Refactor the data model for excutors #38
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I originally thought that it made the most sense to think of rechunking as a chain of chunked array copies. After going through the exercise of trying to write an Executor that doesn't use temporary arrays (pangeo-data#36), I realize now that this was overly generic. *Sometimes* it might make sense to implement a rechunking this way, but Rechunker's algorithm adds more structure than that. There is always a "push/pull" structure that splits read chunks into intermediate chunks, and then combines intermediate chunks into write chunks. This structure wasn't evident from current data model and I needed it for my executor without temporaries, so this PR changes CopySpec to directly keep track of "read/intermediate/write" steps. It's still convenient sometimes to use the "chain of copies" representation, so I've added the utility function `split_into_direct_copies()` for converting to this representation in `rechunker.executors.util`.
Codecov Report
@@ Coverage Diff @@
## master #38 +/- ##
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+ Coverage 93.39% 93.55% +0.16%
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Files 7 7
Lines 318 326 +8
Branches 65 66 +1
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+ Hits 297 305 +8
Misses 10 10
Partials 11 11
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TomAugspurger
approved these changes
Jul 31, 2020
| read : ArrayProxy | ||
| Read proxy with an ``array`` attribute that supports ``__getitem__``. | ||
| intermediate : ArrayProxy | ||
| Intermediate proxy with either an ``array`` that is either ``None`` |
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Thanks for the detailed explanation. I was confused about how direct copies would be represented.
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Thanks @shoyer! |
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I originally thought that it made the most sense to think of rechunking as a chain of chunked array copies.
After going through the exercise of trying to write an Executor that doesn't use temporary arrays (#36), I realize now that this was overly generic. Sometimes it might make sense to implement a rechunking this way, but Rechunker's algorithm adds more structure than that. There is always a "push/pull" structure that splits read chunks into intermediate chunks, and then combines intermediate chunks into write chunks.
This structure wasn't evident from current data model and I needed it for my executor without temporaries, so this PR changes CopySpec to directly keep track of "read/intermediate/write" steps.
I've used this new representation in
api.pyand the dask executor. It's still convenient sometimes to use the "chain of copies" representation, so I've added the utility functionsplit_into_direct_copies()for converting to this representation inrechunker.executors.util.