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Introduce io_max_combine_limit.

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The existing io_combine_limit can be changed by users.  The new
io_max_combine_limit is fixed at server startup time, and functions as a
silent clamp on the user setting.  That in itself is probably quite
useful, but the primary motivation is:

aio_init.c allocates shared memory for all asynchronous IOs including
some per-block data, and we didn't want to waste memory you'd never used
by assuming they could be up to PG_IOV_MAX.  This commit already halves
the size of 'AioHandleIov' and 'AioHandleData'.  A follow-up commit can
now expand PG_IOV_MAX without affecting that.

Since our GUC system doesn't support dependencies or cross-checks
between GUCs, the user-settable one now assigns a "raw" value to
io_combine_limit_guc, and the lower of io_combine_limit_guc and
io_max_combine_limit is maintained in io_combine_limit.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
This commit is contained in:
Thomas Munro
2025-03-19 11:40:56 +13:00
parent 17d8bba6da
commit 10f6646847
8 changed files with 72 additions and 15 deletions
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18
src/backend/commands/variable.c
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@@ -1156,6 +1156,24 @@ assign_maintenance_io_concurrency(int newval, void *extra)
#endif
}
/*
* GUC assign hooks that recompute io_combine_limit whenever
* io_combine_limit_guc and io_max_combine_limit are changed. These are needed
* because the GUC subsystem doesn't support dependencies between GUCs, and
* they may be assigned in either order.
*/
void
assign_io_max_combine_limit(int newval, void *extra)
{
io_max_combine_limit = newval;
io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc);
}
void
assign_io_combine_limit(int newval, void *extra)
{
io_combine_limit_guc = newval;
io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc);
}
/*
* These show hooks just exist because we want to show the values in octal.