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Arrange for read-only accesses to SLRU page buffers to take only a shared

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lock, not exclusive, if the desired page is already in memory.  This can
be demonstrated to be a significant win on the pg_subtrans cache when there
is a large window of open transactions.  It should be useful for pg_clog
as well.  I didn't try to make GetMultiXactIdMembers() use the code, as
that would have taken some restructuring, and what with the local cache
for multixact contents it probably wouldn't really make a difference.
Per my recent proposal.
This commit is contained in:
Tom Lane
2005-12-06 18:10:06 +00:00
parent 953208a34c
commit a615acf555
5 changed files with 145 additions and 28 deletions
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6
src/backend/access/transam/multixact.c
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@@ -42,7 +42,7 @@
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.13 2005/11/22 18:17:07 momjian Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.14 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -1690,12 +1690,12 @@ TruncateMultiXact(void)
int entryno;
MultiXactOffset *offptr;
LWLockAcquire(MultiXactOffsetControlLock, LW_EXCLUSIVE);
/* lock is acquired by SimpleLruReadPage_ReadOnly */
pageno = MultiXactIdToOffsetPage(oldestMXact);
entryno = MultiXactIdToOffsetEntry(oldestMXact);
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, oldestMXact);
slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, oldestMXact);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;
oldestOffset = *offptr;