Avoid creation of the free space map for small heap relations.

Previously, all heaps had FSMs. For very small tables, this means that the
FSM took up more space than the heap did. This is wasteful, so now we
refrain from creating the FSM for heaps with 4 pages or fewer. If the last
known target block has insufficient space, we still try to insert into some
other page before giving up and extending the relation, since doing
otherwise leads to table bloat. Testing showed that trying every page
penalized performance slightly, so we compromise and try every other page.
This way, we visit at most two pages. Any pages with wasted free space
become visible at next relation extension, so we still control table bloat.
As a bonus, directly attempting one or two pages can even be faster than
consulting the FSM would have been.

Once the FSM is created for a heap we don't remove it even if somebody
deletes all the rows from the corresponding relation.  We don't think it is
a useful optimization as it is quite likely that relation will again grow
to the same size.

Author: John Naylor with design inputs and some code contribution by Amit Kapila
Reviewed-by: Amit Kapila
Tested-by: Mithun C Y
Discussion: https://www.postgresql.org/message-id/CAJVSVGWvB13PzpbLEecFuGFc5V2fsO736BsdTakPiPAcdMM5tQ@mail.gmail.com

1 files changed, 269 insertions, 6 deletions

diff --git a/src/backend/storage/freespace/freespace.c b/src/backend/storage/freespace/freespace.c
index eee82860575..5f46391b681 100644
--- a/src/backend/storage/freespace/freespace.c
+++ b/src/backend/storage/freespace/freespace.c
@@ -76,6 +76,14 @@
 #define FSM_ROOT_LEVEL	(FSM_TREE_DEPTH - 1)
 #define FSM_BOTTOM_LEVEL 0
 
+/* Status codes for the local map. */
+
+/* Either already tried, or beyond the end of the relation */
+#define FSM_LOCAL_NOT_AVAIL 0x00
+
+/* Available to try */
+#define FSM_LOCAL_AVAIL		0x01
+
 /*
  * The internal FSM routines work on a logical addressing scheme. Each
  * level of the tree can be thought of as a separately addressable file.
@@ -89,6 +97,17 @@ typedef struct
 /* Address of the root page. */
 static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};
 
+/* Local map of block numbers for small heaps with no FSM. */
+typedef struct
+{
+	BlockNumber nblocks;
+	uint8		map[HEAP_FSM_CREATION_THRESHOLD];
+} FSMLocalMap;
+
+static FSMLocalMap fsm_local_map = {0, {FSM_LOCAL_NOT_AVAIL}};
+
+#define FSM_LOCAL_MAP_EXISTS (fsm_local_map.nblocks > 0)
+
 /* functions to navigate the tree */
 static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
 static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
@@ -107,10 +126,14 @@ static Size fsm_space_cat_to_avail(uint8 cat);
 /* workhorse functions for various operations */
 static int fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
 				   uint8 newValue, uint8 minValue);
+static void fsm_local_set(Relation rel, BlockNumber cur_nblocks);
 static BlockNumber fsm_search(Relation rel, uint8 min_cat);
+static BlockNumber fsm_local_search(void);
 static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
 				BlockNumber start, BlockNumber end,
 				bool *eof);
+static bool fsm_allow_writes(Relation rel, BlockNumber heapblk,
+				BlockNumber nblocks, BlockNumber *get_nblocks);
 
 
 /******** Public API ********/
@@ -127,13 +150,46 @@ static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
  * amount of free space available on that page and then try again (see
  * RecordAndGetPageWithFreeSpace).  If InvalidBlockNumber is returned,
  * extend the relation.
+ *
+ * For very small heap relations that don't have a FSM, we try every other
+ * page before extending the relation.  To keep track of which pages have
+ * been tried, initialize a local in-memory map of pages.
  */
 BlockNumber
-GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
+GetPageWithFreeSpace(Relation rel, Size spaceNeeded, bool check_fsm_only)
 {
 	uint8		min_cat = fsm_space_needed_to_cat(spaceNeeded);
+	BlockNumber target_block,
+				nblocks;
+
+	/* First try the FSM, if it exists. */
+	target_block = fsm_search(rel, min_cat);
+
+	if (target_block == InvalidBlockNumber &&
+		(rel->rd_rel->relkind == RELKIND_RELATION ||
+		 rel->rd_rel->relkind == RELKIND_TOASTVALUE) &&
+		!check_fsm_only)
+	{
+		nblocks = RelationGetNumberOfBlocks(rel);
+
+		if (nblocks > HEAP_FSM_CREATION_THRESHOLD)
+		{
+			/*
+			 * If the FSM knows nothing of the rel, try the last page before
+			 * we give up and extend.  This avoids one-tuple-per-page syndrome
+			 * during bootstrapping or in a recently-started system.
+			 */
+			target_block = nblocks - 1;
+		}
+		else if (nblocks > 0)
+		{
+			/* Create or update local map and get first candidate block. */
+			fsm_local_set(rel, nblocks);
+			target_block = fsm_local_search();
+		}
+	}
 
-	return fsm_search(rel, min_cat);
+	return target_block;
 }
 
 /*
@@ -144,16 +200,47 @@ GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
  * also some effort to return a page close to the old page; if there's a
  * page with enough free space on the same FSM page where the old one page
  * is located, it is preferred.
+ *
+ * For very small heap relations that don't have a FSM, we update the local
+ * map to indicate we have tried a page, and return the next page to try.
  */
 BlockNumber
 RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
 							  Size oldSpaceAvail, Size spaceNeeded)
 {
-	int			old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
-	int			search_cat = fsm_space_needed_to_cat(spaceNeeded);
+	int			old_cat;
+	int			search_cat;
 	FSMAddress	addr;
 	uint16		slot;
 	int			search_slot;
+	BlockNumber nblocks = InvalidBlockNumber;
+
+	/* First try the local map, if it exists. */
+	if (FSM_LOCAL_MAP_EXISTS)
+	{
+		Assert((rel->rd_rel->relkind == RELKIND_RELATION ||
+				rel->rd_rel->relkind == RELKIND_TOASTVALUE) &&
+			   fsm_local_map.map[oldPage] == FSM_LOCAL_AVAIL);
+
+		fsm_local_map.map[oldPage] = FSM_LOCAL_NOT_AVAIL;
+		return fsm_local_search();
+	}
+
+	if (!fsm_allow_writes(rel, oldPage, InvalidBlockNumber, &nblocks))
+	{
+		/*
+		 * If we have neither a local map nor a FSM, we probably just
+		 * tried the target block in the smgr relation entry and failed,
+		 * so we'll need to create the local map.
+		 */
+		fsm_local_set(rel, nblocks);
+		return fsm_local_search();
+	}
+
+	/* Normal FSM logic follows */
+
+	old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
+	search_cat = fsm_space_needed_to_cat(spaceNeeded);
 
 	/* Get the location of the FSM byte representing the heap block */
 	addr = fsm_get_location(oldPage, &slot);
@@ -176,21 +263,42 @@ RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
  * Note that if the new spaceAvail value is higher than the old value stored
  * in the FSM, the space might not become visible to searchers until the next
  * FreeSpaceMapVacuum call, which updates the upper level pages.
+ *
+ * Callers have no need for a local map.
  */
 void
-RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
+RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk,
+						Size spaceAvail, BlockNumber nblocks)
 {
-	int			new_cat = fsm_space_avail_to_cat(spaceAvail);
+	int			new_cat;
 	FSMAddress	addr;
 	uint16		slot;
+	BlockNumber dummy;
+
+	if (!fsm_allow_writes(rel, heapBlk, nblocks, &dummy))
+		/* No FSM to update and no local map either */
+		return;
 
 	/* Get the location of the FSM byte representing the heap block */
 	addr = fsm_get_location(heapBlk, &slot);
 
+	new_cat = fsm_space_avail_to_cat(spaceAvail);
 	fsm_set_and_search(rel, addr, slot, new_cat, 0);
 }
 
 /*
+ * Clear the local map.  We must call this when we have found a block with
+ * enough free space, when we extend the relation, or on transaction abort.
+ */
+void
+FSMClearLocalMap(void)
+{
+	fsm_local_map.nblocks = 0;
+	memset(&fsm_local_map.map, FSM_LOCAL_NOT_AVAIL,
+		   sizeof(fsm_local_map.map));
+}
+
+/*
  * XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
  *		WAL replay
  */
@@ -204,6 +312,30 @@ XLogRecordPageWithFreeSpace(RelFileNode rnode, BlockNumber heapBlk,
 	BlockNumber blkno;
 	Buffer		buf;
 	Page		page;
+	bool		write_to_fsm;
+
+	/* This is meant to mirror the logic in fsm_allow_writes() */
+	if (heapBlk >= HEAP_FSM_CREATION_THRESHOLD)
+		write_to_fsm = true;
+	else
+	{
+		/* Open the relation at smgr level */
+		SMgrRelation smgr = smgropen(rnode, InvalidBackendId);
+
+		if (smgrexists(smgr, FSM_FORKNUM))
+			write_to_fsm = true;
+		else
+		{
+			BlockNumber heap_nblocks = smgrnblocks(smgr, MAIN_FORKNUM);
+			if (heap_nblocks > HEAP_FSM_CREATION_THRESHOLD)
+				write_to_fsm = true;
+			else
+				write_to_fsm = false;
+		}
+	}
+
+	if (!write_to_fsm)
+		return;
 
 	/* Get the location of the FSM byte representing the heap block */
 	addr = fsm_get_location(heapBlk, &slot);
@@ -904,3 +1036,134 @@ fsm_vacuum_page(Relation rel, FSMAddress addr,
 
 	return max_avail;
 }
+
+/*
+ * For heaps, we prevent creation of the FSM unless the number of pages
+ * exceeds HEAP_FSM_CREATION_THRESHOLD.  For tables that don't already have
+ * a FSM, this will save an inode and a few kB of space.
+ *
+ * XXX The API is a little awkward -- if the caller passes a valid nblocks
+ * value, it can avoid invoking a system call.  If the caller passes
+ * InvalidBlockNumber and receives a false return value, it can get an
+ * up-to-date relation size from get_nblocks.  This saves a few cycles in
+ * the caller, which would otherwise need to get the relation size by itself.
+ */
+static bool
+fsm_allow_writes(Relation rel, BlockNumber heapblk,
+				 BlockNumber nblocks, BlockNumber *get_nblocks)
+{
+	bool		skip_get_nblocks;
+
+	if (heapblk >= HEAP_FSM_CREATION_THRESHOLD)
+		return true;
+
+	/* Non-heap rels can always create a FSM. */
+	if (rel->rd_rel->relkind != RELKIND_RELATION &&
+		rel->rd_rel->relkind != RELKIND_TOASTVALUE)
+		return true;
+
+	/*
+	 * If the caller knows nblocks, we can avoid a system call later.
+	 * If it doesn't, maybe we have relpages from a previous VACUUM.
+	 * Since the table may have extended since then, we still have to
+	 * count the pages later if we can't return now.
+	 */
+	if (nblocks != InvalidBlockNumber)
+	{
+		if (nblocks > HEAP_FSM_CREATION_THRESHOLD)
+			return true;
+		else
+			skip_get_nblocks = true;
+	}
+	else
+	{
+		if (rel->rd_rel->relpages != InvalidBlockNumber &&
+			rel->rd_rel->relpages > HEAP_FSM_CREATION_THRESHOLD)
+			return true;
+		else
+			skip_get_nblocks = false;
+	}
+
+	RelationOpenSmgr(rel);
+	if (smgrexists(rel->rd_smgr, FSM_FORKNUM))
+		return true;
+
+	if (skip_get_nblocks)
+		return false;
+
+	/* last resort */
+	*get_nblocks = RelationGetNumberOfBlocks(rel);
+	if (*get_nblocks > HEAP_FSM_CREATION_THRESHOLD)
+		return true;
+	else
+		return false;
+}
+
+/*
+ * Initialize or update the local map of blocks to try, for when there is
+ * no FSM.
+ *
+ * When we initialize the map, the whole heap is potentially available to
+ * try.  Testing revealed that trying every block can cause a small
+ * performance dip compared to when we use a FSM, so we try every other
+ * block instead.
+ */
+static void
+fsm_local_set(Relation rel, BlockNumber cur_nblocks)
+{
+	BlockNumber blkno,
+				cached_target_block;
+
+	/* The local map must not be set already. */
+	Assert(!FSM_LOCAL_MAP_EXISTS);
+
+	/*
+	 * Starting at the current last block in the relation and working
+	 * backwards, mark alternating blocks as available.
+	 */
+	blkno = cur_nblocks - 1;
+	while (true)
+	{
+		fsm_local_map.map[blkno] = FSM_LOCAL_AVAIL;
+		if (blkno >= 2)
+			blkno -= 2;
+		else
+			break;
+	}
+
+	/* Cache the number of blocks. */
+	fsm_local_map.nblocks = cur_nblocks;
+
+	/* Set the status of the cached target block to 'unavailable'. */
+	cached_target_block = RelationGetTargetBlock(rel);
+	if (cached_target_block != InvalidBlockNumber &&
+		cached_target_block < cur_nblocks)
+		fsm_local_map.map[cached_target_block] = FSM_LOCAL_NOT_AVAIL;
+}
+
+/*
+ * Search the local map for an available block to try, in descending order.
+ * As such, there is no heuristic available to decide which order will be
+ * better to try, but the probability of having space in the last block in the
+ * map is higher because that is the most recent block added to the heap.
+ *
+ * This function is used when there is no FSM.
+ */
+static BlockNumber
+fsm_local_search(void)
+{
+	BlockNumber		target_block;
+
+	/* Local map must be set by now. */
+	Assert(FSM_LOCAL_MAP_EXISTS);
+
+	target_block = fsm_local_map.nblocks;
+	do
+	{
+		target_block--;
+		if (fsm_local_map.map[target_block] == FSM_LOCAL_AVAIL)
+			return target_block;
+	} while (target_block > 0);
+
+	return InvalidBlockNumber;
+}