76 lines
5.1 KiB
HTML
76 lines
5.1 KiB
HTML
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<meta name="DC.Title" content="Determine when to use simultaneous multithreading" />
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<meta name="abstract" content="Simultaneous multithreading allows sharing of process facilities to run two applications or two threads of the same application at the same time. Find out more about this concept." />
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<title>Determine when to use simultaneous multithreading</title>
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<body id="rzahxmultithread"><a name="rzahxmultithread"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Determine when to use simultaneous multithreading</h1>
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<div><p>Simultaneous multithreading allows sharing of process facilities
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to run two applications or two threads of the same application at the same
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time. Find out more about this concept.</p>
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<p>Although an operating system gives the impression that it is concurrently
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executing a very large number of tasks, each processor in a symmetric multiprocessor
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(SMP) traditionally executes a single task's instruction stream at any moment
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in time. The QPRCMLTTSK system value controls whether to enable the individual
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SMP processors to concurrently execute multiple instruction streams. Each
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instruction stream belongs to separate tasks or threads. When enabled, each
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individual processor is concurrently executing multiple tasks at the same
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time. The effect of its use will likely increase the performance capacity
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of a system or improve the responsiveness of a multithreaded application.
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Running multiple instruction streams at the same time does not improve the
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performance of any given task. As is the case with any performance recommendations,
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results vary in different environments.</p>
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<p>The way that multithreading is done depends on the hardware model, and
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therefore, the performance capacity gains will vary. IBM<sup>®</sup> eServer™ 5<em>xx</em> models support this
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approach through a concept called simultaneous multithreading (SMT). This
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approach, called hyperthreading on some Intel<sup>®</sup> processors, shares processor facilities
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to execute each task's instructions at the same time. Older processors use
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an approach called hardware multithreading (HMT). In the hardware multithreading
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approach, the hardware switches between the tasks on any long processing delay
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event, for example, a cache miss. Some models do not support any form of multithreading,
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which means the QPRCMLTTSK system value has no performance effect.</p>
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<p>Because the QPRCMLTTSK system value enables the parallel use of shared
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processor resources, the performance gains depend highly on the application
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and the model. Refer to the <cite><span class="keyword">iSeries™</span> Performance
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Capabilities Reference</cite> for guidelines about what performance gains
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might be expected through its use. In some cases, some applications are better
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served by disabling this system value.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzahxtune.htm" title="When you have identified a performance problem, you will want to tune the system to fix it.">Tune performance</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="../rzakz/rzakzqprcmlttsk.htm">QPRCMLTTSK system value</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../books/sc410607.pdf" target="_blank">iSeries Performance Capabilities Reference PDF</a></div>
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</div>
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</div>
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