Allow parallel create index to accumulate buffer usage stats.

Currently, we don't account for buffer usage incurred by parallel workers
for parallel create index.  This commit allows each worker to record the
buffer usage stats and leader backend to accumulate that stats at the
end of the operation.  This will allow pg_stat_statements to display
correct buffer usage stats for (parallel) create index command.

Reported-by: Julien Rouhaud
Author: Sawada Masahiko
Reviewed-by: Dilip Kumar, Julien Rouhaud and Amit Kapila
Backpatch-through: 11, where this was introduced
Discussion: https://postgr.es/m/20200328151721.GB12854@nol

Author: Amit Kapila

src/backend/access/nbtree/nbtsort.c

@@ -71,6 +71,7 @@
 #define PARALLEL_KEY_TUPLESORT_SPOOL2	UINT64CONST(0xA000000000000003)
 #define PARALLEL_KEY_QUERY_TEXT			UINT64CONST(0xA000000000000004)
 #define PARALLEL_KEY_WAL_USAGE			UINT64CONST(0xA000000000000005)
+#define PARALLEL_KEY_BUFFER_USAGE		UINT64CONST(0xA000000000000006)
 
 /*
  * DISABLE_LEADER_PARTICIPATION disables the leader's participation in
@@ -194,6 +195,7 @@ typedef struct BTLeader
 	Sharedsort *sharedsort2;
 	Snapshot	snapshot;
 	WalUsage   *walusage;
+	BufferUsage *bufferusage;
 } BTLeader;
 
 /*
@@ -1457,6 +1459,7 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	BTSpool    *btspool = buildstate->spool;
 	BTLeader   *btleader = (BTLeader *) palloc0(sizeof(BTLeader));
 	WalUsage   *walusage;
+	BufferUsage *bufferusage;
 	bool		leaderparticipates = true;
 	char	   *sharedquery;
 	int			querylen;
@@ -1510,16 +1513,19 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	}
 
 	/*
-	 * Estimate space for WalUsage -- PARALLEL_KEY_WAL_USAGE
+	 * Estimate space for WalUsage and BufferUsage -- PARALLEL_KEY_WAL_USAGE
+	 * and PARALLEL_KEY_BUFFER_USAGE.
 	 *
-	 * WalUsage during execution of maintenance command can be used by an
-	 * extension that reports the WAL usage, such as pg_stat_statements. We
-	 * have no way of knowing whether anyone's looking at pgWalUsage, so do it
-	 * unconditionally.
+	 * If there are no extensions loaded that care, we could skip this.  We
+	 * have no way of knowing whether anyone's looking at pgWalUsage or
+	 * pgBufferUsage, so do it unconditionally.
 	 */
 	shm_toc_estimate_chunk(&pcxt->estimator,
 						   mul_size(sizeof(WalUsage), pcxt->nworkers));
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
+	shm_toc_estimate_chunk(&pcxt->estimator,
+						   mul_size(sizeof(BufferUsage), pcxt->nworkers));
+	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Finally, estimate PARALLEL_KEY_QUERY_TEXT space */
 	querylen = strlen(debug_query_string);
@@ -1592,10 +1598,16 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	memcpy(sharedquery, debug_query_string, querylen + 1);
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, sharedquery);
 
-	/* Allocate space for each worker's WalUsage; no need to initialize */
+	/*
+	 * Allocate space for each worker's WalUsage and BufferUsage; no need to
+	 * initialize.
+	 */
 	walusage = shm_toc_allocate(pcxt->toc,
 								mul_size(sizeof(WalUsage), pcxt->nworkers));
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_WAL_USAGE, walusage);
+	bufferusage = shm_toc_allocate(pcxt->toc,
+								   mul_size(sizeof(BufferUsage), pcxt->nworkers));
+	shm_toc_insert(pcxt->toc, PARALLEL_KEY_BUFFER_USAGE, bufferusage);
 
 	/* Launch workers, saving status for leader/caller */
 	LaunchParallelWorkers(pcxt);
@@ -1608,6 +1620,7 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	btleader->sharedsort2 = sharedsort2;
 	btleader->snapshot = snapshot;
 	btleader->walusage = walusage;
+	btleader->bufferusage = bufferusage;
 
 	/* If no workers were successfully launched, back out (do serial build) */
 	if (pcxt->nworkers_launched == 0)
@@ -1646,7 +1659,7 @@ _bt_end_parallel(BTLeader *btleader)
 	 * or we might get incomplete data.)
 	 */
 	for (i = 0; i < btleader->pcxt->nworkers_launched; i++)
-		InstrAccumParallelQuery(NULL, &btleader->walusage[i]);
+		InstrAccumParallelQuery(&btleader->bufferusage[i], &btleader->walusage[i]);
 
 	/* Free last reference to MVCC snapshot, if one was used */
 	if (IsMVCCSnapshot(btleader->snapshot))
@@ -1779,6 +1792,7 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 	LOCKMODE	heapLockmode;
 	LOCKMODE	indexLockmode;
 	WalUsage   *walusage;
+	BufferUsage *bufferusage;
 	int			sortmem;
 
 #ifdef BTREE_BUILD_STATS
@@ -1848,9 +1862,11 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 	_bt_parallel_scan_and_sort(btspool, btspool2, btshared, sharedsort,
 							   sharedsort2, sortmem, false);
 
-	/* Report WAL usage during parallel execution */
+	/* Report WAL/buffer usage during parallel execution */
+	bufferusage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE, false);
 	walusage = shm_toc_lookup(toc, PARALLEL_KEY_WAL_USAGE, false);
-	InstrEndParallelQuery(NULL, &walusage[ParallelWorkerNumber]);
+	InstrEndParallelQuery(&bufferusage[ParallelWorkerNumber],
+						  &walusage[ParallelWorkerNumber]);
 
 #ifdef BTREE_BUILD_STATS
 	if (log_btree_build_stats)

src/backend/executor/instrument.c

@@ -188,11 +188,8 @@ InstrStartParallelQuery(void)
 void
 InstrEndParallelQuery(BufferUsage *bufusage, WalUsage *walusage)
 {
-	if (bufusage)
-	{
-		memset(bufusage, 0, sizeof(BufferUsage));
-		BufferUsageAccumDiff(bufusage, &pgBufferUsage, &save_pgBufferUsage);
-	}
+	memset(bufusage, 0, sizeof(BufferUsage));
+	BufferUsageAccumDiff(bufusage, &pgBufferUsage, &save_pgBufferUsage);
 	memset(walusage, 0, sizeof(WalUsage));
 	WalUsageAccumDiff(walusage, &pgWalUsage, &save_pgWalUsage);
 }
@@ -201,8 +198,7 @@ InstrEndParallelQuery(BufferUsage *bufusage, WalUsage *walusage)
 void
 InstrAccumParallelQuery(BufferUsage *bufusage, WalUsage *walusage)
 {
-	if (bufusage)
-		BufferUsageAdd(&pgBufferUsage, bufusage);
+	BufferUsageAdd(&pgBufferUsage, bufusage);
 	WalUsageAdd(&pgWalUsage, walusage);
 }