From 06fb5612c970b3af95aca3db5a955669b07537ca Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 19 Mar 2025 15:23:12 +1300 Subject: Increase io_combine_limit range to 1MB. The default of 128kB is unchanged, but the upper limit is changed from 32 blocks to 128 blocks, unless the operating system's IOV_MAX is too low. Some other RDBMSes seem to cap their multi-block buffer pool I/O around this number, and it seems useful to allow experimentation. The concrete change is to our definition of PG_IOV_MAX, which provides the maximum for io_combine_limit and io_max_combine_limit. It also affects a couple of other places that work with arrays of struct iovec or smaller objects on the stack, so we still don't want to use the system IOV_MAX directly without a clamp: it is not under our control and likely to be 1024. 128 seems acceptable for our current usage. For Windows, we can't use real scatter/gather yet, so we continue to define our own IOV_MAX value of 16 and emulate preadv()/pwritev() with loops. Someone would need to research the trade-offs of raising that number. NB if trying to see this working: you might temporarily need to hack BAS_BULKREAD to be bigger, since otherwise the obvious way of "a very big SELECT" is limited by that for now. Suggested-by: Tomas Vondra Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com --- src/backend/utils/misc/postgresql.conf.sample | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'src/backend/utils/misc/postgresql.conf.sample') diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample index 66bda60f4ca..6abd1baeac8 100644 --- a/src/backend/utils/misc/postgresql.conf.sample +++ b/src/backend/utils/misc/postgresql.conf.sample @@ -200,9 +200,9 @@ #backend_flush_after = 0 # measured in pages, 0 disables #effective_io_concurrency = 16 # 1-1000; 0 disables prefetching #maintenance_io_concurrency = 16 # 1-1000; 0 disables prefetching -#io_max_combine_limit = 128kB # usually 1-32 blocks (depends on OS) +#io_max_combine_limit = 128kB # usually 1-128 blocks (depends on OS) # (change requires restart) -#io_combine_limit = 128kB # usually 1-32 blocks (depends on OS) +#io_combine_limit = 128kB # usually 1-128 blocks (depends on OS) #io_method = worker # worker, sync (change requires restart) #io_max_concurrency = -1 # Max number of IOs that one process -- cgit v1.2.3