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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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21
src/include/access/slru.h
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@@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/access/slru.h,v 1.15 2005/11/05 21:19:47 tgl Exp $
* $PostgreSQL: pgsql/src/include/access/slru.h,v 1.16 2005/12/06 18:10:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -46,17 +46,26 @@ typedef struct SlruSharedData
/*
* Info for each buffer slot. Page number is undefined when status is
* EMPTY. lru_count is essentially the number of page switches since last
* use of this page; the page with highest lru_count is the best candidate
* to replace.
* EMPTY.
*/
char *page_buffer[NUM_SLRU_BUFFERS];
SlruPageStatus page_status[NUM_SLRU_BUFFERS];
bool page_dirty[NUM_SLRU_BUFFERS];
int page_number[NUM_SLRU_BUFFERS];
unsigned int page_lru_count[NUM_SLRU_BUFFERS];
int page_lru_count[NUM_SLRU_BUFFERS];
LWLockId buffer_locks[NUM_SLRU_BUFFERS];
/*----------
* We mark a page "most recently used" by setting
* page_lru_count[slotno] = ++cur_lru_count;
* The oldest page is therefore the one with the highest value of
* cur_lru_count - page_lru_count[slotno]
* The counts will eventually wrap around, but this calculation still
* works as long as no page's age exceeds INT_MAX counts.
*----------
*/
int cur_lru_count;
/*
* latest_page_number is the page number of the current end of the log;
* this is not critical data, since we use it only to avoid swapping out
@@ -106,6 +115,8 @@ extern void SimpleLruInit(SlruCtl ctl, const char *name,
LWLockId ctllock, const char *subdir);
extern int SimpleLruZeroPage(SlruCtl ctl, int pageno);
extern int SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid);
extern int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
TransactionId xid);
extern void SimpleLruWritePage(SlruCtl ctl, int slotno, SlruFlush fdata);
extern void SimpleLruFlush(SlruCtl ctl, bool checkpoint);
extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);