Andres Freund wrote:
> On 2013-11-22 12:04:31 -0300, Alvaro Herrera wrote:
> Yes, somewhat: <9.3 GetMultiXactIdMembers() didn't do any work if the
> multixact was old enough:
> if (MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyBackendId]))
> {
> debug_elog2(DEBUG2, "GetMembers: it's too old");
> *xids = NULL;
> return -1;
> }
> so, in OLTP style workloads, doing a heap_lock_tuple() often didn't have
> to perform any IO when locking a tuple that previously had been locked
> using a multixact. We knew that none of the members could still be
> running and thus didn't have to ask the SLRU for them since a
> not-running member cannot still have a row locked.
Right.
> Now, fkey locks changed that because for !HEAP_XMAX_LOCK_ONLY mxacts, we
> need to look up the members to get the update xid and check whether it
> has committed or aborted, even if we know that it isn't currently
> running anymore due do OldestVisibleMXactId. But there's absolutely no
> need to do that for HEAP_XMAX_LOCK_ONLY mxacts, the old reasoning for
> not looking up the members if old is just fine.
Correct. The only difficulty here is that we would need to pass down
the fact that we know for certain that this is only a locking Multixact.
There are some callers that go to it indirectly via MultiXactIdWait or
MultiXactIdExpand, but now that I look I think it's fine for those to
pass false (i.e. assume there might be an update and disable the
optimization), since those aren't hot compared to the other cases.
This patch implements this idea, but I haven't tested it much beyond
compiling and ensuring it passes the existing tests.
Regarding the outdated comment, I had a rewritten version of that
somewhere which I evidently forgot to push :-( I noticed it was
outdated a couple of weeks after I pushed the main patch.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
*** a/contrib/pgrowlocks/pgrowlocks.c
--- b/contrib/pgrowlocks/pgrowlocks.c
***************
*** 168,174 **** pgrowlocks(PG_FUNCTION_ARGS)
allow_old = !(infomask & HEAP_LOCK_MASK) &&
(infomask & HEAP_XMAX_LOCK_ONLY);
! nmembers = GetMultiXactIdMembers(xmax, &members, allow_old);
if (nmembers == -1)
{
values[Atnum_xids] = "{0}";
--- 168,175 ----
allow_old = !(infomask & HEAP_LOCK_MASK) &&
(infomask & HEAP_XMAX_LOCK_ONLY);
! nmembers = GetMultiXactIdMembers(xmax, &members, allow_old,
! false);
if (nmembers == -1)
{
values[Atnum_xids] = "{0}";
*** a/src/backend/access/heap/heapam.c
--- b/src/backend/access/heap/heapam.c
***************
*** 3987,3993 **** l3:
* the case, HeapTupleSatisfiesUpdate would have returned
* MayBeUpdated and we wouldn't be here.
*/
! nmembers = GetMultiXactIdMembers(xwait, &members, false);
for (i = 0; i < nmembers; i++)
{
--- 3987,3995 ----
* the case, HeapTupleSatisfiesUpdate would have returned
* MayBeUpdated and we wouldn't be here.
*/
! nmembers =
! GetMultiXactIdMembers(xwait, &members, false,
! HEAP_XMAX_IS_LOCKED_ONLY(infomask));
for (i = 0; i < nmembers; i++)
{
***************
*** 4008,4014 **** l3:
}
}
! pfree(members);
}
/*
--- 4010,4017 ----
}
}
! if (members)
! pfree(members);
}
/*
***************
*** 4157,4163 **** l3:
* been the case, HeapTupleSatisfiesUpdate would have returned
* MayBeUpdated and we wouldn't be here.
*/
! nmembers = GetMultiXactIdMembers(xwait, &members, false);
if (nmembers <= 0)
{
--- 4160,4168 ----
* been the case, HeapTupleSatisfiesUpdate would have returned
* MayBeUpdated and we wouldn't be here.
*/
! nmembers =
! GetMultiXactIdMembers(xwait, &members, false,
! HEAP_XMAX_IS_LOCKED_ONLY(infomask));
if (nmembers <= 0)
{
***************
*** 5217,5223 **** GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
* We only use this in multis we just created, so they cannot be values
* pre-pg_upgrade.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false);
for (i = 0; i < nmembers; i++)
{
--- 5222,5228 ----
* We only use this in multis we just created, so they cannot be values
* pre-pg_upgrade.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false, false);
for (i = 0; i < nmembers; i++)
{
***************
*** 5293,5299 **** MultiXactIdGetUpdateXid(TransactionId xmax, uint16 t_infomask)
* Since we know the LOCK_ONLY bit is not set, this cannot be a multi from
* pre-pg_upgrade.
*/
! nmembers = GetMultiXactIdMembers(xmax, &members, false);
if (nmembers > 0)
{
--- 5298,5304 ----
* Since we know the LOCK_ONLY bit is not set, this cannot be a multi from
* pre-pg_upgrade.
*/
! nmembers = GetMultiXactIdMembers(xmax, &members, false, false);
if (nmembers > 0)
{
***************
*** 5376,5382 **** Do_MultiXactIdWait(MultiXactId multi, MultiXactStatus status,
int remain = 0;
allow_old = !(infomask & HEAP_LOCK_MASK) && HEAP_XMAX_IS_LOCKED_ONLY(infomask);
! nmembers = GetMultiXactIdMembers(multi, &members, allow_old);
if (nmembers >= 0)
{
--- 5381,5388 ----
int remain = 0;
allow_old = !(infomask & HEAP_LOCK_MASK) && HEAP_XMAX_IS_LOCKED_ONLY(infomask);
! nmembers = GetMultiXactIdMembers(multi, &members, allow_old,
! HEAP_XMAX_IS_LOCKED_ONLY(infomask));
if (nmembers >= 0)
{
*** a/src/backend/access/transam/multixact.c
--- b/src/backend/access/transam/multixact.c
***************
*** 391,397 **** MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
* caller of this function does a check that the multixact is no longer
* running.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false);
if (nmembers < 0)
{
--- 391,397 ----
* caller of this function does a check that the multixact is no longer
* running.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false, false);
if (nmembers < 0)
{
***************
*** 489,495 **** MultiXactIdIsRunning(MultiXactId multi)
* "false" here means we assume our callers have checked that the given
* multi cannot possibly come from a pg_upgraded database.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false);
if (nmembers < 0)
{
--- 489,495 ----
* "false" here means we assume our callers have checked that the given
* multi cannot possibly come from a pg_upgraded database.
*/
! nmembers = GetMultiXactIdMembers(multi, &members, false, false);
if (nmembers < 0)
{
***************
*** 1029,1038 **** GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
* the value currently known as the next to assign, raise an error. Previously
* these also returned -1, but since this can lead to the wrong visibility
* results, it is dangerous to do that.
*/
int
GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
! bool allow_old)
{
int pageno;
int prev_pageno;
--- 1029,1043 ----
* the value currently known as the next to assign, raise an error. Previously
* these also returned -1, but since this can lead to the wrong visibility
* results, it is dangerous to do that.
+ *
+ * onlyLock must be set to true if caller is certain that the given multi
+ * is used only to lock tuples; can be false without loss of correctness,
+ * but passing a true means we can return quickly without checking for
+ * old updates.
*/
int
GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
! bool allow_old, bool onlyLock)
{
int pageno;
int prev_pageno;
***************
*** 1066,1071 **** GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
--- 1071,1089 ----
MultiXactIdSetOldestVisible();
/*
+ * If we know the multi is used only for locking and not for updates,
+ * then we can skip checking if the value is older than our oldest
+ * visible multi. It cannot possibly still be running.
+ */
+ if (onlyLock &&
+ MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyBackendId]))
+ {
+ debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
+ *members = NULL;
+ return -1;
+ }
+
+ /*
* We check known limits on MultiXact before resorting to the SLRU area.
*
* An ID older than MultiXactState->oldestMultiXactId cannot possibly be
***************
*** 2507,2513 **** pg_get_multixact_members(PG_FUNCTION_ARGS)
multi = palloc(sizeof(mxact));
/* no need to allow for old values here */
! multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false);
multi->iter = 0;
tupdesc = CreateTemplateTupleDesc(2, false);
--- 2525,2532 ----
multi = palloc(sizeof(mxact));
/* no need to allow for old values here */
! multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
! false);
multi->iter = 0;
tupdesc = CreateTemplateTupleDesc(2, false);
*** a/src/include/access/multixact.h
--- b/src/include/access/multixact.h
***************
*** 85,91 **** extern MultiXactId ReadNextMultiXactId(void);
extern bool MultiXactIdIsRunning(MultiXactId multi);
extern void MultiXactIdSetOldestMember(void);
extern int GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **xids,
! bool allow_old);
extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
extern void AtEOXact_MultiXact(void);
--- 85,91 ----
extern bool MultiXactIdIsRunning(MultiXactId multi);
extern void MultiXactIdSetOldestMember(void);
extern int GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **xids,
! bool allow_old, bool lockOnly);
extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
extern void AtEOXact_MultiXact(void);
--
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