pg_stat_wal: Accumulate time as instr_time instead of microseconds

In instr_time.h it is stated that:

* When summing multiple measurements, it's recommended to leave the
* running sum in instr_time form (ie, use INSTR_TIME_ADD or
* INSTR_TIME_ACCUM_DIFF) and convert to a result format only at the end.

The reason for that is that converting to microseconds is not cheap, and can
loose precision.  Therefore this commit changes 'PendingWalStats' to use
'instr_time' instead of 'PgStat_Counter' while accumulating 'wal_write_time'
and 'wal_sync_time'.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/1feedb83-7aa9-cb4b-5086-598349d3f555@gmail.com

src/backend/access/transam/xlog.c

@@ -2206,8 +2206,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 					instr_time	duration;
 
 					INSTR_TIME_SET_CURRENT(duration);
-					INSTR_TIME_SUBTRACT(duration, start);
+					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, duration, start);
-					PendingWalStats.wal_write_time += INSTR_TIME_GET_MICROSEC(duration);
 				}
 
 				PendingWalStats.wal_write++;
@@ -8204,8 +8203,7 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 		instr_time	duration;
 
 		INSTR_TIME_SET_CURRENT(duration);
-		INSTR_TIME_SUBTRACT(duration, start);
+		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, duration, start);
-		PendingWalStats.wal_sync_time += INSTR_TIME_GET_MICROSEC(duration);
 	}
 
 	PendingWalStats.wal_sync++;

src/backend/storage/file/buffile.c

@@ -469,8 +469,7 @@ BufFileLoadBuffer(BufFile *file)
 	if (track_io_timing)
 	{
 		INSTR_TIME_SET_CURRENT(io_time);
-		INSTR_TIME_SUBTRACT(io_time, io_start);
+		INSTR_TIME_ACCUM_DIFF(pgBufferUsage.temp_blk_read_time, io_time, io_start);
-		INSTR_TIME_ADD(pgBufferUsage.temp_blk_read_time, io_time);
 	}
 
 	/* we choose not to advance curOffset here */
@@ -544,8 +543,7 @@ BufFileDumpBuffer(BufFile *file)
 		if (track_io_timing)
 		{
 			INSTR_TIME_SET_CURRENT(io_time);
-			INSTR_TIME_SUBTRACT(io_time, io_start);
+			INSTR_TIME_ACCUM_DIFF(pgBufferUsage.temp_blk_write_time, io_time, io_start);
-			INSTR_TIME_ADD(pgBufferUsage.temp_blk_write_time, io_time);
 		}
 
 		file->curOffset += bytestowrite;

src/backend/utils/activity/pgstat_wal.c

@@ -21,7 +21,7 @@
 #include "executor/instrument.h"
 
 
-PgStat_WalStats PendingWalStats = {0};
+PgStat_PendingWalStats PendingWalStats = {0};
 
 /*
  * WAL usage counters saved from pgWALUsage at the previous call to
@@ -70,7 +70,7 @@ bool
 pgstat_flush_wal(bool nowait)
 {
 	PgStatShared_Wal *stats_shmem = &pgStatLocal.shmem->wal;
-	WalUsage	diff = {0};
+	WalUsage	wal_usage_diff = {0};
 
 	Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
 	Assert(pgStatLocal.shmem != NULL &&
@@ -88,25 +88,26 @@ pgstat_flush_wal(bool nowait)
 	 * Calculate how much WAL usage counters were increased by subtracting the
 	 * previous counters from the current ones.
 	 */
-	WalUsageAccumDiff(&diff, &pgWalUsage, &prevWalUsage);
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevWalUsage);
-	PendingWalStats.wal_records = diff.wal_records;
-	PendingWalStats.wal_fpi = diff.wal_fpi;
-	PendingWalStats.wal_bytes = diff.wal_bytes;
 
 	if (!nowait)
 		LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
 	else if (!LWLockConditionalAcquire(&stats_shmem->lock, LW_EXCLUSIVE))
 		return true;
 
-#define WALSTAT_ACC(fld) stats_shmem->stats.fld += PendingWalStats.fld
+#define WALSTAT_ACC(fld, var_to_add) \
-	WALSTAT_ACC(wal_records);
+	(stats_shmem->stats.fld += var_to_add.fld)
-	WALSTAT_ACC(wal_fpi);
+#define WALSTAT_ACC_INSTR_TIME(fld) \
-	WALSTAT_ACC(wal_bytes);
+	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
-	WALSTAT_ACC(wal_buffers_full);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
-	WALSTAT_ACC(wal_write);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
-	WALSTAT_ACC(wal_sync);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
-	WALSTAT_ACC(wal_write_time);
+	WALSTAT_ACC(wal_buffers_full, PendingWalStats);
-	WALSTAT_ACC(wal_sync_time);
+	WALSTAT_ACC(wal_write, PendingWalStats);
+	WALSTAT_ACC(wal_sync, PendingWalStats);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
 #undef WALSTAT_ACC
 
 	LWLockRelease(&stats_shmem->lock);

src/include/pgstat.h

@@ -435,6 +435,21 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+/*
+ * This struct stores wal-related durations as instr_time, which makes it
+ * cheaper and easier to accumulate them, by not requiring type
+ * conversions. During stats flush instr_time will be converted into
+ * microseconds.
+ */
+typedef struct PgStat_PendingWalStats
+{
+	PgStat_Counter wal_buffers_full;
+	PgStat_Counter wal_write;
+	PgStat_Counter wal_sync;
+	instr_time	wal_write_time;
+	instr_time	wal_sync_time;
+} PgStat_PendingWalStats;
+
 
 /*
  * Functions in pgstat.c
@@ -748,7 +763,7 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
  */
 
 /* updated directly by backends and background processes */
-extern PGDLLIMPORT PgStat_WalStats PendingWalStats;
+extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
 
 
 #endif							/* PGSTAT_H */

src/tools/pgindent/typedefs.list

@@ -2053,6 +2053,7 @@ PgStat_Kind
 PgStat_KindInfo
 PgStat_LocalState
 PgStat_PendingDroppedStatsItem
+PgStat_PendingWalStats
 PgStat_SLRUStats
 PgStat_ShmemControl
 PgStat_Snapshot