Runtime variables

OpenBLAS checks the following environment variables on startup:

  • OPENBLAS_NUM_THREADS: the number of threads to use (for non-OpenMP builds of OpenBLAS)
  • OMP_NUM_THREADS: the number of threads to use (for OpenMP builds - note that setting this may also affect any other OpenMP code)

  • OPENBLAS_DEFAULT_NUM_THREADS: the number of threads to use, irrespective if OpenBLAS was built for OpenMP or pthreads

  • OPENBLAS_MAIN_FREE=1: this can be used to disable automatic assignment of cpu affinity in OpenBLAS builds that have it enabled by default

  • OPENBLAS_THREAD_TIMEOUT: this can be used to define the length of time that idle threads should wait before exiting
  • OMP_ADAPTIVE=1: this can be used in OpenMP builds to actually remove any surplus threads when the number of threads is decreased

DYNAMIC_ARCH builds also accept the following:

  • OPENBLAS_VERBOSE:

    • set this to 1 to enable a warning when there is no exact match for the detected cpu in the library
    • set this to 2 to make OpenBLAS print the name of the cpu target it autodetected

  • OPENBLAS_CORETYPE: set this to one of the supported target names to override autodetection, e.g., OPENBLAS_CORETYPE=HASWELL

  • OPENBLAS_L2_SIZE: set this to override the autodetected size of the L2 cache where it is not reported correctly (in virtual environments)

Deprecated variables still recognized for compatibilty:

  • GOTO_NUM_THREADS: equivalent to OPENBLAS_NUM_THREADS
  • GOTOBLAS_MAIN_FREE: equivalent to OPENBLAS_MAIN_FREE
  • OPENBLAS_BLOCK_FACTOR: this applies a scale factor to the GEMM "P" parameter of the block matrix code, see file driver/others/parameter.c