Defer flushing of SLRU files.

Previously, we called fsync() after writing out individual pg_xact,
pg_multixact and pg_commit_ts pages due to cache pressure, leading to
regular I/O stalls in user backends and recovery.  Collapse requests for
the same file into a single system call as part of the next checkpoint,
as we already did for relation files, using the infrastructure developed
by commit 3eb77eba.  This can cause a significant improvement to
recovery performance, especially when it's otherwise CPU-bound.

Hoist ProcessSyncRequests() up into CheckPointGuts() to make it clearer
that it applies to all the SLRU mini-buffer-pools as well as the main
buffer pool.  Rearrange things so that data collected in CheckpointStats
includes SLRU activity.

Also remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.  (I'm not sure if they were ever needed, but
they aren't now.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (parts)
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

1 files changed, 3 insertions, 22 deletions

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index a50f60b99af..0111e867c79 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -193,9 +193,7 @@ SUBTRANSShmemInit(void)
 	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
 	SimpleLruInit(SubTransCtl, "Subtrans", NUM_SUBTRANS_BUFFERS, 0,
 				  SubtransSLRULock, "pg_subtrans",
-				  LWTRANCHE_SUBTRANS_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	SubTransCtl->do_fsync = false;
+				  LWTRANCHE_SUBTRANS_BUFFER, SYNC_HANDLER_NONE);
 }
 
 /*
@@ -279,37 +277,20 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 }
 
 /*
- * This must be called ONCE during postmaster or standalone-backend shutdown
- */
-void
-ShutdownSUBTRANS(void)
-{
-	/*
-	 * Flush dirty SUBTRANS pages to disk
-	 *
-	 * This is not actually necessary from a correctness point of view. We do
-	 * it merely as a debugging aid.
-	 */
-	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(false);
-	SimpleLruFlush(SubTransCtl, false);
-	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(false);
-}
-
-/*
  * Perform a checkpoint --- either during shutdown, or on-the-fly
  */
 void
 CheckPointSUBTRANS(void)
 {
 	/*
-	 * Flush dirty SUBTRANS pages to disk
+	 * Write dirty SUBTRANS pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely to improve the odds that writing of dirty pages is done by
 	 * the checkpoint process and not by backends.
 	 */
 	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(true);
-	SimpleLruFlush(SubTransCtl, true);
+	SimpleLruWriteAll(SubTransCtl, true);
 	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(true);
 }