db/etl: stop the bufio read benchmarks allocating per field - #23618
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db/etl: stop the bufio read benchmarks allocating per field#23618AskAlexSharov wants to merge 1 commit into
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readFieldBufioU16 and U32 read the length prefix into a local array and hand it to io.ReadFull, which takes an io.Reader - so the array escapes and every field read costs an allocation. The mmap readers next to them slice the mapping directly and pay nothing, so the comparison the file exists to make was handing the bufio side a penalty that is not about buffered I/O at all. The caller owns lenBuf now. BenchmarkSequentialRead/val32/bufio_u16, two alternating rounds each: 57.3ms and 59.5ms before, 42.8ms and 42.0ms after - 8.4MB over 3947589 allocations becomes 525KB over 7.
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readFieldBufioU16andreadFieldBufioU32read the length prefix into a local[2]byte/[4]byteand pass it toio.ReadFull. That takes anio.Reader, so the array escapes and every field read costs an allocation.The mmap readers beside them slice the mapping directly and pay nothing, so the comparison this file exists to make was handing the bufio side a penalty that has nothing to do with buffered I/O.
The caller owns
lenBufnow.BenchmarkSequentialRead/val32/bufio_u16, two alternating rounds each:Benchmark-only; no production code touched. Found while auditing allocations for #23599.