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docs: Correct README claim that OpenMP thread limits are process-wide #210

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10 changes: 6 additions & 4 deletions README.md
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Expand Up @@ -322,10 +322,12 @@ https://github.com/xianyi/OpenBLAS/issues/2985).
and workarounds:
https://github.com/joblib/threadpoolctl/blob/master/multiple_openmp.md

- Setting the maximum number of threads of the OpenMP and BLAS libraries has a global
effect and impacts the whole Python process. There is no thread level isolation as
these libraries do not offer thread-local APIs to configure the number of threads to
use in nested parallel calls.
- Setting the maximum number of threads of BLAS libraries such as OpenBLAS with
the pthreads threading layer and MKL has a global effect and impacts the whole
Python process. By contrast, for OpenMP runtimes such as libgomp and libomp,
`omp_set_num_threads` applies to the calling thread. Therefore, when
`threadpool_limits` limits OpenMP libraries, the limit is applied on the
current thread and does not propagate to threads spawned afterward.


## Maintainers
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