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<title>GNU libgomp: GOMP_CPU_AFFINITY</title>
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Next: <a href="GOMP_005fDEBUG.html#GOMP_005fDEBUG" accesskey="n" rel="next">GOMP_DEBUG</a>, Previous: <a href="OMP_005fWAIT_005fPOLICY.html#OMP_005fWAIT_005fPOLICY" accesskey="p" rel="prev">OMP_WAIT_POLICY</a>, Up: <a href="Environment-Variables.html#Environment-Variables" accesskey="u" rel="up">Environment Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p>
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<a name="GOMP_005fCPU_005fAFFINITY-_002d_002d-Bind-threads-to-specific-CPUs"></a>
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<h3 class="section">3.15 <code>GOMP_CPU_AFFINITY</code> – Bind threads to specific CPUs</h3>
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<a name="index-Environment-Variable-14"></a>
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<dl compact="compact">
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<dt><em>Description</em>:</dt>
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<dd><p>Binds threads to specific CPUs. The variable should contain a space-separated
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or comma-separated list of CPUs. This list may contain different kinds of
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entries: either single CPU numbers in any order, a range of CPUs (M-N)
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or a range with some stride (M-N:S). CPU numbers are zero based. For example,
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<code>GOMP_CPU_AFFINITY="0 3 1-2 4-15:2"</code> will bind the initial thread
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to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to
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CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12,
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and 14 respectively and then start assigning back from the beginning of
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the list. <code>GOMP_CPU_AFFINITY=0</code> binds all threads to CPU 0.
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</p>
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<p>There is no libgomp library routine to determine whether a CPU affinity
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specification is in effect. As a workaround, language-specific library
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functions, e.g., <code>getenv</code> in C or <code>GET_ENVIRONMENT_VARIABLE</code> in
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Fortran, may be used to query the setting of the <code>GOMP_CPU_AFFINITY</code>
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environment variable. A defined CPU affinity on startup cannot be changed
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or disabled during the runtime of the application.
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</p>
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<p>If both <code>GOMP_CPU_AFFINITY</code> and <code>OMP_PROC_BIND</code> are set,
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<code>OMP_PROC_BIND</code> has a higher precedence. If neither has been set and
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<code>OMP_PROC_BIND</code> is unset, or when <code>OMP_PROC_BIND</code> is set to
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<code>FALSE</code>, the host system will handle the assignment of threads to CPUs.
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</p>
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</dd>
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<dt><em>See also</em>:</dt>
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<dd><p><a href="OMP_005fPLACES.html#OMP_005fPLACES">OMP_PLACES</a>, <a href="OMP_005fPROC_005fBIND.html#OMP_005fPROC_005fBIND">OMP_PROC_BIND</a>
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</p></dd>
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