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Restructure subtransaction handling to reduce resource consumption,

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as per recent discussions.  Invent SubTransactionIds that are managed like
CommandIds (ie, counter is reset at start of each top transaction), and
use these instead of TransactionIds to keep track of subtransaction status
in those modules that need it.  This means that a subtransaction does not
need an XID unless it actually inserts/modifies rows in the database.
Accordingly, don't assign it an XID nor take a lock on the XID until it
tries to do that.  This saves a lot of overhead for subtransactions that
are only used for error recovery (eg plpgsql exceptions).  Also, arrange
to release a subtransaction's XID lock as soon as the subtransaction
exits, in both the commit and abort cases.  This avoids holding many
unique locks after a long series of subtransactions.  The price is some
additional overhead in XactLockTableWait, but that seems acceptable.
Finally, restructure the state machine in xact.c to have a more orthogonal
set of states for subtransactions.
This commit is contained in:
Tom Lane
2004-09-16 16:58:44 +00:00
parent 42c0d1f3cd
commit 8f9f198603
34 changed files with 1220 additions and 945 deletions
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62
src/backend/storage/lmgr/lmgr.c
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@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.69 2004/08/29 05:06:48 momjian Exp $
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.70 2004/09/16 16:58:33 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -21,6 +21,7 @@
#include "catalog/catalog.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
#include "storage/sinval.h"
#include "utils/inval.h"
@@ -311,9 +312,6 @@ UnlockPage(Relation relation, BlockNumber blkno, LOCKMODE lockmode)
* Insert a lock showing that the given transaction ID is running ---
* this is done during xact startup. The lock can then be used to wait
* for the transaction to finish.
*
* We need no corresponding unlock function, since the lock will always
* be released implicitly at transaction commit/abort, never any other way.
*/
void
XactLockTableInsert(TransactionId xid)
@@ -325,19 +323,43 @@ XactLockTableInsert(TransactionId xid)
tag.dbId = InvalidOid; /* xids are globally unique */
tag.objId.xid = xid;
if (!LockAcquire(LockTableId, &tag, xid,
if (!LockAcquire(LockTableId, &tag, GetTopTransactionId(),
ExclusiveLock, false))
elog(ERROR, "LockAcquire failed");
}
/*
* XactLockTableDelete
*
* Delete the lock showing that the given transaction ID is running.
* (This is never used for main transaction IDs; those locks are only
* released implicitly at transaction end. But we do use it for subtrans
* IDs.)
*/
void
XactLockTableDelete(TransactionId xid)
{
LOCKTAG tag;
MemSet(&tag, 0, sizeof(tag));
tag.relId = XactLockTableId;
tag.dbId = InvalidOid; /* xids are globally unique */
tag.objId.xid = xid;
LockRelease(LockTableId, &tag, GetTopTransactionId(), ExclusiveLock);
}
/*
* XactLockTableWait
*
* Wait for the specified transaction to commit or abort.
*
* Note that this does the right thing for subtransactions: if we
* wait on a subtransaction, we will be awakened as soon as it aborts
* or its parent commits.
* Note that this does the right thing for subtransactions: if we wait on a
* subtransaction, we will exit as soon as it aborts or its top parent commits.
* It takes some extra work to ensure this, because to save on shared memory
* the XID lock of a subtransaction is released when it ends, whether
* successfully or unsuccessfully. So we have to check if it's "still running"
* and if so wait for its parent.
*/
void
XactLockTableWait(TransactionId xid)
@@ -345,18 +367,24 @@ XactLockTableWait(TransactionId xid)
LOCKTAG tag;
TransactionId myxid = GetTopTransactionId();
Assert(!TransactionIdEquals(xid, myxid));
for (;;)
{
Assert(TransactionIdIsValid(xid));
Assert(!TransactionIdEquals(xid, myxid));
MemSet(&tag, 0, sizeof(tag));
tag.relId = XactLockTableId;
tag.dbId = InvalidOid;
tag.objId.xid = xid;
MemSet(&tag, 0, sizeof(tag));
tag.relId = XactLockTableId;
tag.dbId = InvalidOid;
tag.objId.xid = xid;
if (!LockAcquire(LockTableId, &tag, myxid,
ShareLock, false))
elog(ERROR, "LockAcquire failed");
if (!LockAcquire(LockTableId, &tag, myxid, ShareLock, false))
elog(ERROR, "LockAcquire failed");
LockRelease(LockTableId, &tag, myxid, ShareLock);
LockRelease(LockTableId, &tag, myxid, ShareLock);
if (!TransactionIdIsInProgress(xid))
break;
xid = SubTransGetParent(xid);
}
/*
* Transaction was committed/aborted/crashed - we have to update