Further reduce memory footprint of CLOBBER_CACHE_ALWAYS testing.

Some buildfarm members using CLOBBER_CACHE_ALWAYS have been having OOM
problems of late.  Commit 2455ab488 addressed this problem by recovering
space transiently used within RelationBuildPartitionDesc, but it turns
out that leaves quite a lot on the table, because other subroutines of
RelationBuildDesc also leak memory like mad.  Let's move the temp-context
management into RelationBuildDesc so that leakage from the other
subroutines is also recovered.

I examined this issue by arranging for postgres.c to dump the size of
MessageContext just before resetting it in each command cycle, and
then running the update.sql regression test (which is one of the two
that are seeing buildfarm OOMs) with and without CLOBBER_CACHE_ALWAYS.
Before 2455ab488, the peak space usage with CCA was as much as 250MB.
That patch got it down to ~80MB, but with this patch it's about 0.5MB,
and indeed the space usage now seems nearly indistinguishable from a
non-CCA build.

RelationBuildDesc's traditional behavior of not worrying about leaking
transient data is of many years' standing, so I'm pretty hesitant to
change that without more evidence that it'd be useful in a normal build.
(So far as I can see, non-CCA memory consumption is about the same with
or without this change, whuch if anything suggests that it isn't useful.)
Hence, configure the patch so that we recover space only when
CLOBBER_CACHE_ALWAYS or CLOBBER_CACHE_RECURSIVELY is defined.  However,
that choice can be overridden at compile time, in case somebody would
like to do some performance testing and try to develop evidence for
changing that decision.

It's possible that we ought to back-patch this change, but in the
absence of back-branch OOM problems in the buildfarm, I'm not in
a hurry to do that.

Discussion: https://postgr.es/m/CA+TgmoY3bRmGB6-DUnoVy5fJoreiBJ43rwMrQRCdPXuKt4Ykaw@mail.gmail.com

1 files changed, 4 insertions, 19 deletions

diff --git a/src/backend/partitioning/partdesc.c b/src/backend/partitioning/partdesc.c
index 5f349ad46ad..a65cc526da5 100644
--- a/src/backend/partitioning/partdesc.c
+++ b/src/backend/partitioning/partdesc.c
@@ -67,20 +67,9 @@ RelationBuildPartitionDesc(Relation rel)
 				nparts;
 	PartitionKey key = RelationGetPartitionKey(rel);
 	MemoryContext oldcxt;
-	MemoryContext rbcontext;
 	int		   *mapping;
 
 	/*
-	 * While building the partition descriptor, we create various temporary
-	 * data structures; in CLOBBER_CACHE_ALWAYS mode, at least, it's important
-	 * not to leak them, since this can get called a lot.
-	 */
-	rbcontext = AllocSetContextCreate(CurrentMemoryContext,
-									  "RelationBuildPartitionDesc",
-									  ALLOCSET_DEFAULT_SIZES);
-	oldcxt = MemoryContextSwitchTo(rbcontext);
-
-	/*
 	 * Get partition oids from pg_inherits.  This uses a single snapshot to
 	 * fetch the list of children, so while more children may be getting
 	 * added concurrently, whatever this function returns will be accurate
@@ -197,14 +186,15 @@ RelationBuildPartitionDesc(Relation rel)
 	/* If there are no partitions, the rest of the partdesc can stay zero */
 	if (nparts > 0)
 	{
-		/* Create PartitionBoundInfo, using the temporary context. */
+		/* Create PartitionBoundInfo, using the caller's context. */
 		boundinfo = partition_bounds_create(boundspecs, nparts, key, &mapping);
 
 		/* Now copy all info into relcache's partdesc. */
-		MemoryContextSwitchTo(rel->rd_pdcxt);
+		oldcxt = MemoryContextSwitchTo(rel->rd_pdcxt);
 		partdesc->boundinfo = partition_bounds_copy(boundinfo, key);
 		partdesc->oids = (Oid *) palloc(nparts * sizeof(Oid));
 		partdesc->is_leaf = (bool *) palloc(nparts * sizeof(bool));
+		MemoryContextSwitchTo(oldcxt);
 
 		/*
 		 * Assign OIDs from the original array into mapped indexes of the
@@ -214,9 +204,8 @@ RelationBuildPartitionDesc(Relation rel)
 		 *
 		 * Also record leaf-ness of each partition.  For this we use
 		 * get_rel_relkind() which may leak memory, so be sure to run it in
-		 * the temporary context.
+		 * the caller's context.
 		 */
-		MemoryContextSwitchTo(rbcontext);
 		for (i = 0; i < nparts; i++)
 		{
 			int			index = mapping[i];
@@ -228,10 +217,6 @@ RelationBuildPartitionDesc(Relation rel)
 	}
 
 	rel->rd_partdesc = partdesc;
-
-	/* Return to caller's context, and blow away the temporary context. */
-	MemoryContextSwitchTo(oldcxt);
-	MemoryContextDelete(rbcontext);
 }
 
 /*