Reduce page locking in GIN vacuum

GIN vacuum during cleaning posting tree can lock this whole tree for a long
time with by holding LockBufferForCleanup() on root. Patch changes it with
two ways: first, cleanup lock will be taken only if there is an empty page
(which should be deleted) and, second, it tries to lock only subtree, not the
whole posting tree.

Author: Andrey Borodin with minor editorization by me
Reviewed-by: Jeff Davis, me

https://commitfest.postgresql.org/13/896/

Author: feodor

src/backend/access/gin/README

@@ -314,10 +314,17 @@ deleted.
 The previous paragraph's reasoning only applies to searches, and only to
 posting trees. To protect from inserters following a downlink to a deleted
 page, vacuum simply locks out all concurrent insertions to the posting tree,
-by holding a super-exclusive lock on the posting tree root. Inserters hold a
-pin on the root page, but searches do not, so while new searches cannot begin
-while root page is locked, any already-in-progress scans can continue
-concurrently with vacuum. In the entry tree, we never delete pages.
+by holding a super-exclusive lock on the parent page of subtree with deletable
+pages. Inserters hold a pin on the root page, but searches do not, so while
+new searches cannot begin while root page is locked, any already-in-progress
+scans can continue concurrently with vacuum in corresponding subtree of
+posting tree. To exclude interference with readers vacuum takes exclusive
+locks in a depth-first scan in left-to-right order of page tuples. Leftmost
+page is never deleted. Thus before deleting any page we obtain exclusive
+lock on any left page, effectively excluding deadlock with any reader, despite
+taking parent lock before current and left lock after current. We take left
+lock not for a concurrency reasons, but rather in need to mark page dirty.
+In the entry tree, we never delete pages.
 
 (This is quite different from the mechanism the btree indexam uses to make
 page-deletions safe; it stamps the deleted pages with an XID and keeps the

src/backend/access/gin/ginbtree.c

@@ -31,7 +31,7 @@ static void ginFinishSplit(GinBtree btree, GinBtreeStack *stack,
 /*
  * Lock buffer by needed method for search.
  */
-static int
+int
 ginTraverseLock(Buffer buffer, bool searchMode)
 {
 	Page		page;

src/backend/access/gin/ginvacuum.c

@@ -109,75 +109,17 @@ xlogVacuumPage(Relation index, Buffer buffer)
 	PageSetLSN(page, recptr);
 }
 
-static bool
-ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno, bool isRoot, Buffer *rootBuffer)
-{
-	Buffer		buffer;
-	Page		page;
-	bool		hasVoidPage = FALSE;
-	MemoryContext oldCxt;
-
-	buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
-								RBM_NORMAL, gvs->strategy);
-	page = BufferGetPage(buffer);
-
-	/*
-	 * We should be sure that we don't concurrent with inserts, insert process
-	 * never release root page until end (but it can unlock it and lock
-	 * again). New scan can't start but previously started ones work
-	 * concurrently.
-	 */
-	if (isRoot)
-		LockBufferForCleanup(buffer);
-	else
-		LockBuffer(buffer, GIN_EXCLUSIVE);
-
-	Assert(GinPageIsData(page));
 
-	if (GinPageIsLeaf(page))
-	{
-		oldCxt = MemoryContextSwitchTo(gvs->tmpCxt);
-		ginVacuumPostingTreeLeaf(gvs->index, buffer, gvs);
-		MemoryContextSwitchTo(oldCxt);
-		MemoryContextReset(gvs->tmpCxt);
-
-		/* if root is a leaf page, we don't desire further processing */
-		if (!isRoot && !hasVoidPage && GinDataLeafPageIsEmpty(page))
-			hasVoidPage = TRUE;
-	}
-	else
-	{
-		OffsetNumber i;
-		bool		isChildHasVoid = FALSE;
-
-		for (i = FirstOffsetNumber; i <= GinPageGetOpaque(page)->maxoff; i++)
-		{
-			PostingItem *pitem = GinDataPageGetPostingItem(page, i);
-
-			if (ginVacuumPostingTreeLeaves(gvs, PostingItemGetBlockNumber(pitem), FALSE, NULL))
-				isChildHasVoid = TRUE;
-		}
-
-		if (isChildHasVoid)
-			hasVoidPage = TRUE;
-	}
+typedef struct DataPageDeleteStack
+{
+	struct DataPageDeleteStack *child;
+	struct DataPageDeleteStack *parent;
 
-	/*
-	 * if we have root and there are empty pages in tree, then we don't
-	 * release lock to go further processing and guarantee that tree is unused
-	 */
-	if (!(isRoot && hasVoidPage))
-	{
-		UnlockReleaseBuffer(buffer);
-	}
-	else
-	{
-		Assert(rootBuffer);
-		*rootBuffer = buffer;
-	}
+	BlockNumber blkno;			/* current block number */
+	BlockNumber leftBlkno;		/* rightest non-deleted page on left */
+	bool		isRoot;
+} DataPageDeleteStack;
 
-	return hasVoidPage;
-}
 
 /*
  * Delete a posting tree page.
@@ -194,8 +136,13 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	BlockNumber rightlink;
 
 	/*
-	 * Lock the pages in the same order as an insertion would, to avoid
-	 * deadlocks: left, then right, then parent.
+	 * This function MUST be called only if someone of parent pages hold
+	 * exclusive cleanup lock. This guarantees that no insertions currently
+	 * happen in this subtree. Caller also acquire Exclusive lock on deletable
+	 * page and is acquiring and releasing exclusive lock on left page before.
+	 * Left page was locked and released. Then parent and this page are locked.
+	 * We acquire left page lock here only to mark page dirty after changing
+	 * right pointer.
 	 */
 	lBuffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, leftBlkno,
 								 RBM_NORMAL, gvs->strategy);
@@ -205,10 +152,6 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 								 RBM_NORMAL, gvs->strategy);
 
 	LockBuffer(lBuffer, GIN_EXCLUSIVE);
-	LockBuffer(dBuffer, GIN_EXCLUSIVE);
-	if (!isParentRoot)			/* parent is already locked by
-								 * LockBufferForCleanup() */
-		LockBuffer(pBuffer, GIN_EXCLUSIVE);
 
 	START_CRIT_SECTION();
 
@@ -272,26 +215,15 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 		PageSetLSN(BufferGetPage(lBuffer), recptr);
 	}
 
-	if (!isParentRoot)
-		LockBuffer(pBuffer, GIN_UNLOCK);
 	ReleaseBuffer(pBuffer);
 	UnlockReleaseBuffer(lBuffer);
-	UnlockReleaseBuffer(dBuffer);
+	ReleaseBuffer(dBuffer);
 
 	END_CRIT_SECTION();
 
 	gvs->result->pages_deleted++;
 }
 
-typedef struct DataPageDeleteStack
-{
-	struct DataPageDeleteStack *child;
-	struct DataPageDeleteStack *parent;
-
-	BlockNumber blkno;			/* current block number */
-	BlockNumber leftBlkno;		/* rightest non-deleted page on left */
-	bool		isRoot;
-} DataPageDeleteStack;
 
 /*
  * scans posting tree and deletes empty pages
@@ -325,6 +257,10 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
 
 	buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
 								RBM_NORMAL, gvs->strategy);
+
+	if(!isRoot)
+		LockBuffer(buffer, GIN_EXCLUSIVE);
+
 	page = BufferGetPage(buffer);
 
 	Assert(GinPageIsData(page));
@@ -359,6 +295,9 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
 		}
 	}
 
+	if(!isRoot)
+			LockBuffer(buffer, GIN_UNLOCK);
+
 	ReleaseBuffer(buffer);
 
 	if (!meDelete)
@@ -367,37 +306,124 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
 	return meDelete;
 }
 
-static void
-ginVacuumPostingTree(GinVacuumState *gvs, BlockNumber rootBlkno)
+
+/*
+ * Scan through posting tree, delete empty tuples from leaf pages.
+ * Also, this function collects empty subtrees (with all empty leafs).
+ * For parents of these subtrees CleanUp lock is taken, then we call
+ * ScanToDelete. This is done for every inner page, which points to
+ * empty subtree.
+ */
+static bool
+ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno, bool isRoot)
 {
-	Buffer		rootBuffer = InvalidBuffer;
-	DataPageDeleteStack root,
-			   *ptr,
-			   *tmp;
+	Buffer		buffer;
+	Page		page;
+	bool		hasVoidPage = FALSE;
+	MemoryContext oldCxt;
+
+	buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
+								RBM_NORMAL, gvs->strategy);
+	page = BufferGetPage(buffer);
+
+	ginTraverseLock(buffer,false);
 
-	if (ginVacuumPostingTreeLeaves(gvs, rootBlkno, TRUE, &rootBuffer) == FALSE)
+	Assert(GinPageIsData(page));
+
+	if (GinPageIsLeaf(page))
 	{
-		Assert(rootBuffer == InvalidBuffer);
-		return;
+		oldCxt = MemoryContextSwitchTo(gvs->tmpCxt);
+		ginVacuumPostingTreeLeaf(gvs->index, buffer, gvs);
+		MemoryContextSwitchTo(oldCxt);
+		MemoryContextReset(gvs->tmpCxt);
+
+		/* if root is a leaf page, we don't desire further processing */
+		if (GinDataLeafPageIsEmpty(page))
+			hasVoidPage = TRUE;
+
+		UnlockReleaseBuffer(buffer);
+
+		return hasVoidPage;
 	}
+	else
+	{
+		OffsetNumber 	i;
+		bool			hasEmptyChild = FALSE;
+		bool			hasNonEmptyChild = FALSE;
+		OffsetNumber	maxoff = GinPageGetOpaque(page)->maxoff;
+		BlockNumber*	children = palloc(sizeof(BlockNumber) * (maxoff + 1));
+
+		/*
+		 * Read all children BlockNumbers.
+		 * Not sure it is safe if there are many concurrent vacuums.
+		 */
 
-	memset(&root, 0, sizeof(DataPageDeleteStack));
-	root.leftBlkno = InvalidBlockNumber;
-	root.isRoot = TRUE;
+		for (i = FirstOffsetNumber; i <= maxoff; i++)
+		{
+			PostingItem *pitem = GinDataPageGetPostingItem(page, i);
 
-	vacuum_delay_point();
+			children[i] = PostingItemGetBlockNumber(pitem);
+		}
 
-	ginScanToDelete(gvs, rootBlkno, TRUE, &root, InvalidOffsetNumber);
+		UnlockReleaseBuffer(buffer);
 
-	ptr = root.child;
-	while (ptr)
-	{
-		tmp = ptr->child;
-		pfree(ptr);
-		ptr = tmp;
+		for (i = FirstOffsetNumber; i <= maxoff; i++)
+		{
+			if (ginVacuumPostingTreeLeaves(gvs, children[i], FALSE))
+				hasEmptyChild = TRUE;
+			else
+				hasNonEmptyChild = TRUE;
+		}
+
+		pfree(children);
+
+		vacuum_delay_point();
+
+		/*
+		 * All subtree is empty - just return TRUE to indicate that parent must
+		 * do a cleanup. Unless we are ROOT an there is way to go upper.
+		 */
+
+		if(hasEmptyChild && !hasNonEmptyChild && !isRoot)
+			return TRUE;
+
+		if(hasEmptyChild)
+		{
+			DataPageDeleteStack root,
+					   *ptr,
+					   *tmp;
+
+			buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
+											RBM_NORMAL, gvs->strategy);
+			LockBufferForCleanup(buffer);
+
+			memset(&root, 0, sizeof(DataPageDeleteStack));
+				root.leftBlkno = InvalidBlockNumber;
+				root.isRoot = TRUE;
+
+			ginScanToDelete(gvs, blkno, TRUE, &root, InvalidOffsetNumber);
+
+			ptr = root.child;
+
+			while (ptr)
+			{
+				tmp = ptr->child;
+				pfree(ptr);
+				ptr = tmp;
+			}
+
+			UnlockReleaseBuffer(buffer);
+		}
+
+		/* Here we have deleted all empty subtrees */
+		return FALSE;
 	}
+}
 
-	UnlockReleaseBuffer(rootBuffer);
+static void
+ginVacuumPostingTree(GinVacuumState *gvs, BlockNumber rootBlkno)
+{
+	ginVacuumPostingTreeLeaves(gvs, rootBlkno, TRUE);
 }
 
 /*

src/include/access/gin_private.h

@@ -471,4 +471,6 @@ ginCompareItemPointers(ItemPointer a, ItemPointer b)
 		return -1;
 }
 
+extern int ginTraverseLock(Buffer buffer, bool searchMode);
+
 #endif   /* GIN_PRIVATE_H */