Spurious standby query cancellations
In ResolveRecoveryConflictWithLock, there is the comment:
/*
* If blowing away everybody with conflicting locks doesn't work, after
* the first two attempts then we just start blowing everybody away
until
* it does work.
But what it does is something different than that.
At least under some conditions, it will wait patiently for all initially
conflicting locks to go away. If that doesn't work twice (because new
lockers joined while we were waiting for the old ones to go away), then it
will wait patiently for all transactions throughout the system to go away
even if they don't conflict, perhaps not even realizing that the lock has
become free in the mean time.
Then when its patience runs out, it kills everything on the system. But it
never tried to blow away just the conflicting locks, instead it just tried
to wait them out.
The fact that trying to wait them out didn't work (twice) doesn't mean that
killing them wouldn't have worked.
I think that it was intended that num_attempts would get incremented only
once WaitExceedsMaxStandbyDelay becomes true, but that is not what happens
with the current code.
Currently WaitExceedsMaxStandbyDelay only has one caller. I think it we
should take the sleep code out of that function and move it into the
existing call site, and then have ResolveRecoveryConflictWithLock check
with WaitExceedsMaxStandbyDelay before incrementing num_attempts.
Cheers,
Jeff
Attachments:
standby_lock_cancel_v1.patchapplication/octet-stream; name=standby_lock_cancel_v1.patchDownload+20-16
On 27 August 2015 at 22:55, Jeff Janes <jeff.janes@gmail.com> wrote:
In ResolveRecoveryConflictWithLock, there is the comment:
/*
* If blowing away everybody with conflicting locks doesn't work, after
* the first two attempts then we just start blowing everybody away
until
* it does work.But what it does is something different than that.
At least under some conditions, it will wait patiently for all initially
conflicting locks to go away. If that doesn't work twice (because new
lockers joined while we were waiting for the old ones to go away), then it
will wait patiently for all transactions throughout the system to go away
even if they don't conflict, perhaps not even realizing that the lock has
become free in the mean time.Then when its patience runs out, it kills everything on the system. But
it never tried to blow away just the conflicting locks, instead it just
tried to wait them out.The fact that trying to wait them out didn't work (twice) doesn't mean
that killing them wouldn't have worked.I think that it was intended that num_attempts would get incremented only
once WaitExceedsMaxStandbyDelay becomes true, but that is not what happens
with the current code.Currently WaitExceedsMaxStandbyDelay only has one caller. I think it we
should take the sleep code out of that function and move it into the
existing call site, and then have ResolveRecoveryConflictWithLock check
with WaitExceedsMaxStandbyDelay before incrementing num_attempts.
I agree with the problem in the current code, thank you for spotting it.
I also agree that the proposed solution would work-ish, if we are
suggesting backpatching this.
It's now possible to fix this by putting a lock wait on the actual lock
request, which wasn't available when I first wrote that, hence the crappy
wait loop. Using the timeout handler would now be the preferred way to
solve this. We can backpatch that to 9.3 if needed, where they were
introduced.
There's an example of how to use lock waits further down
on ResolveRecoveryConflictWithBufferPin(). Could you have a look at doing
it that way?
We probably also need to resurrect my earlier patch to avoid logging
AccessExclusiveLocks on temp tables.
--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On Fri, Sep 4, 2015 at 3:25 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
On 27 August 2015 at 22:55, Jeff Janes <jeff.janes@gmail.com> wrote:
In ResolveRecoveryConflictWithLock, there is the comment:
/*
* If blowing away everybody with conflicting locks doesn't work,
after
* the first two attempts then we just start blowing everybody away
until
* it does work.But what it does is something different than that.
At least under some conditions, it will wait patiently for all initially
conflicting locks to go away. If that doesn't work twice (because new
lockers joined while we were waiting for the old ones to go away), then it
will wait patiently for all transactions throughout the system to go away
even if they don't conflict, perhaps not even realizing that the lock has
become free in the mean time.Then when its patience runs out, it kills everything on the system. But
it never tried to blow away just the conflicting locks, instead it just
tried to wait them out.The fact that trying to wait them out didn't work (twice) doesn't mean
that killing them wouldn't have worked.I think that it was intended that num_attempts would get incremented only
once WaitExceedsMaxStandbyDelay becomes true, but that is not what happens
with the current code.Currently WaitExceedsMaxStandbyDelay only has one caller. I think it we
should take the sleep code out of that function and move it into the
existing call site, and then have ResolveRecoveryConflictWithLock check
with WaitExceedsMaxStandbyDelay before incrementing num_attempts.I agree with the problem in the current code, thank you for spotting it.
I also agree that the proposed solution would work-ish, if we are
suggesting backpatching this.
I wasn't specifically thinking of backpatching it, but if it doesn't
operate the way you intended it to, it might make sense to do that.
I stumbled on this while experimentally looking into a different issue (the
ProcArrayLock contention issue in HeapTupleSatisfiesMVCC that was recently
ameliorated in 8a7d0701814, but the variant of that issue reported
in "[PERFORM] Strange query stalls on replica in 9.3.9"). I haven't heard
of any complaints from the field about too-frequent query cancellations,
but I also don't have my "ear to the ground" in that area.
It's now possible to fix this by putting a lock wait on the actual lock
request, which wasn't available when I first wrote that, hence the crappy
wait loop. Using the timeout handler would now be the preferred way to
solve this. We can backpatch that to 9.3 if needed, where they were
introduced.There's an example of how to use lock waits further down
on ResolveRecoveryConflictWithBufferPin(). Could you have a look at doing
it that way?
It looks like this will take some major surgery. The heavy weight lock
manager doesn't play well with others when it comes to timeouts other than
its own. LockBufferForCleanup is a simple retry loop, but the lock manager
is much more complicated than that.
Here are some approaches I can think of:
Approach one, replace LockAcquireExtended's dontWait boolean with a
"waitUntil" timeout value. Queue the lock like ordinary
(non-recovery-backend) lock requests are queued so that new requestors are
not granted new locks which conflict with what we want. If the timeout
expires without the lock being acquired, dequeue ourselves and clean up,
and then return LOCKACQUIRE_NOT_AVAIL. I think something would have to be
done about deadlocks, as well, as I don't think the regular deadlock
detectors works in the standby startup process.
Approach two, same as one but when the timeout expires we invoke a callback
to terminate the blocking processes without dequeueing ourselves. That way
no one can sneak in and grab the lock during the time we were doing the
terminations. This way we are guaranteed to succeed after one round of
terminations, and there is no need to terminate innocent bystanders. I
think this is my preferred solution to have. (But I haven't had much luck
implementing it beyond the hand-wavy stages.)
Approach three, ask for the lock with dontWait as we do now, but if we
don't get the lock then somehow arrange for us to be signalled when the
lock becomes free, without being queued for it. Then we would have to
retry, with no guarantee the retry would be successful. Functionally this
would be no different than the simple patch I gave, except the polling loop
is much more efficient. You still have to terminate processes if a stream
of new requestors take the lock in a constantly overlapping basis.
We probably also need to resurrect my earlier patch to avoid logging
AccessExclusiveLocks on temp tables.
I couldn't find that, can you provide a link?
Another source of frequent AccessExclusiveLocks is vacuum truncation
attempts. If a table has some occupied pages right at the end which are
marked as all-visible, then forward scan doesn't visit them. Since it
didn't visit them, it assumes they might be empty and truncatable and so
takes the AccessExclusiveLock, only to immediately see that the last page
is not empty. Perhaps the forward scan could be special-cased to never
skip the final block of a table. That way if it is not empty, the
truncation is abandoned before taking the AccessExclusiveLock.
Cheers,
Jeff
On 14 September 2015 at 12:00, Jeff Janes <jeff.janes@gmail.com> wrote:
It's now possible to fix this by putting a lock wait on the actual lock
request, which wasn't available when I first wrote that, hence the crappy
wait loop. Using the timeout handler would now be the preferred way to
solve this. We can backpatch that to 9.3 if needed, where they were
introduced.There's an example of how to use lock waits further down
on ResolveRecoveryConflictWithBufferPin(). Could you have a look at doing
it that way?It looks like this will take some major surgery. The heavy weight lock
manager doesn't play well with others when it comes to timeouts other than
its own. LockBufferForCleanup is a simple retry loop, but the lock manager
is much more complicated than that.
Not sure I understand this objection. I can't see a reason that my proposal
wouldn't work.
We probably also need to resurrect my earlier patch to avoid logging
AccessExclusiveLocks on temp tables.I couldn't find that, can you provide a link?
/messages/by-id/CA+U5nMJYGYS+kccJ+=aQxuC_G09hF6zsD-xhu5W9oqGNkbE6nQ@mail.gmail.com
Another source of frequent AccessExclusiveLocks is vacuum truncation
attempts. If a table has some occupied pages right at the end which are
marked as all-visible, then forward scan doesn't visit them. Since it
didn't visit them, it assumes they might be empty and truncatable and so
takes the AccessExclusiveLock, only to immediately see that the last page
is not empty. Perhaps the forward scan could be special-cased to never
skip the final block of a table. That way if it is not empty, the
truncation is abandoned before taking the AccessExclusiveLock.
We can probably get rid of that lock, but it will require lots of smoke and
mirrors to persuade scans that they don't actually need to scan as far as
they thought they did, because a VACUUM has checked and it knows those
blocks do not contain tuples visible to the scan. Which sounds a little
hairy.
I think Andres is working on putting an end of data watermark in shared
memory rather than using the end of physical file, which sounds like a
better plan.
--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On Wed, Sep 16, 2015 at 2:44 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
On 14 September 2015 at 12:00, Jeff Janes <jeff.janes@gmail.com> wrote:
It's now possible to fix this by putting a lock wait on the actual lock
request, which wasn't available when I first wrote that, hence the crappy
wait loop. Using the timeout handler would now be the preferred way to
solve this. We can backpatch that to 9.3 if needed, where they were
introduced.There's an example of how to use lock waits further down
on ResolveRecoveryConflictWithBufferPin(). Could you have a look at doing
it that way?It looks like this will take some major surgery. The heavy weight lock
manager doesn't play well with others when it comes to timeouts other than
its own. LockBufferForCleanup is a simple retry loop, but the lock manager
is much more complicated than that.Not sure I understand this objection. I can't see a reason that my
proposal wouldn't work.
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock
to be called back from the lmgr code so that is it like
ResolveRecoveryConflictWithBufferPin
code. It does not try to cancel the conflicting lock holders from the
signal handler, rather it just loops an extra time and cancels the
transactions on the next call.
It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, so I
didn't do a timeout for deadlock detection purposes.
Cheers,
Jeff
Attachments:
standby_lock_cancel_v2.patchtext/x-patch; charset=US-ASCII; name=standby_lock_cancel_v2.patchDownload+145-110
On Thu, Sep 24, 2015 at 3:33 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Sep 16, 2015 at 2:44 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
On 14 September 2015 at 12:00, Jeff Janes <jeff.janes@gmail.com> wrote:
It's now possible to fix this by putting a lock wait on the actual lock
request, which wasn't available when I first wrote that, hence the crappy
wait loop. Using the timeout handler would now be the preferred way to solve
this. We can backpatch that to 9.3 if needed, where they were introduced.There's an example of how to use lock waits further down on
ResolveRecoveryConflictWithBufferPin(). Could you have a look at doing it
that way?It looks like this will take some major surgery. The heavy weight lock
manager doesn't play well with others when it comes to timeouts other than
its own. LockBufferForCleanup is a simple retry loop, but the lock manager
is much more complicated than that.Not sure I understand this objection. I can't see a reason that my
proposal wouldn't work.On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code so that
is it like ResolveRecoveryConflictWithBufferPin code. It does not try to
cancel the conflicting lock holders from the signal handler, rather it just
loops an extra time and cancels the transactions on the next call.It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, so I
didn't do a timeout for deadlock detection purposes.
Patch moved to next CF because of a lack of reviews. Simon is
registered as reviewer, hence I guess that the ball is on his side of
the field.
--
Michael
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On Wed, Sep 23, 2015 at 11:33 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code so that
is it like ResolveRecoveryConflictWithBufferPin code. It does not try to
cancel the conflicting lock holders from the signal handler, rather it just
loops an extra time and cancels the transactions on the next call.It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, so I
didn't do a timeout for deadlock detection purposes.
I was testing that this still applies to git HEAD. And it doesn't due
to the re-factoring of lock.h into lockdef.h. (And apparently it
never actually did, because that refactoring happened long before I
wrote this patch. I guess I must have done this work against
9_5_STABLE.)
standby.h cannot include lock.h because standby.h is included
indirectly by pg_xlogdump. But it has to get LOCKTAG which is only in
lock.h.
Does this mean that standby.h also needs to get parts spun off into a
new standbydef.h that can be included from front-end code?
standby.h doesn't need to know the internals of LOCKTAG. It just
needs to declare a function that receives it as an opaque pointer. I
don't know if that info helps resolve the situation, though.
Cheers,
Jeff
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On Wed, Dec 23, 2015 at 9:40 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Sep 23, 2015 at 11:33 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code so that
is it like ResolveRecoveryConflictWithBufferPin code. It does not try to
cancel the conflicting lock holders from the signal handler, rather it just
loops an extra time and cancels the transactions on the next call.It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, so I
didn't do a timeout for deadlock detection purposes.I was testing that this still applies to git HEAD. And it doesn't due
to the re-factoring of lock.h into lockdef.h. (And apparently it
never actually did, because that refactoring happened long before I
wrote this patch. I guess I must have done this work against
9_5_STABLE.)standby.h cannot include lock.h because standby.h is included
indirectly by pg_xlogdump. But it has to get LOCKTAG which is only in
lock.h.Does this mean that standby.h also needs to get parts spun off into a
new standbydef.h that can be included from front-end code?
That is how I've done it.
The lock cancel patch applies over the header split patch.
Cheers,
Jeff
On 24 December 2015 at 20:15, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Dec 23, 2015 at 9:40 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Sep 23, 2015 at 11:33 PM, Jeff Janes <jeff.janes@gmail.com>
wrote:
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code sothat
is it like ResolveRecoveryConflictWithBufferPin code. It does not try
to
cancel the conflicting lock holders from the signal handler, rather it
just
loops an extra time and cancels the transactions on the next call.
It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, soI
didn't do a timeout for deadlock detection purposes.
I was testing that this still applies to git HEAD. And it doesn't due
to the re-factoring of lock.h into lockdef.h. (And apparently it
never actually did, because that refactoring happened long before I
wrote this patch. I guess I must have done this work against
9_5_STABLE.)standby.h cannot include lock.h because standby.h is included
indirectly by pg_xlogdump. But it has to get LOCKTAG which is only in
lock.h.Does this mean that standby.h also needs to get parts spun off into a
new standbydef.h that can be included from front-end code?That is how I've done it.
The lock cancel patch applies over the header split patch.
This looks good to me, apart from some WhitespaceCrime.
Header split applied, will test and apply the main patch this week.
--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Simon Riggs wrote:
This looks good to me, apart from some WhitespaceCrime.
Header split applied, will test and apply the main patch this week.
Since the patch already appears to have a committer's attention, it's
okay to move it to the next commitfest; if Simon happens to commit ahead
of time, that would be a great outcome too.
--
�lvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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On Fri, Dec 25, 2015 at 1:45 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Dec 23, 2015 at 9:40 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Sep 23, 2015 at 11:33 PM, Jeff Janes <jeff.janes@gmail.com>
wrote:
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code
so that
is it like ResolveRecoveryConflictWithBufferPin code. It does not try
to
cancel the conflicting lock holders from the signal handler, rather it
just
loops an extra time and cancels the transactions on the next call.
It looks like the deadlock detection is adequately handled within
normal
lmgr code within the back-ends of the other parties to the deadlock,
so I
didn't do a timeout for deadlock detection purposes.
I was testing that this still applies to git HEAD. And it doesn't due
to the re-factoring of lock.h into lockdef.h. (And apparently it
never actually did, because that refactoring happened long before I
wrote this patch. I guess I must have done this work against
9_5_STABLE.)standby.h cannot include lock.h because standby.h is included
indirectly by pg_xlogdump. But it has to get LOCKTAG which is only in
lock.h.Does this mean that standby.h also needs to get parts spun off into a
new standbydef.h that can be included from front-end code?That is how I've done it.
The lock cancel patch applies over the header split patch.
Review Comments -
1.
/*
* If somebody wakes us between LWLockRelease and WaitLatch, the latch
--- 1081,1103 ----
* If
LockTimeout is set, also enable the timeout for that. We can save a
* few cycles by enabling both
timeout sources in one call.
*/
! if (!InHotStandby)
{
! if (LockTimeout > 0)
!
{
! EnableTimeoutParams timeouts[2];
!
! timeouts[0].id =
DEADLOCK_TIMEOUT;
! timeouts[0].type = TMPARAM_AFTER;
! timeouts
[0].delay_ms = DeadlockTimeout;
! timeouts[1].id = LOCK_TIMEOUT;
!
timeouts[1].type = TMPARAM_AFTER;
! timeouts[1].delay_ms = LockTimeout;
!
enable_timeouts(timeouts, 2);
! }
! else
! enable_timeout_after
(DEADLOCK_TIMEOUT, DeadlockTimeout);
}If you are enabling the timeout only in non-hotstandby mode, then
later in code (in same function) it should be disabled under the same
condition, otherwise it will lead to assertion failure.
2.
+ /*
+ * Clear any timeout requests established above. We assume here that the
+ * Startup
process doesn't have any other outstanding timeouts than what
+ * this function
+ * uses. If
that stops being true, we could cancel the timeouts
+ * individually, but that'd be slower.
+ */Comment seems to be misaligned.
3.
+ void
+ StandbyLockTimeoutHandler(void)
+ {
+ /* forget any pending STANDBY_DEADLOCK_TIMEOUT request */
+
disable_timeout(STANDBY_DEADLOCK_TIMEOUT, false);
+ }Is there any reason to disable deadlock timeout when it is not
enabled by ResolveRecoveryConflictWithLock()?
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
On 24 December 2015 at 20:15, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Dec 23, 2015 at 9:40 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Sep 23, 2015 at 11:33 PM, Jeff Janes <jeff.janes@gmail.com>
wrote:
On further thought, neither do I. The attached patch inverts
ResolveRecoveryConflictWithLock to be called back from the lmgr code sothat
is it like ResolveRecoveryConflictWithBufferPin code. It does not try
to
cancel the conflicting lock holders from the signal handler, rather it
just
loops an extra time and cancels the transactions on the next call.
It looks like the deadlock detection is adequately handled within normal
lmgr code within the back-ends of the other parties to the deadlock, soI
didn't do a timeout for deadlock detection purposes.
That is how I've done it.
It's taken me a while to figure this out.
My testing showed a bug in disable_timeout(), which turns out to be a
double-disable, which I've fixed. I'll submit a different patch to put in
some diagnostics if such cases show up again, which could happen now we
have user-defined timeouts.
What surprises me is that I can't see this patch ever worked as submitted,
when run on an assert-enabled build.
If you want this backpatched, please submit versions that apply cleanly and
test them. I'm less inclined to do that myself, just regard this as an
improvement.
--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services