Simplify distance heuristics in read_stream.c. Make the distance control heuristics simpler and more aggressive in preparation for asynchronous I/O.
The v17 version of read_stream.c made a conservative choice to limit the look-ahead distance when streaming sequential blocks, because it couldn't benefit very much from looking ahead further yet. It had a three-behavior model where only random I/O would rapidly increase the look-ahead distance, to support read-ahead advice. Sequential I/O would move it towards the io_combine_limit setting, just enough to build one full-sized synchronous I/O at a time, and then expect kernel read-ahead to avoid I/O stalls. That already left I/O performance on the table with advice-based I/O concurrency, since sequential blocks could be followed by random jumps, eg with the proposed streaming Bitmap Heap Scan patch. It is time to delete the cautious middle option and adjust the distance based on recent I/O needs only, since asynchronous reads will need to be started ahead of time whether random or sequential. It is still limited by io_combine_limit, *_io_concurrency, buffer availability and strategy ring size, as before. Reviewed-by: Andres Freund <and...@anarazel.de> (earlier version) Tested-by: Melanie Plageman <melanieplage...@gmail.com> Discussion: https://postgr.es/m/CA%2BhUKGK_%3D4CVmMHvsHjOVrK6t4F%3DLBpFzsrr3R%2BaJYN8kcTfWg%40mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/799959dc7cf0e2462601bea8d07b6edec3fa0c4f Modified Files -------------- src/backend/storage/aio/read_stream.c | 68 +++++++++-------------------------- 1 file changed, 16 insertions(+), 52 deletions(-)
