OMP_PLACES (GNU libgomp)

### 4.14 `OMP_PLACES` – Specifies on which CPUs the threads should be placed ¶

ICV: place-partition-var
Scope: implicit tasks
Description:

  The thread placement can be either specified using an abstract name or by an explicit list of the places. The abstract names `threads`, `cores`, `sockets`, `ll_caches` and `numa_domains` can be optionally followed by a positive number in parentheses, which denotes the how many places shall be created. With `threads` each place corresponds to a single hardware thread; `cores` to a single core with the corresponding number of hardware threads; with `sockets` the place corresponds to a single socket; with `ll_caches` to a set of cores that shares the last level cache on the device; and `numa_domains` to a set of cores for which their closest memory on the device is the same memory and at a similar distance from the cores. The resulting placement can be shown by setting the `OMP_DISPLAY_ENV` environment variable.

  Alternatively, the placement can be specified explicitly as comma-separated list of places. A place is specified by set of nonnegative numbers in curly braces, denoting the hardware threads. The curly braces can be omitted when only a single number has been specified. The hardware threads belonging to a place can either be specified as comma-separated list of nonnegative thread numbers or using an interval. Multiple places can also be either specified by a comma-separated list of places or by an interval. To specify an interval, a colon followed by the count is placed after the hardware thread number or the place. Optionally, the length can be followed by a colon and the stride number – otherwise a unit stride is assumed. Placing an exclamation mark (`!`) directly before a curly brace or numbers inside the curly braces (excluding intervals) excludes those hardware threads.

  For instance, the following specifies the same places list: `"{0,1,2}, {3,4,6}, {7,8,9}, {10,11,12}"`; `"{0:3}, {3:3}, {7:3}, {10:3}"`; and `"{0:2}:4:3"`.

  If `OMP_PLACES` and `GOMP_CPU_AFFINITY` are unset and `OMP_PROC_BIND` is either unset or `false`, threads may be moved between CPUs following no placement policy.

See also:

  `OMP_PROC_BIND` – Whether threads may be moved between CPUs, `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs, `omp_get_proc_bind` – Whether threads may be moved between CPUs, `OMP_DISPLAY_ENV` – Show OpenMP version and environment variables

Reference:

  OpenMP specification v4.5, Section 4.5