Setting number of threads in python

Question

I have a python script which consist of numpy and scipy functions. I was trying to check the scaling for my code.

numpy.show_config()

The configuration for the numpy installed in my system shows the following information.

blas_mkl_info:
    libraries = ['mkl_rt']
    library_dirs = ['Library\\lib']
    define_macros = [('SCIPY_MKL_H', None), ('HAVE_CBLAS', None)]
    include_dirs = ['Library\\include']
blas_opt_info:
    libraries = ['mkl_rt']
    library_dirs = ['Library\\lib']
    define_macros = [('SCIPY_MKL_H', None), ('HAVE_CBLAS', None)]
    include_dirs = ['Library\\include']
lapack_mkl_info:
    libraries = ['mkl_rt']
    library_dirs = ['Library\\lib']
    define_macros = [('SCIPY_MKL_H', None), ('HAVE_CBLAS', None)]
    include_dirs = ['Library\\include']
lapack_opt_info:
    libraries = ['mkl_rt']
    library_dirs = ['Library\\lib']
    define_macros = [('SCIPY_MKL_H', None), ('HAVE_CBLAS', None)]
    include_dirs = ['Library\\include']
Supported SIMD extensions in this NumPy install:
    baseline = SSE,SSE2,SSE3
    found = SSSE3,SSE41,POPCNT,SSE42,AVX,F16C,FMA3,AVX2,AVX512F,AVX512CD,AVX512_SKX,AVX512_CLX,AVX512_CNL
    not found =

So I tried setting the following environment variables before importing numpy:

import os    
os.environ["OMP_NUM_THREADS"] = '16' 
os.environ["OPENBLAS_NUM_THREADS"] = '16' 
os.environ["MKL_NUM_THREADS"] = '16'

But still, my code is only using 1 thread and there is not difference in the time of execution of the program.

I have also tried setting mkl.set_num_threads(16) but no difference.

I am aware that python has GIL which doesn't allow multiple threads to execute simultaneously as u expect in C. Is there any other way to set the number of threads to be used in python?

score 0 · Accepted Answer · answered Aug 19 '22 at 06:32

It works but you have to set the environment variables before the first time you load a module in your script (including any the sub modules)

To be safe you should do this in your main program before any other import

import os
nthreads = 1
os.environ["OMP_NUM_THREADS"] = str(nthreads) 
os.environ["OPENBLAS_NUM_THREADS"] = str(nthreads) 
os.environ["MKL_NUM_THREADS"] = str(nthreads)
import numpy

An alternative is to set the environment variables before running the script as suggested by this answer.

One thing I thought was that reloading the module using import lib could do the trick to allow setting it dynamically, but no, it doesn't work.

If you are can use pytorch instead of numpy then you may use torch.set_num_threads that is effective, check an example usage here.

Setting number of threads in python

1 Answers1