
sys 0m0.438s
|
|||
|
|
</pre></div>
|
|||
|
|
</div>
|
|||
|
|
<p>First, note that the parallel version took more total CPU time (almost 10
|
|||
|
|
minutes); this kind of overhead is typical, as there are overhead performance
|
|||
|
|
penalties for using multiple threads. Second, the real time was only about 1/3
|
|||
|
|
of the user time, not 1/4. In practice, no system ever achieves perfect parallel
|
|||
|
|
execution. Chapter 9 will explain the limits of parallelism in more detail.</p>
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
<div class="container">
|
|||
|
|
|
|||
|
|
<div class="mt-4 container center">
|
|||
|
|
«  <a id="prevmod1" href="Extended6Input.html">6.7. Extended Example: Keyboard Input Listener</a>
|
|||
|
|
  ::  
|
|||
|
|
<a class="uplink" href="index.html">Contents</a>
|
|||
|
|
  ::  
|
|||
|
|
<a id="nextmod1" href="SynchOverview.html">7.1. Synchronization Primitives</a>  »
|
|||
|
|
</div>
|
|||
|
|
|
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