Truncate predicate lock manager's SLRU lazily at checkpoint. That's safer

than doing it aggressively whenever the tail-XID pointer is advanced, because
this way we don't need to do it while holding SerializableXactHashLock.

This also fixes bug #5915 spotted by YAMAMOTO Takashi, and removes an
obsolete comment spotted by Kevin Grittner.

src/backend/access/transam/xlog.c

@@ -48,6 +48,7 @@
 #include "storage/ipc.h"
 #include "storage/latch.h"
 #include "storage/pmsignal.h"
+#include "storage/predicate.h"
 #include "storage/procarray.h"
 #include "storage/reinit.h"
 #include "storage/smgr.h"
@@ -7875,6 +7876,7 @@ CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
 	CheckPointCLOG();
 	CheckPointSUBTRANS();
 	CheckPointMultiXact();
+	CheckPointPredicate();
 	CheckPointRelationMap();
 	CheckPointBuffers(flags);	/* performs all required fsyncs */
 	/* We deliberately delay 2PC checkpointing as long as possible */

src/backend/storage/lmgr/predicate.c

@@ -323,10 +323,9 @@ static SlruCtlData OldSerXidSlruCtlData;
 
 typedef struct OldSerXidControlData
 {
-	int			headPage;
-	int			tailSegment;
-	TransactionId headXid;
-	TransactionId tailXid;
+	int			headPage;		/* newest initialized page */
+	TransactionId headXid;		/* newest valid Xid in the SLRU */
+	TransactionId tailXid;		/* oldest xmin we might be interested in */
 	bool		warningIssued;
 } OldSerXidControlData;
 
@@ -711,7 +710,6 @@ OldSerXidInit(void)
 		 * Set control information to reflect empty SLRU.
 		 */
 		oldSerXidControl->headPage = -1;
-		oldSerXidControl->tailSegment = -1;
 		oldSerXidControl->headXid = InvalidTransactionId;
 		oldSerXidControl->tailXid = InvalidTransactionId;
 		oldSerXidControl->warningIssued = false;
@@ -722,10 +720,6 @@ OldSerXidInit(void)
  * Record a committed read write serializable xid and the minimum
  * commitSeqNo of any transactions to which this xid had a rw-conflict out.
  * A zero seqNo means that there were no conflicts out from xid.
- *
- * The return value is normally false -- true means that we're about to
- * wrap around our space for tracking these xids, so the caller might want
- * to take action to prevent that.
  */
 static void
 OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
@@ -733,7 +727,7 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	TransactionId tailXid;
 	int			targetPage;
 	int			slotno;
-	int			page;
+	int			firstZeroPage;
 	int			xidSpread;
 	bool		isNewPage;
 
@@ -745,30 +739,34 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 
 	/*
 	 * If no serializable transactions are active, there shouldn't be anything
-	 * to push out to this SLRU.  Hitting this assert would mean there's
+	 * to push out to the SLRU.  Hitting this assert would mean there's
 	 * something wrong with the earlier cleanup logic.
 	 */
 	tailXid = oldSerXidControl->tailXid;
 	Assert(TransactionIdIsValid(tailXid));
 
+	/*
+	 * If the SLRU is currently unused, zero out the whole active region
+	 * from tailXid to headXid before taking it into use. Otherwise zero
+	 * out only any new pages that enter the tailXid-headXid range as we
+	 * advance headXid.
+	 */
 	if (oldSerXidControl->headPage < 0)
 	{
-		page = OldSerXidPage(tailXid);
-		oldSerXidControl->tailSegment = OldSerXidSegment(page);
-		page = oldSerXidControl->tailSegment * OLDSERXID_ENTRIESPERPAGE;
+		firstZeroPage = OldSerXidPage(tailXid);
 		isNewPage = true;
 	}
 	else
 	{
-		page = OldSerXidNextPage(oldSerXidControl->headPage);
-		isNewPage = OldSerXidPagePrecedesLogically(oldSerXidControl->headPage, targetPage);
+		firstZeroPage = OldSerXidNextPage(oldSerXidControl->headPage);
+		isNewPage = OldSerXidPagePrecedesLogically(oldSerXidControl->headPage,
+												   targetPage);
 	}
 
 	if (!TransactionIdIsValid(oldSerXidControl->headXid)
 		|| TransactionIdFollows(xid, oldSerXidControl->headXid))
 		oldSerXidControl->headXid = xid;
-	if (oldSerXidControl->headPage < 0
-	|| OldSerXidPagePrecedesLogically(oldSerXidControl->headPage, targetPage))
+	if (isNewPage)
 		oldSerXidControl->headPage = targetPage;
 
 	xidSpread = (((uint32) xid) - ((uint32) tailXid));
@@ -788,10 +786,10 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	if (isNewPage)
 	{
 		/* Initialize intervening pages. */
-		while (page != targetPage)
+		while (firstZeroPage != targetPage)
 		{
-			(void) SimpleLruZeroPage(OldSerXidSlruCtl, page);
-			page = OldSerXidNextPage(page);
+			(void) SimpleLruZeroPage(OldSerXidSlruCtl, firstZeroPage);
+			firstZeroPage = OldSerXidNextPage(firstZeroPage);
 		}
 		slotno = SimpleLruZeroPage(OldSerXidSlruCtl, targetPage);
 	}
@@ -846,31 +844,24 @@ OldSerXidGetMinConflictCommitSeqNo(TransactionId xid)
 /*
  * Call this whenever there is a new xmin for active serializable
  * transactions.  We don't need to keep information on transactions which
- * preceed that.  InvalidTransactionId means none active, so everything in
- * the SLRU should be discarded.
+ * precede that.  InvalidTransactionId means none active, so everything in
+ * the SLRU can be discarded.
  */
 static void
 OldSerXidSetActiveSerXmin(TransactionId xid)
 {
-	int			newTailPage;
-	int			newTailSegment;
-
 	LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);
 
 	/*
 	 * When no sxacts are active, nothing overlaps, set the xid values to
-	 * invalid to show that there are no valid entries.  Don't clear the
-	 * segment/page information, though.  A new xmin might still land in an
-	 * existing segment, and we don't want to repeatedly delete and re-create
-	 * the same segment file.
+	 * invalid to show that there are no valid entries.  Don't clear headPage,
+	 * though.  A new xmin might still land on that page, and we don't want
+	 * to repeatedly zero out the same page.
 	 */
 	if (!TransactionIdIsValid(xid))
 	{
-		if (TransactionIdIsValid(oldSerXidControl->tailXid))
-		{
-			oldSerXidControl->headXid = InvalidTransactionId;
 		oldSerXidControl->tailXid = InvalidTransactionId;
-		}
+		oldSerXidControl->headXid = InvalidTransactionId;
 		LWLockRelease(OldSerXidLock);
 		return;
 	}
@@ -886,7 +877,9 @@ OldSerXidSetActiveSerXmin(TransactionId xid)
 		Assert(oldSerXidControl->headPage < 0);
 		if (!TransactionIdIsValid(oldSerXidControl->tailXid)
 			|| TransactionIdPrecedes(xid, oldSerXidControl->tailXid))
+		{
 			oldSerXidControl->tailXid = xid;
+		}
 		LWLockRelease(OldSerXidLock);
 		return;
 	}
@@ -896,37 +889,57 @@ OldSerXidSetActiveSerXmin(TransactionId xid)
 
 	oldSerXidControl->tailXid = xid;
 
-	/* Exit quickly if there are no segments active. */
-	if (oldSerXidControl->headPage < 0)
-	{
 	LWLockRelease(OldSerXidLock);
-		return;
-	}
+}
 
-	newTailPage = OldSerXidPage(xid);
-	newTailSegment = OldSerXidSegment(newTailPage);
+/*
+ * Perform a checkpoint --- either during shutdown, or on-the-fly
+ *
+ * We don't have any data that needs to survive a restart, but this is a
+ * convenient place to truncate the SLRU.
+ */
+void
+CheckPointPredicate(void)
+{
+	int tailPage;
+
+	LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);
 
-	/* Exit quickly if we're still on the same segment. */
-	if (newTailSegment == oldSerXidControl->tailSegment)
+	/* Exit quickly if the SLRU is currently not in use. */
+	if (oldSerXidControl->headPage < 0)
 	{
 		LWLockRelease(OldSerXidLock);
 		return;
 	}
 
-	oldSerXidControl->tailSegment = newTailSegment;
-
-	/* See if that has cleared the last segment. */
-	if (OldSerXidPagePrecedesLogically(oldSerXidControl->headPage,
-									newTailSegment * SLRU_PAGES_PER_SEGMENT))
+	if (TransactionIdIsValid(oldSerXidControl->tailXid))
 	{
-		oldSerXidControl->headXid = InvalidTransactionId;
+		/* We can truncate the SLRU up to the page containing tailXid */
+		tailPage = OldSerXidPage(oldSerXidControl->tailXid);
+	}
+	else
+	{
+		/*
+		 * The SLRU is no longer needed. Truncate everything but the last
+		 * page. We don't dare to touch the last page in case the SLRU is
+		 * taken back to use, and the new tail falls on the same page.
+		 */
+		tailPage = oldSerXidControl->headPage;
 		oldSerXidControl->headPage = -1;
-		oldSerXidControl->tailSegment = -1;
 	}
 
 	LWLockRelease(OldSerXidLock);
 
-	SimpleLruTruncate(OldSerXidSlruCtl, newTailPage);
+	/*
+	 * Flush dirty SLRU pages to disk
+	 *
+	 * This is not actually necessary from a correctness point of view. We do
+	 * it merely as a debugging aid.
+	 */
+	SimpleLruFlush(OldSerXidSlruCtl, true);
+
+	/* Truncate away pages that are no longer required */
+	SimpleLruTruncate(OldSerXidSlruCtl, tailPage);
 }
 
 /*------------------------------------------------------------------------*/

src/include/storage/predicate.h

@@ -36,6 +36,8 @@ extern int	max_predicate_locks_per_xact;
 extern void InitPredicateLocks(void);
 extern Size PredicateLockShmemSize(void);
 
+extern void CheckPointPredicate(void);
+
 /* predicate lock reporting */
 extern bool PageIsPredicateLocked(const Relation relation, const BlockNumber blkno);