Python helpers to limit the number of threads used in native libraries that handle their own internal threadpool (BLAS and OpenMP implementations)





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Python helpers to limit the number of threads used in the threadpool-backed of common native libraries used for scientific computing and data science (e.g. BLAS and OpenMP).

Fine control of the underlying thread-pool size can be useful in workloads that involve nested parallelism so as to mitigate oversubscription issues.


  • For users, install the last published version from PyPI:

    pip install threadpoolctl
  • For contributors, install from the source repository in developer mode:

    pip install -r dev-requirements.txt
    flit install --symlink

    then you run the tests with pytest:



Command Line Interface

Get a JSON description of thread-pools initialized when importing python packages such as numpy or scipy for instance:

python -m threadpoolctl -i numpy scipy.linalg
    "filepath": "/home/ogrisel/miniconda3/envs/tmp/lib/",
    "prefix": "libmkl_rt",
    "user_api": "blas",
    "internal_api": "mkl",
    "version": "2019.0.4",
    "num_threads": 2,
    "threading_layer": "intel"
    "filepath": "/home/ogrisel/miniconda3/envs/tmp/lib/",
    "prefix": "libiomp",
    "user_api": "openmp",
    "internal_api": "openmp",
    "version": null,
    "num_threads": 4

The JSON information is written on STDOUT. If some of the packages are missing, a warning message is displayed on STDERR.

Python Runtime Programmatic Introspection

Introspect the current state of the threadpool-enabled runtime libraries that are loaded when importing Python packages:

>>> from threadpoolctl import threadpool_info
>>> from pprint import pprint
>>> pprint(threadpool_info())

>>> import numpy
>>> pprint(threadpool_info())
[{'filepath': '/home/ogrisel/miniconda3/envs/tmp/lib/',
  'internal_api': 'mkl',
  'num_threads': 2,
  'prefix': 'libmkl_rt',
  'threading_layer': 'intel',
  'user_api': 'blas',
  'version': '2019.0.4'},
 {'filepath': '/home/ogrisel/miniconda3/envs/tmp/lib/',
  'internal_api': 'openmp',
  'num_threads': 4,
  'prefix': 'libiomp',
  'user_api': 'openmp',
  'version': None}]

>>> import xgboost
>>> pprint(threadpool_info())
[{'filepath': '/home/ogrisel/miniconda3/envs/tmp/lib/',
  'internal_api': 'mkl',
  'num_threads': 2,
  'prefix': 'libmkl_rt',
  'threading_layer': 'intel',
  'user_api': 'blas',
  'version': '2019.0.4'},
 {'filepath': '/home/ogrisel/miniconda3/envs/tmp/lib/',
  'internal_api': 'openmp',
  'num_threads': 4,
  'prefix': 'libiomp',
  'user_api': 'openmp',
  'version': None},
 {'filepath': '/home/ogrisel/miniconda3/envs/tmp/lib/',
  'internal_api': 'openmp',
  'num_threads': 4,
  'prefix': 'libgomp',
  'user_api': 'openmp',
  'version': None}]

In the above example, numpy was installed from the default anaconda channel and comes with the MKL and its Intel OpenMP (libiomp5) implementation while xgboost was installed from and links against GNU OpenMP (libgomp) so both OpenMP runtimes are loaded in the same Python program.

Setting the Maximum Size of Thread-Pools

Control the number of threads used by the underlying runtime libraries in specific sections of your Python program:

from threadpoolctl import threadpool_limits
import numpy as np

with threadpool_limits(limits=1, user_api='blas'):
    # In this block, calls to blas implementation (like openblas or MKL)
    # will be limited to use only one thread. They can thus be used jointly
    # with thread-parallelism.
    a = np.random.randn(1000, 1000)
    a_squared = a @ a

Known Limitations

  • threadpool_limits can fail to limit the number of inner threads when nesting parallel loops managed by distinct OpenMP runtime implementations (for instance libgomp from GCC and libomp from clang/llvm or libiomp from ICC).

    See the test_openmp_nesting function in tests/ for an example. More information can be found at:

    Note however that this problem does not happen when threadpool_limits is used to limit the number of threads used internally by BLAS calls that are themselves nested under OpenMP parallel loops. threadpool_limits works as expected, even if the inner BLAS implementation relies on a distinct OpenMP implementation.

  • Using Intel OpenMP (ICC) and LLVM OpenMP (clang) in the same Python program under Linux is known to cause problems. See the following guide for more details and workarounds:


To make a release:

Bump the version number (__version__) in

Build the distribution archives:

pip install flit
flit build

Check the contents of dist/.

If everything is fine, make a commit for the release, tag it, push the tag to github and then:

flit publish


The initial dynamic library introspection code was written by @anton-malakhov for the smp package available at .

threadpoolctl extends this for other operating systems. Contrary to smp, threadpoolctl does not attempt to limit the size of Python multiprocessing pools (threads or processes) or set operating system-level CPU affinity constraints: threadpoolctl only interacts with native libraries via their public runtime APIs.

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