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| <a name="GOMP_005fCPU_005fAFFINITY-_002d_002d-Bind-threads-to-specific-CPUs"></a> |
| <h3 class="section">3.14 <code>GOMP_CPU_AFFINITY</code> – Bind threads to specific CPUs</h3> |
| <a name="index-Environment-Variable-13"></a> |
| <dl compact="compact"> |
| <dt><em>Description</em>:</dt> |
| <dd><p>Binds threads to specific CPUs. The variable should contain a space-separated |
| or comma-separated list of CPUs. This list may contain different kinds of |
| entries: either single CPU numbers in any order, a range of CPUs (M-N) |
| or a range with some stride (M-N:S). CPU numbers are zero based. For example, |
| <code>GOMP_CPU_AFFINITY="0 3 1-2 4-15:2"</code> will bind the initial thread |
| to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to |
| CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12, |
| and 14 respectively and then start assigning back from the beginning of |
| the list. <code>GOMP_CPU_AFFINITY=0</code> binds all threads to CPU 0. |
| </p> |
| <p>There is no GNU OpenMP library routine to determine whether a CPU affinity |
| specification is in effect. As a workaround, language-specific library |
| functions, e.g., <code>getenv</code> in C or <code>GET_ENVIRONMENT_VARIABLE</code> in |
| Fortran, may be used to query the setting of the <code>GOMP_CPU_AFFINITY</code> |
| environment variable. A defined CPU affinity on startup cannot be changed |
| or disabled during the runtime of the application. |
| </p> |
| <p>If both <code>GOMP_CPU_AFFINITY</code> and <code>OMP_PROC_BIND</code> are set, |
| <code>OMP_PROC_BIND</code> has a higher precedence. If neither has been set and |
| <code>OMP_PROC_BIND</code> is unset, or when <code>OMP_PROC_BIND</code> is set to |
| <code>FALSE</code>, the host system will handle the assignment of threads to CPUs. |
| </p> |
| </dd> |
| <dt><em>See also</em>:</dt> |
| <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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