Introduce io_max_combine_limit.

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 <[email protected]> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com

1 files changed, 18 insertions, 0 deletions

diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index 4ad6e236d69..f550a3c0c63 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -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.