[WIP] Stop using ReadaheadRandomAccessFile in TableCache #4052
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Summary: Improve the iterators performance when the user explicitly sets the readahead size via `ReadOptions.readahead_size`. 1. Stop creating new table readers when the user explicitly sets readahead size. 2. Make use of an internal buffer based on `FilePrefetchBuffer` instead of using `ReadaheadRandomAccessFileReader`, to handle the user readahead requests (for both buffered and direct io cases). 3. Add `readahead_size` to db_bench. **Benchmarks:** https://gist.github.com/sagar0/53693edc320a18abeaeca94ca32f5737 For 1 MB readahead, Buffered IO performance improves by 28% and Direct IO performance improves by 50%. For 512KB readahead, Buffered IO performance improves by 30% and Direct IO performance improves by 67%. **Test Plan:** Updated `DBIteratorTest.ReadAhead` test to make sure that: - no new table readers are created for iterators on setting ReadOptions.readahead_size - At least "readahead" number of bytes are actually getting read on each iterator read. TODO later: - Use similar logic for compactions as well. - This ties in nicely with #4052 and paves the way for removing ReadaheadRandomAcessFile later. Pull Request resolved: #5246 Differential Revision: D15107946 Pulled By: sagar0 fbshipit-source-id: 2c1149729ca7d779e4e8b7710ba6f4e8cbfd3bea
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Summary: Improve the iterators performance when the user explicitly sets the readahead size via `ReadOptions.readahead_size`. 1. Stop creating new table readers when the user explicitly sets readahead size. 2. Make use of an internal buffer based on `FilePrefetchBuffer` instead of using `ReadaheadRandomAccessFileReader`, to handle the user readahead requests (for both buffered and direct io cases). 3. Add `readahead_size` to db_bench. **Benchmarks:** https://gist.github.com/sagar0/53693edc320a18abeaeca94ca32f5737 For 1 MB readahead, Buffered IO performance improves by 28% and Direct IO performance improves by 50%. For 512KB readahead, Buffered IO performance improves by 30% and Direct IO performance improves by 67%. **Test Plan:** Updated `DBIteratorTest.ReadAhead` test to make sure that: - no new table readers are created for iterators on setting ReadOptions.readahead_size - At least "readahead" number of bytes are actually getting read on each iterator read. TODO later: - Use similar logic for compactions as well. - This ties in nicely with facebook#4052 and paves the way for removing ReadaheadRandomAcessFile later. Pull Request resolved: facebook#5246 Differential Revision: D15107946 Pulled By: sagar0 fbshipit-source-id: 2c1149729ca7d779e4e8b7710ba6f4e8cbfd3bea
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Note:
This is still a poc and NOT ready for submission, as I haven't thought though all the cases where it might cause performance regression (too many cases to think through).
Things I might be missing so far:
for_compactiontoNewIndexIteratorinBlockBasedTable::NewIterator. Otherwise we are not doing any readahead for indexes, which would be pretty bad.use_mmap_reads=true?Summary:
Remove ReadaheadRandomAccessFile usage in
TableCache::GetTableReader. Instead let BlockBasedTableIterator and FilePrefetchBuffer handle all the readahead logic.Test Plan:
make check -- all tests succeed.
DBIteratorTest.ReadAheadtest failed since it wasn't aware of Prefetch. It started passing once I addedPrefetchtoCountingFile.