Implement COMMIT_SIBLINGS parameter to allow pre-commit delay to occur

only if at least N other backends currently have open transactions.  This
is not a great deal of intelligence about whether a delay might be
profitable ... but it beats no intelligence at all.  Note that the default
COMMIT_DELAY is still zero --- this new code does nothing unless that
setting is changed.
Also, mark ENABLEFSYNC as a system-wide setting.  It's no longer safe to
allow that to be set per-backend, since we may be relying on some other
backend's fsync to have synced the WAL log.

2 files changed, 15 insertions, 11 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 37eee5ebfd7..0af25826589 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.97 2001/02/18 04:50:43 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.98 2001/02/26 00:50:07 tgl Exp $
  *
  * NOTES
  *		Transaction aborts can now occur two ways:
@@ -157,6 +157,7 @@
 #include <sys/time.h>
 
 #include "access/nbtree.h"
+#include "access/xact.h"
 #include "catalog/heap.h"
 #include "catalog/index.h"
 #include "commands/async.h"
@@ -177,8 +178,6 @@
 
 extern bool SharedBufferChanged;
 
-void RecordTransactionCommit(void);
-
 static void AbortTransaction(void);
 static void AtAbort_Cache(void);
 static void AtAbort_Locks(void);
@@ -216,12 +215,14 @@ TransactionStateData CurrentTransactionStateData = {
 
 TransactionState CurrentTransactionState = &CurrentTransactionStateData;
 
+/*
+ * User-tweakable parameters
+ */
 int			DefaultXactIsoLevel = XACT_READ_COMMITTED;
 int			XactIsoLevel;
 
-#include "access/xlogutils.h"
-
-int			CommitDelay = 0;	/* in microseconds */
+int			CommitDelay = 0;	/* precommit delay in microseconds */
+int			CommitSiblings = 5;	/* number of concurrent xacts needed to sleep */
 
 static void (*_RollbackFunc)(void*) = NULL;
 static void *_RollbackData = NULL;
@@ -687,10 +688,15 @@ RecordTransactionCommit()
 		 * Sleep before commit! So we can flush more than one
 		 * commit records per single fsync.  (The idea is some other
 		 * backend may do the XLogFlush while we're sleeping.  This
-		 * needs work however, because on most Unixen, the minimum
+		 * needs work still, because on most Unixen, the minimum
 		 * select() delay is 10msec or more, which is way too long.)
+		 *
+		 * We do not sleep if enableFsync is not turned on, nor if there
+		 * are fewer than CommitSiblings other backends with active
+		 * transactions.
 		 */
-		if (CommitDelay > 0)
+		if (CommitDelay > 0 && enableFsync &&
+			CountActiveBackends() >= CommitSiblings)
 		{
 			struct timeval	delay;
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index fbc4223034a..fc861aba123 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.54 2001/02/18 04:39:42 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.55 2001/02/26 00:50:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2096,8 +2096,6 @@ ShutdownXLOG()
 	elog(LOG, "database system is shut down");
 }
 
-extern XLogRecPtr	GetUndoRecPtr(void);
-
 void
 CreateCheckPoint(bool shutdown)
 {