Make large sequential scans and VACUUMs work in a limited-size "ring" of

buffers, rather than blowing out the whole shared-buffer arena.  Aside from
avoiding cache spoliation, this fixes the problem that VACUUM formerly tended
to cause a WAL flush for every page it modified, because we had it hacked to
use only a single buffer.  Those flushes will now occur only once per
ring-ful.  The exact ring size, and the threshold for seqscans to switch into
the ring usage pattern, remain under debate; but the infrastructure seems
done.  The key bit of infrastructure is a new optional BufferAccessStrategy
object that can be passed to ReadBuffer operations; this replaces the former
StrategyHintVacuum API.

This patch also changes the buffer usage-count methodology a bit: we now
advance usage_count when first pinning a buffer, rather than when last
unpinning it.  To preserve the behavior that a buffer's lifetime starts to
decrease when it's released, the clock sweep code is modified to not decrement
usage_count of pinned buffers.

Work not done in this commit: teach GiST and GIN indexes to use the vacuum
BufferAccessStrategy for vacuum-driven fetches.

Original patch by Simon, reworked by Heikki and again by Tom.

1 files changed, 31 insertions, 1 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 3dc00499bfb..4ca4aa754c6 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.269 2007/05/20 21:08:19 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.270 2007/05/30 20:11:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1800,6 +1800,36 @@ XLogFlush(XLogRecPtr record)
 }
 
 /*
+ * Test whether XLOG data has been flushed up to (at least) the given position.
+ *
+ * Returns true if a flush is still needed.  (It may be that someone else
+ * is already in process of flushing that far, however.)
+ */
+bool
+XLogNeedsFlush(XLogRecPtr record)
+{
+	/* Quick exit if already known flushed */
+	if (XLByteLE(record, LogwrtResult.Flush))
+		return false;
+
+	/* read LogwrtResult and update local state */
+	{
+		/* use volatile pointer to prevent code rearrangement */
+		volatile XLogCtlData *xlogctl = XLogCtl;
+
+		SpinLockAcquire(&xlogctl->info_lck);
+		LogwrtResult = xlogctl->LogwrtResult;
+		SpinLockRelease(&xlogctl->info_lck);
+	}
+
+	/* check again */
+	if (XLByteLE(record, LogwrtResult.Flush))
+		return false;
+
+	return true;
+}
+
+/*
  * Create a new XLOG file segment, or open a pre-existing one.
  *
  * log, seg: identify segment to be created/opened.