Subject: [PATCH v8 05/10] heapam: Add batch mode mvcc check and use it in page mode

From: Andres Freund <[email-masked]>
Date: Thu, 17 Oct 2024 13:16:36 -0400

There are two reasons for doing so:

1) It is generally faster to perform checks in a batched fashion and making
   sequential scans faster is nice.

2) We would like to stop setting hint bits while pages are being written
   out. The necessary locking becomes visible for page mode scans if done for
   every tuple. With batching the overhead can be amortized to only happen
   once per page.

There are substantial further optimization opportunities along these
lines:

- Right now HeapTupleSatisfiesMVCCBatch() simply uses the single-tuple
  HeapTupleSatisfiesMVCC(), relying on the compiler to inline it. We could
  instead write an explicitly optimized version that avoids repeated xid
  tests.

- Introduce batched version of the serializability test

- Introduce batched version of HeapTupleSatisfiesVacuum

Reviewed-by: Melanie Plageman <[email-masked]>
Discussion: https://postgr.es/m/6rgb2nvhyvnszz4ul3wfzlf5rheb2kkwrglthnna7qhe24onwr@vw27225tkyar
---
 src/include/access/heapam.h                 | 17 +++++
 src/backend/access/heap/heapam.c            | 84 ++++++++++++++++-----
 src/backend/access/heap/heapam_visibility.c | 42 +++++++++++
 src/tools/pgindent/typedefs.list            |  1 +
 4 files changed, 124 insertions(+), 20 deletions(-)

diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index f7e4ae3843c..cf4cd3e4dbd 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -449,6 +449,23 @@ extern bool HeapTupleHeaderIsOnlyLocked(HeapTupleHeader tuple);
 extern bool HeapTupleIsSurelyDead(HeapTuple htup,
 								  GlobalVisState *vistest);
 
+/*
+ * The output of HeapTupleSatisfiesMVCCBatch() is passed via this struct, as
+ * otherwise the increased number of arguments to
+ * HeapTupleSatisfiesMVCCBatch() leads to on-stack argument passing on x86-64,
+ * which causes a small regression.
+ */
+typedef struct BatchMVCCState
+{
+	HeapTupleData tuples[MaxHeapTuplesPerPage];
+	bool		visible[MaxHeapTuplesPerPage];
+} BatchMVCCState;
+
+extern int	HeapTupleSatisfiesMVCCBatch(Snapshot snapshot, Buffer buffer,
+										int ntups,
+										BatchMVCCState *batchmvcc,
+										OffsetNumber *vistuples_dense);
+
 /*
  * To avoid leaking too much knowledge about reorderbuffer implementation
  * details this is implemented in reorderbuffer.c not heapam_visibility.c
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 6daf4a87dec..513a9b275a2 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -519,42 +519,86 @@ page_collect_tuples(HeapScanDesc scan, Snapshot snapshot,
 					BlockNumber block, int lines,
 					bool all_visible, bool check_serializable)
 {
+	Oid			relid = RelationGetRelid(scan->rs_base.rs_rd);
 	int			ntup = 0;
-	OffsetNumber lineoff;
+	int			nvis = 0;
+	BatchMVCCState batchmvcc;
 
-	for (lineoff = FirstOffsetNumber; lineoff <= lines; lineoff++)
+	/* page at a time should have been disabled otherwise */
+	Assert(IsMVCCSnapshot(snapshot));
+
+	/* first find all tuples on the page */
+	for (OffsetNumber lineoff = FirstOffsetNumber; lineoff <= lines; lineoff++)
 	{
 		ItemId		lpp = PageGetItemId(page, lineoff);
-		HeapTupleData loctup;
-		bool		valid;
+		HeapTuple	tup;
 
-		if (!ItemIdIsNormal(lpp))
+		if (unlikely(!ItemIdIsNormal(lpp)))
 			continue;
 
-		loctup.t_data = (HeapTupleHeader) PageGetItem(page, lpp);
-		loctup.t_len = ItemIdGetLength(lpp);
-		loctup.t_tableOid = RelationGetRelid(scan->rs_base.rs_rd);
-		ItemPointerSet(&(loctup.t_self), block, lineoff);
+		/*
+		 * If the page is not all-visible or we need to check serializability,
+		 * maintain enough state to be able to refind the tuple efficiently,
+		 * without again first needing to fetch the item and then via that the
+		 * tuple.
+		 */
+		if (!all_visible || check_serializable)
+		{
+			tup = &batchmvcc.tuples[ntup];
 
+			tup->t_data = (HeapTupleHeader) PageGetItem(page, lpp);
+			tup->t_len = ItemIdGetLength(lpp);
+			tup->t_tableOid = relid;
+			ItemPointerSet(&(tup->t_self), block, lineoff);
+		}
+
+		/*
+		 * If the page is all visible, these fields otherwise won't be
+		 * populated in loop below.
+		 */
 		if (all_visible)
-			valid = true;
-		else
-			valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);
-
-		if (check_serializable)
-			HeapCheckForSerializableConflictOut(valid, scan->rs_base.rs_rd,
-												&loctup, buffer, snapshot);
-
-		if (valid)
 		{
+			if (check_serializable)
+			{
+				batchmvcc.visible[ntup] = true;
+			}
 			scan->rs_vistuples[ntup] = lineoff;
-			ntup++;
 		}
+
+		ntup++;
 	}
 
 	Assert(ntup <= MaxHeapTuplesPerPage);
 
-	return ntup;
+	/*
+	 * Unless the page is all visible, test visibility for all tuples one
+	 * go. That is considerably more efficient than calling
+	 * HeapTupleSatisfiesMVCC() one-by-one.
+	 */
+	if (all_visible)
+		nvis = ntup;
+	else
+		nvis = HeapTupleSatisfiesMVCCBatch(snapshot, buffer,
+										   ntup,
+										   &batchmvcc,
+										   scan->rs_vistuples);
+
+	/*
+	 * So far we don't have batch API for testing serializabilty, so do so
+	 * one-by-one.
+	 */
+	if (check_serializable)
+	{
+		for (int i = 0; i < ntup; i++)
+		{
+			HeapCheckForSerializableConflictOut(batchmvcc.visible[i],
+												scan->rs_base.rs_rd,
+												&batchmvcc.tuples[i],
+												buffer, snapshot);
+		}
+	}
+
+	return nvis;
 }
 
 /*
diff --git a/src/backend/access/heap/heapam_visibility.c b/src/backend/access/heap/heapam_visibility.c
index debf5d56b95..04284c4b2eb 100644
--- a/src/backend/access/heap/heapam_visibility.c
+++ b/src/backend/access/heap/heapam_visibility.c
@@ -1598,6 +1598,48 @@ HeapTupleSatisfiesHistoricMVCC(HeapTuple htup, Snapshot snapshot,
 		return true;
 }
 
+/*
+ * Perform HeaptupleSatisfiesMVCC() on each passed in tuple. This is more
+ * efficient than doing HeapTupleSatisfiesMVCC() one-by-one.
+ *
+ * To be checked tuples are passed via BatchMVCCState->tuples. Each tuple's
+ * visibility is stored in batchmvcc->visible[]. In addition,
+ * ->vistuples_dense is set to contain the offsets of visible tuples.
+ *
+ * The reason this is more efficient than HeapTupleSatisfiesMVCC() is that it
+ * avoids a cross-translation-unit function call for each tuple. In the future
+ * it will also allow more efficient setting of hint bits.
+ *
+ * Returns the number of visible tuples.
+ */
+int
+HeapTupleSatisfiesMVCCBatch(Snapshot snapshot, Buffer buffer,
+							int ntups,
+							BatchMVCCState *batchmvcc,
+							OffsetNumber *vistuples_dense)
+{
+	int			nvis = 0;
+
+	Assert(IsMVCCSnapshot(snapshot));
+
+	for (int i = 0; i < ntups; i++)
+	{
+		bool		valid;
+		HeapTuple	tup = &batchmvcc->tuples[i];
+
+		valid = HeapTupleSatisfiesMVCC(tup, snapshot, buffer);
+		batchmvcc->visible[i] = valid;
+
+		if (likely(valid))
+		{
+			vistuples_dense[nvis] = tup->t_self.ip_posid;
+			nvis++;
+		}
+	}
+
+	return nvis;
+}
+
 /*
  * HeapTupleSatisfiesVisibility
  *		True iff heap tuple satisfies a time qual.
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 04845d5e680..2ffdf364386 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -255,6 +255,7 @@ Barrier
 BaseBackupCmd
 BaseBackupTargetHandle
 BaseBackupTargetType
+BatchMVCCState
 BeginDirectModify_function
 BeginForeignInsert_function
 BeginForeignModify_function
-- 
2.48.1.76.g4e746b1a31.dirty

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