Fix ColumnTransformer in parallel with joblib's auto memmapping #28822
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I opened #28824 to discuss the read-only situation more globally. |
jeremiedbb
commented
Apr 12, 2024
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| with parallel_backend("loky", max_nbytes=1): | ||
| Xt = transformer.fit_transform(X) | ||
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CI is failing because this is only doable in joblib>=1.13 and our min is 1.12.
I can use a bigger array for now
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we could skip the test on that joblib though.
adrinjalali
approved these changes
Apr 15, 2024
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| with parallel_backend("loky", max_nbytes=1): | ||
| Xt = transformer.fit_transform(X) | ||
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we could skip the test on that joblib though.
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Fixes #28781
When running in parallel, ColumnTransformer will crash if joblib's auto memmap triggers and copies are not made in time.
Currently we index X when declaring the jobs. It means we have copy then read-only memmap. Then if the transformer fails to do inplace transfo, or fails earlier in case of dataframe (see #28781 (comment)).
The fix here proposes to index X within each job instead. This way we have read-only memmap then copy, and the transformer can do inplace transfo.
Disclaimer: it doesn't solve the underlying problem completely. If you select columns by slice it still fails because it creates a view and not a copy. I'm starting to think that the issue is more profound, and lies between the
copyparameter andcheck_array, for all estimators. I thinkcheck_arrayshould always make a copy if the array is read-only, even ifcopy=Falsebecause when an estimator has acopyparameter, it's because it wants to do inplace modifications.