Re-add GUC track_wal_io_timing

This commit is a rework of 2421e9a51d20, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads.  Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths.  The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal.  pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled.  The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu

1 files changed, 8 insertions, 6 deletions

diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index e7362b52a37..ba11545a17f 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -83,20 +83,22 @@ pgstat_count_io_op(IOObject io_object, IOContext io_context, IOOp io_op,
 }
 
 /*
- * Initialize the internal timing for an IO operation.
+ * Initialize the internal timing for an IO operation, depending on an
+ * IO timing GUC.
  */
 instr_time
-pgstat_prepare_io_time(void)
+pgstat_prepare_io_time(bool track_io_guc)
 {
 	instr_time	io_start;
 
-	if (track_io_timing)
+	if (track_io_guc)
 		INSTR_TIME_SET_CURRENT(io_start);
 	else
 	{
 		/*
-		 * There is no need to set io_start when an IO timing GUC is disabled,
-		 * still initialize it to zero to avoid compiler warnings.
+		 * There is no need to set io_start when an IO timing GUC is disabled.
+		 * Initialize it to zero to avoid compiler warnings and to let
+		 * pgstat_count_io_op_time() know that timings should be ignored.
 		 */
 		INSTR_TIME_SET_ZERO(io_start);
 	}
@@ -119,7 +121,7 @@ void
 pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 						instr_time start_time, uint32 cnt, uint64 bytes)
 {
-	if (track_io_timing)
+	if (!INSTR_TIME_IS_ZERO(start_time))
 	{
 		instr_time	io_time;