threadpoolctl: Slower without `OMP_NUM_THREADS=1` than with `OMP_NUM_THREADS=1`

I tried with threadpool_limits(1, user_api=None): with a not so simple case : https://gitlab.com/paugier/tsp-pythran (branch threadpoolctl) on Debian.

The case uses Pythran (through Transonic but I don’t see how it could change anything for this) to get an extension accelerated with OpenMP. Pythran uses the system OpenBlas library.

To reproduce (sorry, I don’t use Git):

hg clone https://gitlab.com/paugier/tsp-pythran.git
cd tsp-pythran
hg up threadpoolctl
# compile the extension with openmp
transonic tsp.py -pf "-march=native -DUSE_XSIMD -fopenmp"
# wait to get the extension ready
python run-test-omp.py
OMP_NUM_THREADS=1 python run-test-omp.py

The good news is that threadpoolctl manages to reduce the number of threads used with OpenMP. However, I get something strange that I don’t understand:

I’m not sure it’s an issue, but I get something slower with python run-test-omp.py (or OMP_NUM_THREADS=2 python run-test-omp.py) than with OMP_NUM_THREADS=1 python run-test-omp.py.

I actually get the same behavior if the extension is built without OpenMP, i.e. just with transonic tsp.py.

OMP_NUM_THREADS=1 python run-test-omp.py
[{'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/home/users/augier3pi/.pyenv/versions/3.7.2/lib/python3.7/site-packages/numpy/.libs/libopenblasp-r0-382c8f3a.3.5.dev.so',
  'n_thread': 1,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': '0.3.5.dev'},
 {'filename_prefixes': ('libiomp', 'libgomp', 'libomp', 'vcomp'),
  'internal_api': 'openmp',
  'module_path': '/usr/lib/x86_64-linux-gnu/libgomp.so.1',
  'n_thread': 1,
  'prefix': 'libgomp',
  'user_api': 'openmp',
  'version': None},
 {'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/home/users/augier3pi/.pyenv/versions/3.7.2/lib/python3.7/site-packages/scipy/.libs/libopenblasp-r0-8dca6697.3.0.dev.so',
  'n_thread': 1,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': None},
 {'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/usr/lib/libopenblas.so.0',
  'n_thread': 1,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': None}]
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run time = 0.43 s
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run time = 0.43 s
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run time = 0.44 s
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run time = 0.46 s


python run-test-omp.py
[{'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/home/users/augier3pi/.pyenv/versions/3.7.2/lib/python3.7/site-packages/numpy/.libs/libopenblasp-r0-382c8f3a.3.5.dev.so',
  'n_thread': 4,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': '0.3.5.dev'},
 {'filename_prefixes': ('libiomp', 'libgomp', 'libomp', 'vcomp'),
  'internal_api': 'openmp',
  'module_path': '/usr/lib/x86_64-linux-gnu/libgomp.so.1',
  'n_thread': 4,
  'prefix': 'libgomp',
  'user_api': 'openmp',
  'version': None},
 {'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/home/users/augier3pi/.pyenv/versions/3.7.2/lib/python3.7/site-packages/scipy/.libs/libopenblasp-r0-8dca6697.3.0.dev.so',
  'n_thread': 4,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': None},
 {'filename_prefixes': ('libopenblas',),
  'internal_api': 'openblas',
  'module_path': '/usr/lib/libopenblas.so.0',
  'n_thread': 4,
  'prefix': 'libopenblas',
  'user_api': 'blas',
  'version': None}]
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run time = 0.57 s
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run time = 0.59 s
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run time = 0.58 s
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run time = 0.58 s