Re: Some problems of recovery conflict wait events

On Thu, 2 Apr 2020 at 15:34, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>
>
>
> On 2020/04/02 14:25, Masahiko Sawada wrote:
> > On Wed, 1 Apr 2020 at 22:32, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
> >>
> >>
> >>
> >> On 2020/03/30 20:10, Masahiko Sawada wrote:
> >>> On Fri, 27 Mar 2020 at 17:54, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2020/03/04 14:31, Masahiko Sawada wrote:
> >>>>> On Wed, 4 Mar 2020 at 13:48, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 2020/03/04 13:27, Michael Paquier wrote:
> >>>>>>> On Wed, Mar 04, 2020 at 01:13:19PM +0900, Masahiko Sawada wrote:
> >>>>>>>> Yeah, so 0001 patch sets existing wait events to recovery conflict
> >>>>>>>> resolution. For instance, it sets (PG_WAIT_LOCK | LOCKTAG_TRANSACTION)
> >>>>>>>> to the recovery conflict on a snapshot. 0003 patch improves these wait
> >>>>>>>> events by adding the new type of wait event such as
> >>>>>>>> WAIT_EVENT_RECOVERY_CONFLICT_SNAPSHOT. Therefore 0001 (and 0002) patch
> >>>>>>>> is the fix for existing versions and 0003 patch is an improvement for
> >>>>>>>> only PG13. Did you mean even 0001 patch doesn't fit for back-patching?
> >>>>>>
> >>>>>> Yes, it looks like a improvement rather than bug fix.
> >>>>>>
> >>>>>
> >>>>> Okay, understand.
> >>>>>
> >>>>>>> I got my eyes on this patch set. The full patch set is in my opinion
> >>>>>>> just a set of improvements, and not bug fixes, so I would refrain from
> >>>>>>> back-backpatching.
> >>>>>>
> >>>>>> I think that the issue (i.e., "waiting" is reported twice needlessly
> >>>>>> in PS display) that 0002 patch tries to fix is a bug. So it should be
> >>>>>> fixed even in the back branches.
> >>>>>
> >>>>> So we need only two patches: one fixes process title issue and another
> >>>>> improve wait event. I've attached updated patches.
> >>>>
> >>>> I started reading v2-0002-Improve-wait-events-for-recovery-conflict-resolut.patch.
> >>>>
> >>>> - ProcWaitForSignal(PG_WAIT_BUFFER_PIN);
> >>>> + ProcWaitForSignal(WAIT_EVENT_RECOVERY_CONFLICT_BUFFER_PIN);
> >>>>
> >>>> Currently the wait event indicating the wait for buffer pin has already
> >>>> been reported. But the above change in the patch changes the name of
> >>>> wait event for buffer pin only in the startup process. Is this really useful?
> >>>> Isn't the existing wait event for buffer pin enough?
> >>>>
> >>>> - /* Wait to be signaled by the release of the Relation Lock */
> >>>> - ProcWaitForSignal(PG_WAIT_LOCK | locktag.locktag_type);
> >>>> + /* Wait to be signaled by the release of the Relation Lock */
> >>>> + ProcWaitForSignal(WAIT_EVENT_RECOVERY_CONFLICT_LOCK);
> >>>>
> >>>> Same as above. Isn't the existing wait event enough?
> >>>
> >>> Yeah, we can use the existing wait events for buffer pin and lock.
> >>>
> >>>>
> >>>> - /*
> >>>> - * Progressively increase the sleep times, but not to more than 1s, since
> >>>> - * pg_usleep isn't interruptible on some platforms.
> >>>> - */
> >>>> - standbyWait_us *= 2;
> >>>> - if (standbyWait_us > 1000000)
> >>>> - standbyWait_us = 1000000;
> >>>> + WaitLatch(MyLatch,
> >>>> + WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT,
> >>>> + STANDBY_WAIT_MS,
> >>>> + wait_event_info);
> >>>> + ResetLatch(MyLatch);
> >>>>
> >>>> ResetLatch() should be called before WaitLatch()?
> >>>
> >>> Fixed.
> >>>
> >>>>
> >>>> Could you tell me why you dropped the "increase-sleep-times" logic?
> >>>
> >>> I thought we can remove it because WaitLatch is interruptible but my
> >>> observation was not correct. The waiting startup process is not
> >>> necessarily woken up by signal. I think it's still better to not wait
> >>> more than 1 sec even if it's an interruptible wait.
> >>
> >> So we don't need to use WaitLatch() there, i.e., it's ok to keep using
> >> pg_usleep()?
> >>
> >>> Attached patch fixes the above and introduces only two wait events of
> >>> conflict resolution: snapshot and tablespace.
> >>
> >> Many thanks for updating the patch!
> >>
> >> - /* Wait to be signaled by the release of the Relation Lock */
> >> - ProcWaitForSignal(PG_WAIT_LOCK | locktag.locktag_type);
> >> + /* Wait to be signaled by the release of the Relation Lock */
> >> + ProcWaitForSignal(PG_WAIT_LOCK | locktag.locktag_type);
> >> + }
> >>
> >> Is this change really valid? What happens if the latch is set during
> >> ResolveRecoveryConflictWithVirtualXIDs()?
> >> ResolveRecoveryConflictWithVirtualXIDs() can return after the latch
> >> is set but before WaitLatch() in WaitExceedsMaxStandbyDelay() is reached.
> >
> > Thank you for reviewing the patch!
> >
> > You're right. It's better to keep using pg_usleep() and set the wait
> > event by pgstat_report_wait_start().
> >
> >>
> >> + default:
> >> + event_name = "unknown wait event";
> >> + break;
> >>
> >> Seems this default case should be removed. Please see other
> >> pgstat_get_wait_xxx() function, so there is no such code.
> >>
> >>> I also removed the wait
> >>> event of conflict resolution of database since it's unlikely to become
> >>> a user-visible and a long sleep as we discussed before.
> >>
> >> Is it worth defining new wait event type RecoveryConflict only for
> >> those two events? ISTM that it's ok to use IPC event type here.
> >>
> >
> > I dropped a new wait even type and added them to IPC wait event type.
> >
> > I've attached the new version patch.
>
> Thanks for updating the patch! The patch looks good to me except
> the following mior things.
>
> + <row>
> + <entry><literal>RecoveryConflictSnapshot</literal></entry>
> + <entry>Waiting for recovery conflict resolution on a physical cleanup.</entry>
> + </row>
> + <row>
> + <entry><literal>RecoveryConflictTablespace</literal></entry>
> + <entry>Waiting for recovery conflict resolution on dropping tablespace.</entry>
> + </row>
>
> You need to increment the value of "morerows" in
> "<entry morerows="38"><literal>IPC</literal></entry>".
>
> The descriptions of those two events should be placed in alphabetical order
> for event name. That is, they should be placed above RecoveryPause.
>
> "vacuum cleanup" is better than "physical cleanup"?

Agreed.

I've attached the updated version patch.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
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