Optimize DropRelFileNodeBuffers() for recovery.

The recovery path of DropRelFileNodeBuffers() is optimized so that
scanning of the whole buffer pool can be avoided when the number of
blocks to be truncated in a relation is below a certain threshold. For
such cases, we find the buffers by doing lookups in BufMapping table.
This improves the performance by more than 100 times in many cases
when several small tables (tested with 1000 relations) are truncated
and where the server is configured with a large value of shared
buffers (greater than equal to 100GB).

This optimization helps cases (a) when vacuum or autovacuum truncated off
any of the empty pages at the end of a relation, or (b) when the relation is
truncated in the same transaction in which it was created.

This commit introduces a new API smgrnblocks_cached which returns a cached
value for the number of blocks in a relation fork. This helps us to determine
the exact size of relation which is required to apply this optimization. The
exact size is required to ensure that we don't leave any buffer for the
relation being dropped as otherwise the background writer or checkpointer
can lead to a PANIC error while flushing buffers corresponding to files that
don't exist.

Author: Kirk Jamison based on ideas by Amit Kapila
Reviewed-by: Kyotaro Horiguchi, Takayuki Tsunakawa, and Amit Kapila
Tested-By: Haiying Tang
Discussion: https://postgr.es/m/OSBPR01MB3207DCA7EC725FDD661B3EDAEF660@OSBPR01MB3207.jpnprd01.prod.outlook.com

1 files changed, 24 insertions, 6 deletions

diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index 0f31ff3822..af603c3db3 100644
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -549,6 +549,28 @@ smgrnblocks(SMgrRelation reln, ForkNumber forknum)
 {
 	BlockNumber result;
 
+	/* Check and return if we get the cached value for the number of blocks. */
+	result = smgrnblocks_cached(reln, forknum);
+	if (result != InvalidBlockNumber)
+		return result;
+
+	result = smgrsw[reln->smgr_which].smgr_nblocks(reln, forknum);
+
+	reln->smgr_cached_nblocks[forknum] = result;
+
+	return result;
+}
+
+/*
+ *	smgrnblocks_cached() -- Get the cached number of blocks in the supplied
+ *							relation.
+ *
+ * Returns an InvalidBlockNumber when not in recovery and when the relation
+ * fork size is not cached.
+ */
+BlockNumber
+smgrnblocks_cached(SMgrRelation reln, ForkNumber forknum)
+{
 	/*
 	 * For now, we only use cached values in recovery due to lack of a shared
 	 * invalidation mechanism for changes in file size.
@@ -556,11 +578,7 @@ smgrnblocks(SMgrRelation reln, ForkNumber forknum)
 	if (InRecovery && reln->smgr_cached_nblocks[forknum] != InvalidBlockNumber)
 		return reln->smgr_cached_nblocks[forknum];
 
-	result = smgrsw[reln->smgr_which].smgr_nblocks(reln, forknum);
-
-	reln->smgr_cached_nblocks[forknum] = result;
-
-	return result;
+	return InvalidBlockNumber;
 }
 
 /*
@@ -582,7 +600,7 @@ smgrtruncate(SMgrRelation reln, ForkNumber *forknum, int nforks, BlockNumber *nb
 	 * Get rid of any buffers for the about-to-be-deleted blocks. bufmgr will
 	 * just drop them without bothering to write the contents.
 	 */
-	DropRelFileNodeBuffers(reln->smgr_rnode, forknum, nforks, nblocks);
+	DropRelFileNodeBuffers(reln, forknum, nforks, nblocks);
 
 	/*
 	 * Send a shared-inval message to force other backends to close any smgr