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Split the shared-memory array of PGPROC pointers out of the sinval
communication structure, and make it its own module with its own lock. This should reduce contention at least a little, and it definitely makes the code seem cleaner. Per my recent proposal.
This commit is contained in:
Tom Lane
@@ -1,4 +1,4 @@
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$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.2 2004/09/16 16:58:26 tgl Exp $
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$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.3 2005/05/19 21:35:45 tgl Exp $
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The Transaction System
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----------------------
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@@ -246,7 +246,7 @@ but since we allow arbitrary nesting of subtransactions, we can't fit all Xids
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in shared memory, so we have to store them on disk. Note, however, that for
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each transaction we keep a "cache" of Xids that are known to be part of the
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transaction tree, so we can skip looking at pg_subtrans unless we know the
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cache has been overflowed. See storage/ipc/sinval.c for the gory details.
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cache has been overflowed. See storage/ipc/procarray.c for the gory details.
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slru.c is the supporting mechanism for both pg_clog and pg_subtrans. It
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implements the LRU policy for in-memory buffer pages. The high-level routines
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@@ -31,7 +31,7 @@
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* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.3 2005/05/07 18:14:25 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/multixact.c,v 1.4 2005/05/19 21:35:45 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -44,7 +44,7 @@
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#include "utils/memutils.h"
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#include "storage/backendid.h"
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#include "storage/lmgr.h"
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#include "storage/sinval.h"
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#include "storage/procarray.h"
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/*
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@@ -383,8 +383,8 @@ MultiXactIdIsRunning(MultiXactId multi)
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}
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/*
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* This could be made better by having a special entry point in sinval.c,
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* walking the PGPROC array only once for the whole array. But in most
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* This could be made faster by having another entry point in procarray.c,
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* walking the PGPROC array only once for all the members. But in most
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* cases nmembers should be small enough that it doesn't much matter.
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*/
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for (i = 0; i < nmembers; i++)
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@@ -22,7 +22,7 @@
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* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.7 2004/12/31 21:59:29 pgsql Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.8 2005/05/19 21:35:45 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -30,7 +30,6 @@
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#include "access/slru.h"
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#include "access/subtrans.h"
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#include "storage/sinval.h"
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#include "utils/tqual.h"
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@@ -6,7 +6,7 @@
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* Copyright (c) 2000-2005, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.63 2005/04/13 18:54:56 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.64 2005/05/19 21:35:45 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -107,7 +107,7 @@ GetNewTransactionId(bool isSubXact)
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* nextXid are already present in PGPROC. Else we have a race
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* condition.
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*
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* XXX by storing xid into MyProc without acquiring SInvalLock, we are
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* XXX by storing xid into MyProc without acquiring ProcArrayLock, we are
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* relying on fetch/store of an xid to be atomic, else other backends
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* might see a partially-set xid here. But holding both locks at once
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* would be a nasty concurrency hit (and in fact could cause a
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@@ -120,8 +120,7 @@ GetNewTransactionId(bool isSubXact)
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*
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* A solution to the atomic-store problem would be to give each PGPROC
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* its own spinlock used only for fetching/storing that PGPROC's xid
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* and related fields. (SInvalLock would then mean primarily that
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* PGPROCs couldn't be added/removed while holding the lock.)
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* and related fields.
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*
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* If there's no room to fit a subtransaction XID into PGPROC, set the
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* cache-overflowed flag instead. This forces readers to look in
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@@ -10,7 +10,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.200 2005/04/28 21:47:10 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.201 2005/05/19 21:35:45 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -34,7 +34,7 @@
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#include "miscadmin.h"
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#include "storage/fd.h"
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#include "storage/proc.h"
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#include "storage/sinval.h"
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#include "storage/procarray.h"
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#include "storage/smgr.h"
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#include "utils/flatfiles.h"
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#include "utils/guc.h"
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@@ -1503,16 +1503,18 @@ CommitTransaction(void)
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* this must be done _before_ releasing locks we hold and _after_
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* RecordTransactionCommit.
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*
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* LWLockAcquire(SInvalLock) is required: UPDATE with xid 0 is blocked by
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* xid 1' UPDATE, xid 1 is doing commit while xid 2 gets snapshot - if
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* xid 2' GetSnapshotData sees xid 1 as running then it must see xid 0
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* as running as well or it will see two tuple versions - one deleted
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* by xid 1 and one inserted by xid 0. See notes in GetSnapshotData.
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* LWLockAcquire(ProcArrayLock) is required; consider this example:
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* UPDATE with xid 0 is blocked by xid 1's UPDATE.
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* xid 1 is doing commit while xid 2 gets snapshot.
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* If xid 2's GetSnapshotData sees xid 1 as running then it must see
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* xid 0 as running as well, or it will be able to see two tuple versions
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* - one deleted by xid 1 and one inserted by xid 0. See notes in
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* GetSnapshotData.
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*/
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if (MyProc != NULL)
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{
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/* Lock SInvalLock because that's what GetSnapshotData uses. */
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LWLockAcquire(SInvalLock, LW_EXCLUSIVE);
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/* Lock ProcArrayLock because that's what GetSnapshotData uses. */
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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MyProc->xid = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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@@ -1520,7 +1522,7 @@ CommitTransaction(void)
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MyProc->subxids.nxids = 0;
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MyProc->subxids.overflowed = false;
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LWLockRelease(SInvalLock);
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LWLockRelease(ProcArrayLock);
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}
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/*
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@@ -1688,8 +1690,8 @@ AbortTransaction(void)
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*/
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if (MyProc != NULL)
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{
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/* Lock SInvalLock because that's what GetSnapshotData uses. */
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LWLockAcquire(SInvalLock, LW_EXCLUSIVE);
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/* Lock ProcArrayLock because that's what GetSnapshotData uses. */
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LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
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MyProc->xid = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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@@ -1697,7 +1699,7 @@ AbortTransaction(void)
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MyProc->subxids.nxids = 0;
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MyProc->subxids.overflowed = false;
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LWLockRelease(SInvalLock);
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LWLockRelease(ProcArrayLock);
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}
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/*
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@@ -7,7 +7,7 @@
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* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.191 2005/05/10 22:27:29 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.192 2005/05/19 21:35:45 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@@ -38,7 +38,7 @@
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#include "storage/lwlock.h"
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#include "storage/pmsignal.h"
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#include "storage/proc.h"
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#include "storage/sinval.h"
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#include "storage/procarray.h"
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#include "storage/spin.h"
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#include "utils/builtins.h"
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#include "utils/guc.h"
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@@ -776,7 +776,7 @@ begin:;
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if (MyLastRecPtr.xrecoff == 0 && !no_tran)
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{
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/*
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* We do not acquire SInvalLock here because of possible deadlock.
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* We do not acquire ProcArrayLock here because of possible deadlock.
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* Anyone who wants to inspect other procs' logRec must acquire
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* WALInsertLock, instead. A better solution would be a per-PROC
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* spinlock, but no time for that before 7.2 --- tgl 12/19/01.
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@@ -4887,11 +4887,11 @@ CreateCheckPoint(bool shutdown, bool force)
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* commits after REDO point).
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*
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* XXX temporarily ifdef'd out to avoid three-way deadlock condition:
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* GetUndoRecPtr needs to grab SInvalLock to ensure that it is looking
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* at a stable set of proc records, but grabbing SInvalLock while
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* GetUndoRecPtr needs to grab ProcArrayLock to ensure that it is looking
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* at a stable set of proc records, but grabbing ProcArrayLock while
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* holding WALInsertLock is no good. GetNewTransactionId may cause a
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* WAL record to be written while holding XidGenLock, and
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* GetSnapshotData needs to get XidGenLock while holding SInvalLock,
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* GetSnapshotData needs to get XidGenLock while holding ProcArrayLock,
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* so there's a risk of deadlock. Need to find a better solution. See
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* pgsql-hackers discussion of 17-Dec-01.
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*
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