Reuse the same reader across ReadChunk calls so that prefetched blocks are not discared.#82
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Reuse the same reader across ReadChunk calls so that prefetched blocks are not discared.#82copybara-service[bot] wants to merge 1 commit intomainfrom
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…s are not discared. In the sequential non-parallel path (ReadRecord->ReadAheadFromBuffer->ReadChunk) ReadChunk reads the whole chunk into memory. Most readers will round up the read to the blocksize and thereby prefetch some data from the next chunk. Currently ReadChunk recreates the reader each time and the previously prefetched data is lost and must be read again. Instead use get_backing_reader() directly each time. We can do this since ReadChunk is only called in single threaded context (Initialize, ReadRecord (parallelism disabled)) so there cannot be any concurrent access to the underlying reader. ReadTrace (before): ``` offset=64, size=1M <-- 1st chunk offset=1M, size=1M ... offset=8M, size=1M <-- block aligned read crosses chunk boundary offset=8.3M, size=0.7M <-- redundant read for 2nd chunk offset=9M, size=1M ... ``` ReadTrace (after): ``` offset=64, size=1M <-- 1st chunk offset=1M, size=1M ... offset=8M, size=1M <-- block aligned read crosses chunk boundary and is reused for the 2nd chunk offset=9M, size=1M ... ``` PiperOrigin-RevId: 577312179
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Reuse the same reader across ReadChunk calls so that prefetched blocks are not discared.
In the sequential non-parallel path (ReadRecord->ReadAheadFromBuffer->ReadChunk) ReadChunk reads the whole chunk into memory. Most readers will round up the read to the blocksize and thereby prefetch some data from the next chunk.
Currently ReadChunk recreates the reader each time and the previously prefetched data is lost and must be read again.
Instead use get_backing_reader() directly each time. We can do this since ReadChunk is only called in single threaded context (Initialize, ReadRecord (parallelism disabled)) so there cannot be any concurrent access to the underlying reader.
ReadTrace (before):
ReadTrace (after):