From 06fb5612c970b3af95aca3db5a955669b07537ca Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 19 Mar 2025 15:23:12 +1300 Subject: [PATCH] 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 --- doc/src/sgml/config.sgml | 4 ++++ src/backend/storage/aio/read_stream.c | 7 ++++--- src/backend/utils/misc/postgresql.conf.sample | 4 ++-- src/include/port/pg_iovec.h | 8 ++++++-- 4 files changed, 16 insertions(+), 7 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 2988865b116..4fd377d1c5f 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2638,6 +2638,8 @@ include_dir 'conf.d' This parameter can only be set in the postgresql.conf file or on the server command line. + The maximum possible size depends on the operating system and block + size, but is typically 1MB on Unix and 128kB on Windows. The default is 128kB. @@ -2655,6 +2657,8 @@ include_dir 'conf.d' higher than the io_max_combine_limit parameter, the lower value will silently be used instead, so both may need to be raised to increase the I/O size. + The maximum possible size depends on the operating system and block + size, but is typically 1MB on Unix and 128kB on Windows. The default is 128kB. diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index d65fa07b44c..45bdf819d57 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -515,9 +515,10 @@ read_stream_begin_impl(int flags, * finishes we don't want to have to wait for its buffers to be consumed * before starting a new one. * - * Be careful not to allow int16 to overflow (even though that's not - * possible with the current GUC range limits), allowing also for the - * spare entry and the overflow space. + * Be careful not to allow int16 to overflow. That is possible with the + * current GUC range limits, so this is an artificial limit of ~32k + * buffers and we'd need to adjust the types to exceed that. We also have + * to allow for the spare entry and the overflow space. */ max_pinned_buffers = (max_ios + 1) * io_combine_limit; max_pinned_buffers = Min(max_pinned_buffers, 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 diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h index d9891d3805d..df40c7208be 100644 --- a/src/include/port/pg_iovec.h +++ b/src/include/port/pg_iovec.h @@ -33,8 +33,12 @@ struct iovec #endif -/* Define a reasonable maximum that is safe to use on the stack. */ -#define PG_IOV_MAX Min(IOV_MAX, 32) +/* + * Define a reasonable maximum that is safe to use on the stack in arrays of + * struct iovec and other small types. The operating system could limit us to + * a number as low as 16, but most systems have 1024. + */ +#define PG_IOV_MAX Min(IOV_MAX, 128) /* * Like preadv(), but with a prefix to remind us of a side-effect: on Windows