Improve read_stream.c advice for dense streams. read_stream.c tries not to issue read-ahead advice when it thinks the kernel's own read-ahead should be active, ie when using buffered I/O and reading sequential blocks. It previously gave up too easily, and issued advice only for the first read of up to io_combine_limit blocks in a larger range of sequential blocks after random jump. The following read could suffer an avoidable I/O stall.
Fix, by continuing to issue advice until the corresponding preadv() calls catch up with the start of the region we're currently issuing advice for, if ever. That's when the kernel actually sees the sequential pattern. Advice is now disabled only when the stream is entirely sequential as far as we can see in the look-ahead window, or in other words, when a sequential region is larger than we can cover with the current io_concurrency and io_combine_limit settings. While refactoring the advice control logic, also get rid of the "suppress_advice" argument that was passed around between functions to skip useless posix_fadvise() calls immediately followed by preadv(). read_stream_start_pending_read() can figure that out, so let's concentrate knowledge of advice heuristics in fewer places (our goal being to make advice-based I/O concurrency a legacy mode soon). The problem cases were revealed by Tomas Vondra's extensive regression testing with many different disk access patterns using Melanie Plageman's streaming Bitmap Heap Scan patch, in a battle against the venerable always-issue-advice-and-always-one-block-at-a-time code. Reviewed-by: Andres Freund <and...@anarazel.de> (earlier version) Reported-by: Melanie Plageman <melanieplage...@gmail.com> Reported-by: Tomas Vondra <to...@vondra.me> Reported-by: Andres Freund <and...@anarazel.de> Tested-by: Melanie Plageman <melanieplage...@gmail.com> Discussion: https://postgr.es/m/CA%2BhUKGK_%3D4CVmMHvsHjOVrK6t4F%3DLBpFzsrr3R%2BaJYN8kcTfWg%40mail.gmail.com Discussion: https://postgr.es/m/CA%2BhUKGJ3HSWciQCz8ekP1Zn7N213RfA4nbuotQawfpq23%2Bw-5Q%40mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/7ea8cd15661e3b0da4b57be2f25fdd512951576f Modified Files -------------- src/backend/storage/aio/read_stream.c | 65 ++++++++++++++++++++++++----------- 1 file changed, 45 insertions(+), 20 deletions(-)
