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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/clog.c
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@ -24,7 +24,7 @@
* 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/clog.c,v 1.34 2005/11/05 21:19:47 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/clog.c,v 1.35 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -128,9 +128,9 @@ TransactionIdGetStatus(TransactionId xid)
char *byteptr;
XidStatus status;
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/* lock is acquired by SimpleLruReadPage_ReadOnly */
slotno = SimpleLruReadPage(ClogCtl, pageno, xid);
slotno = SimpleLruReadPage_ReadOnly(ClogCtl, pageno, xid);
byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;