8000 ElasticNetCV fails under loky backend · Issue #14249 · scikit-learn/scikit-learn · GitHub
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pierreglaser opened this issue Jul 3, 2019 · 1 comment · Fixed by #14264
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ElasticNetCV fails under loky backend #14249

pierreglaser opened this issue Jul 3, 2019 · 1 comment · Fixed by #14264
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@pierreglaser
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running this script:

from sklearn.linear_model import ElasticNetCV
from sklearn.datasets import make_regression
from joblib import parallel_backend
X, y = make_regression(20000, 10)
clf = ElasticNetCV(n_jobs=2, cv=20)
with parallel_backend(backend='loky'):
    clf.fit(X, y)

fails with:

"""
Traceback (most recent call last):
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/externals/loky/process_executor.py", line 418, in _process_worker
    r = call_item()
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/externals/loky/process_executor.py", line 272, in __call__
    return self.fn(*self.args, **self.kwargs)
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/_parallel_backends.py", line 567, in __call__
    return self.func(*args, **kwargs)
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/parallel.py", line 225, in __call__
    for func, args, kwargs in self.items]
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/parallel.py", line 225, in <listcomp>
    for func, args, kwargs in self.items]
  File "/home/pierreglaser/repos/scikit-learn/sklearn/linear_model/coordinate_descent.py", line 997, in _path_residuals
    copy=False)
  File "/home/pierreglaser/repos/scikit-learn/sklearn/linear_model/base.py", line 527, in _pre_fit
    check_input=check_input)
  File "/home/pierreglaser/repos/scikit-learn/sklearn/linear_model/base.py", line 158, in _preprocess_data
    X -= X_offset
ValueError: output array is read-only
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "bug.py", line 7, in <module>
    clf.fit(X, y)
  File "/home/pierreglaser/repos/scikit-learn/sklearn/linear_model/coordinate_descent.py", line 1195, in fit
    **_joblib_parallel_args(prefer="threads"))(jobs)
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/parallel.py", line 934, in __call__
    self.retrieve()
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/parallel.py", line 833, in retrieve
    self._output.extend(job.get(timeout=self.timeout))
  File "/home/pierreglaser/.virtualenvs/sklearn_py36/lib/python3.6/site-packages/joblib/_parallel_backends.py", line 521, in wrap_future_result
    return future.result(timeout=timeout)
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
ValueError: output array is read-only

The problem does not happen if the data is smaller -- My guess is that _pre_fit tries to modify in-place the input (in this case, _pre_fit is called with copy=False), which using loky and large data, is memmaped, thus read-only. Hence the error.
@rth rth added the Bug label Jul 3, 2019
@pierreglaser
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This bug also affects LassoCV.

@jeremiedbb jeremiedbb mentioned this issue Jul 5, 2019
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[MRG] Fix LinearModelsCV for loky backend. jeremiedbb/scikit-learn
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@rth @pierreglaser
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