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Tweak indexscan and seqscan code to arrange that steps from one page to

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the next are handled by ReleaseAndReadBuffer rather than separate
ReleaseBuffer and ReadBuffer calls.  This cuts the number of acquisitions
of the BufMgrLock by a factor of 2 (possibly more, if an indexscan happens
to pull successive rows from the same heap page).  Unfortunately this
doesn't seem enough to get us out of the recently discussed context-switch
storm problem, but it's surely worth doing anyway.
This commit is contained in:
Tom Lane
2004-04-21 18:24:26 +00:00
parent 95a03e9cdf
commit 37fa3b6c89
11 changed files with 132 additions and 65 deletions
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44
src/backend/access/nbtree/nbtpage.c
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@@ -9,7 +9,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.74 2003/12/21 01:23:06 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.75 2004/04/21 18:24:25 tgl Exp $
*
* NOTES
* Postgres btree pages look like ordinary relation pages. The opaque
@@ -262,11 +262,15 @@ _bt_getroot(Relation rel, int access)
Assert(rootblkno != P_NONE);
rootlevel = metad->btm_fastlevel;
_bt_relbuf(rel, metabuf); /* done with the meta page */
/*
* We are done with the metapage; arrange to release it via
* first _bt_relandgetbuf call
*/
rootbuf = metabuf;
for (;;)
{
rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
rootpage = BufferGetPage(rootbuf);
rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -278,8 +282,6 @@ _bt_getroot(Relation rel, int access)
elog(ERROR, "no live root page found in \"%s\"",
RelationGetRelationName(rel));
rootblkno = rootopaque->btpo_next;
_bt_relbuf(rel, rootbuf);
}
/* Note: can't check btpo.level on deleted pages */
@@ -352,11 +354,15 @@ _bt_gettrueroot(Relation rel)
rootblkno = metad->btm_root;
rootlevel = metad->btm_level;
_bt_relbuf(rel, metabuf); /* done with the meta page */
/*
* We are done with the metapage; arrange to release it via
* first _bt_relandgetbuf call
*/
rootbuf = metabuf;
for (;;)
{
rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
rootpage = BufferGetPage(rootbuf);
rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -368,8 +374,6 @@ _bt_gettrueroot(Relation rel)
elog(ERROR, "no live root page found in \"%s\"",
RelationGetRelationName(rel));
rootblkno = rootopaque->btpo_next;
_bt_relbuf(rel, rootbuf);
}
/* Note: can't check btpo.level on deleted pages */
@@ -492,6 +496,28 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
return buf;
}
/*
* _bt_relandgetbuf() -- release a locked buffer and get another one.
*
* This is equivalent to _bt_relbuf followed by _bt_getbuf, with the
* exception that blkno may not be P_NEW. Also, if obuf is InvalidBuffer
* then it reduces to just _bt_getbuf; allowing this case simplifies some
* callers. The motivation for using this is to avoid two entries to the
* bufmgr when one will do.
*/
Buffer
_bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
{
Buffer buf;
Assert(blkno != P_NEW);
if (BufferIsValid(obuf))
LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
buf = ReleaseAndReadBuffer(obuf, rel, blkno);
LockBuffer(buf, access);
return buf;
}
/*
* _bt_relbuf() -- release a locked buffer.
*