Delete empty pages in each pass during GIST VACUUM.

Amit Kapila · Amit Kapila · commit 4e514c6180fb · 2020-01-13T07:59:44.000+05:30
Earlier, we use to postpone deleting empty pages till the second stage of vacuum to amortize the cost of scanning internal pages. However, that can sometimes (say vacuum is canceled or errored between first and second stage) delay the pages to be recycled. Another thing is that to facilitate deleting empty pages in the second stage, we need to share the information about internal and empty pages between different stages of vacuum. It will be quite tricky to share this information via DSM which is required for the upcoming parallel vacuum patch. Also, it will bring the logic to reclaim deleted pages closer to nbtree where we delete empty pages in each pass. Overall, the advantages of deleting empty pages in each pass outweigh the advantages of postponing the same. Author: Dilip Kumar, with changes by Amit Kapila Reviewed-by: Sawada Masahiko and Amit Kapila Discussion: https://postgr.es/m/CAA4eK1LGr+MN0xHZpJ2dfS8QNQ1a_aROKowZB+MPNep8FVtwAA@mail.gmail.com
diff --git a/src/backend/access/gist/README b/src/backend/access/gist/README
@@ -429,18 +429,17 @@ splits during searches, we don't need a "vacuum cycle ID" concept for that
 like B-tree does.
 
 While we scan all the pages, we also make note of any completely empty leaf
-pages. We will try to unlink them from the tree in the second stage. We also
-record the block numbers of all internal pages; they are needed in the second
-stage, to locate parents of the empty pages.
-
-In the second stage, we try to unlink any empty leaf pages from the tree, so
-that their space can be reused. In order to delete an empty page, its
-downlink must be removed from the parent. We scan all the internal pages,
-whose block numbers we memorized in the first stage, and look for downlinks
-to pages that we have memorized as being empty. Whenever we find one, we
-acquire a lock on the parent and child page, re-check that the child page is
-still empty. Then, we remove the downlink and mark the child as deleted, and
-release the locks.
+pages. We will try to unlink them from the tree after the scan. We also record
+the block numbers of all internal pages; they are needed to locate parents of
+the empty pages while unlinking them.
+
+We try to unlink any empty leaf pages from the tree, so that their space can
+be reused. In order to delete an empty page, its downlink must be removed from
+the parent. We scan all the internal pages, whose block numbers we memorized
+in the first stage, and look for downlinks to pages that we have memorized as
+being empty. Whenever we find one, we acquire a lock on the parent and child
+page, re-check that the child page is still empty. Then, we remove the
+downlink and mark the child as deleted, and release the locks.
 
 The insertion algorithm would get confused, if an internal page was completely
 empty. So we never delete the last child of an internal page, even if it's
diff --git a/src/backend/access/gist/gistvacuum.c b/src/backend/access/gist/gistvacuum.c
@@ -24,74 +24,48 @@
 #include "storage/lmgr.h"
 #include "utils/memutils.h"
 
-/*
- * State kept across vacuum stages.
- */
+/* Working state needed by gistbulkdelete */
 typedef struct
 {
-	IndexBulkDeleteResult stats;	/* must be first */
+	IndexVacuumInfo *info;
+	IndexBulkDeleteResult *stats;
+	IndexBulkDeleteCallback callback;
+	void	   *callback_state;
+	GistNSN		startNSN;
 
 	/*
-	 * These are used to memorize all internal and empty leaf pages in the 1st
-	 * vacuum stage.  They are used in the 2nd stage, to delete all the empty
-	 * pages.
+	 * These are used to memorize all internal and empty leaf pages.  They are
+	 * used for deleting all the empty pages.
 	 */
 	IntegerSet *internal_page_set;
 	IntegerSet *empty_leaf_set;
 	MemoryContext page_set_context;
-} GistBulkDeleteResult;
-
-/* Working state needed by gistbulkdelete */
-typedef struct
-{
-	IndexVacuumInfo *info;
-	GistBulkDeleteResult *stats;
-	IndexBulkDeleteCallback callback;
-	void	   *callback_state;
-	GistNSN		startNSN;
 } GistVacState;
 
-static void gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
+static void gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 						   IndexBulkDeleteCallback callback, void *callback_state);
 static void gistvacuumpage(GistVacState *vstate, BlockNumber blkno,
 						   BlockNumber orig_blkno);
 static void gistvacuum_delete_empty_pages(IndexVacuumInfo *info,
-										  GistBulkDeleteResult *stats);
-static bool gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
+										  GistVacState *vstate);
+static bool gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 						   Buffer buffer, OffsetNumber downlink,
 						   Buffer leafBuffer);
 
-/* allocate the 'stats' struct that's kept over vacuum stages */
-static GistBulkDeleteResult *
-create_GistBulkDeleteResult(void)
-{
-	GistBulkDeleteResult *gist_stats;
-
-	gist_stats = (GistBulkDeleteResult *) palloc0(sizeof(GistBulkDeleteResult));
-	gist_stats->page_set_context =
-		GenerationContextCreate(CurrentMemoryContext,
-								"GiST VACUUM page set context",
-								16 * 1024);
-
-	return gist_stats;
-}
-
 /*
  * VACUUM bulkdelete stage: remove index entries.
  */
 IndexBulkDeleteResult *
 gistbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 			   IndexBulkDeleteCallback callback, void *callback_state)
 {
-	GistBulkDeleteResult *gist_stats = (GistBulkDeleteResult *) stats;
-
 	/* allocate stats if first time through, else re-use existing struct */
-	if (gist_stats == NULL)
-		gist_stats = create_GistBulkDeleteResult();
+	if (stats == NULL)
+		stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
 
-	gistvacuumscan(info, gist_stats, callback, callback_state);
+	gistvacuumscan(info, stats, callback, callback_state);
 
-	return (IndexBulkDeleteResult *) gist_stats;
+	return stats;
 }
 
 /*
@@ -100,8 +74,6 @@ gistbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 IndexBulkDeleteResult *
 gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 {
-	GistBulkDeleteResult *gist_stats = (GistBulkDeleteResult *) stats;
-
 	/* No-op in ANALYZE ONLY mode */
 	if (info->analyze_only)
 		return stats;
@@ -111,24 +83,12 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	 * stats from the latest gistbulkdelete call.  If it wasn't called, we
 	 * still need to do a pass over the index, to obtain index statistics.
 	 */
-	if (gist_stats == NULL)
+	if (stats == NULL)
 	{
-		gist_stats = create_GistBulkDeleteResult();
-		gistvacuumscan(info, gist_stats, NULL, NULL);
+		stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
+		gistvacuumscan(info, stats, NULL, NULL);
 	}
 
-	/*
-	 * If we saw any empty pages, try to unlink them from the tree so that
-	 * they can be reused.
-	 */
-	gistvacuum_delete_empty_pages(info, gist_stats);
-
-	/* we don't need the internal and empty page sets anymore */
-	MemoryContextDelete(gist_stats->page_set_context);
-	gist_stats->page_set_context = NULL;
-	gist_stats->internal_page_set = NULL;
-	gist_stats->empty_leaf_set = NULL;
-
 	/*
 	 * It's quite possible for us to be fooled by concurrent page splits into
 	 * double-counting some index tuples, so disbelieve any total that exceeds
@@ -137,11 +97,11 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	 */
 	if (!info->estimated_count)
 	{
-		if (gist_stats->stats.num_index_tuples > info->num_heap_tuples)
-			gist_stats->stats.num_index_tuples = info->num_heap_tuples;
+		if (stats->num_index_tuples > info->num_heap_tuples)
+			stats->num_index_tuples = info->num_heap_tuples;
 	}
 
-	return (IndexBulkDeleteResult *) gist_stats;
+	return stats;
 }
 
 /*
@@ -153,15 +113,16 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
  * occurred).
  *
  * This also makes note of any empty leaf pages, as well as all internal
- * pages.  The second stage, gistvacuum_delete_empty_pages(), needs that
- * information.  Any deleted pages are added directly to the free space map.
- * (They should've been added there when they were originally deleted, already,
- * but it's possible that the FSM was lost at a crash, for example.)
+ * pages while looping over all index pages.  After scanning all the pages, we
+ * remove the empty pages so that they can be reused.  Any deleted pages are
+ * added directly to the free space map.  (They should've been added there
+ * when they were originally deleted, already, but it's possible that the FSM
+ * was lost at a crash, for example.)
  *
  * The caller is responsible for initially allocating/zeroing a stats struct.
  */
 static void
-gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
+gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 			   IndexBulkDeleteCallback callback, void *callback_state)
 {
 	Relation	rel = info->index;
@@ -175,21 +136,23 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
 	 * Reset counts that will be incremented during the scan; needed in case
 	 * of multiple scans during a single VACUUM command.
 	 */
-	stats->stats.estimated_count = false;
-	stats->stats.num_index_tuples = 0;
-	stats->stats.pages_deleted = 0;
-	stats->stats.pages_free = 0;
-	MemoryContextReset(stats->page_set_context);
+	stats->estimated_count = false;
+	stats->num_index_tuples = 0;
+	stats->pages_deleted = 0;
+	stats->pages_free = 0;
 
 	/*
 	 * Create the integer sets to remember all the internal and the empty leaf
 	 * pages in page_set_context.  Internally, the integer set will remember
 	 * this context so that the subsequent allocations for these integer sets
 	 * will be done from the same context.
 	 */
-	oldctx = MemoryContextSwitchTo(stats->page_set_context);
-	stats->internal_page_set = intset_create();
-	stats->empty_leaf_set = intset_create();
+	vstate.page_set_context = GenerationContextCreate(CurrentMemoryContext,
+													  "GiST VACUUM page set context",
+													  16 * 1024);
+	oldctx = MemoryContextSwitchTo(vstate.page_set_context);
+	vstate.internal_page_set = intset_create();
+	vstate.empty_leaf_set = intset_create();
 	MemoryContextSwitchTo(oldctx);
 
 	/* Set up info to pass down to gistvacuumpage */
@@ -257,11 +220,23 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
 	 * Note that if no recyclable pages exist, we don't bother vacuuming the
 	 * FSM at all.
 	 */
-	if (stats->stats.pages_free > 0)
+	if (stats->pages_free > 0)
 		IndexFreeSpaceMapVacuum(rel);
 
 	/* update statistics */
-	stats->stats.num_pages = num_pages;
+	stats->num_pages = num_pages;
+
+	/*
+	 * If we saw any empty pages, try to unlink them from the tree so that
+	 * they can be reused.
+	 */
+	gistvacuum_delete_empty_pages(info, &vstate);
+
+	/* we don't need the internal and empty page sets anymore */
+	MemoryContextDelete(vstate.page_set_context);
+	vstate.page_set_context = NULL;
+	vstate.internal_page_set = NULL;
+	vstate.empty_leaf_set = NULL;
 }
 
 /*
@@ -278,7 +253,6 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
 static void
 gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 {
-	GistBulkDeleteResult *stats = vstate->stats;
 	IndexVacuumInfo *info = vstate->info;
 	IndexBulkDeleteCallback callback = vstate->callback;
 	void	   *callback_state = vstate->callback_state;
@@ -307,13 +281,13 @@ gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 	{
 		/* Okay to recycle this page */
 		RecordFreeIndexPage(rel, blkno);
-		stats->stats.pages_free++;
-		stats->stats.pages_deleted++;
+		vstate->stats->pages_free++;
+		vstate->stats->pages_deleted++;
 	}
 	else if (GistPageIsDeleted(page))
 	{
 		/* Already deleted, but can't recycle yet */
-		stats->stats.pages_deleted++;
+		vstate->stats->pages_deleted++;
 	}
 	else if (GistPageIsLeaf(page))
 	{
@@ -388,7 +362,7 @@ gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 
 			END_CRIT_SECTION();
 
-			stats->stats.tuples_removed += ntodelete;
+			vstate->stats->tuples_removed += ntodelete;
 			/* must recompute maxoff */
 			maxoff = PageGetMaxOffsetNumber(page);
 		}
@@ -405,10 +379,10 @@ gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 			 * it up.
 			 */
 			if (blkno == orig_blkno)
-				intset_add_member(stats->empty_leaf_set, blkno);
+				intset_add_member(vstate->empty_leaf_set, blkno);
 		}
 		else
-			stats->stats.num_index_tuples += nremain;
+			vstate->stats->num_index_tuples += nremain;
 	}
 	else
 	{
@@ -443,7 +417,7 @@ gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 		 * parents of empty leaf pages.
 		 */
 		if (blkno == orig_blkno)
-			intset_add_member(stats->internal_page_set, blkno);
+			intset_add_member(vstate->internal_page_set, blkno);
 	}
 
 	UnlockReleaseBuffer(buffer);
@@ -466,7 +440,7 @@ gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
  * Scan all internal pages, and try to delete their empty child pages.
  */
 static void
-gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats)
+gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistVacState *vstate)
 {
 	Relation	rel = info->index;
 	BlockNumber empty_pages_remaining;
@@ -475,10 +449,10 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
 	/*
 	 * Rescan all inner pages to find those that have empty child pages.
 	 */
-	empty_pages_remaining = intset_num_entries(stats->empty_leaf_set);
-	intset_begin_iterate(stats->internal_page_set);
+	empty_pages_remaining = intset_num_entries(vstate->empty_leaf_set);
+	intset_begin_iterate(vstate->internal_page_set);
 	while (empty_pages_remaining > 0 &&
-		   intset_iterate_next(stats->internal_page_set, &blkno))
+		   intset_iterate_next(vstate->internal_page_set, &blkno))
 	{
 		Buffer		buffer;
 		Page		page;
@@ -521,7 +495,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
 			BlockNumber leafblk;
 
 			leafblk = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
-			if (intset_is_member(stats->empty_leaf_set, leafblk))
+			if (intset_is_member(vstate->empty_leaf_set, leafblk))
 			{
 				leafs_to_delete[ntodelete] = leafblk;
 				todelete[ntodelete++] = off;
@@ -561,7 +535,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
 			gistcheckpage(rel, leafbuf);
 
 			LockBuffer(buffer, GIST_EXCLUSIVE);
-			if (gistdeletepage(info, stats,
+			if (gistdeletepage(info, vstate->stats,
 							   buffer, todelete[i] - deleted,
 							   leafbuf))
 				deleted++;
@@ -573,7 +547,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
 		ReleaseBuffer(buffer);
 
 		/* update stats */
-		stats->stats.pages_removed += deleted;
+		vstate->stats->pages_removed += deleted;
 
 		/*
 		 * We can stop the scan as soon as we have seen the downlinks, even if
@@ -596,7 +570,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
  * prevented it.
  */
 static bool
-gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
+gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 			   Buffer parentBuffer, OffsetNumber downlink,
 			   Buffer leafBuffer)
 {
@@ -665,7 +639,7 @@ gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
 	/* mark the page as deleted */
 	MarkBufferDirty(leafBuffer);
 	GistPageSetDeleted(leafPage, txid);
-	stats->stats.pages_deleted++;
+	stats->pages_deleted++;
 
 	/* remove the downlink from the parent */
 	MarkBufferDirty(parentBuffer);
