Re: Some problems of recovery conflict wait events - Mailing list pgsql-hackers

On Mon, 9 Mar 2020 at 13:24, Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
>
>
> On 2020/03/08 13:52, Masahiko Sawada wrote:
> > On Thu, 5 Mar 2020 at 20:16, Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>
> >>
> >>
> >> On 2020/03/05 16:58, Masahiko Sawada wrote:
> >>> On Wed, 4 Mar 2020 at 15:21, Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2020/03/04 14:31, Masahiko Sawada wrote:
> >>>>> On Wed, 4 Mar 2020 at 13:48, Fujii Masao <masao.fujii@oss.nttdata.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.
> >>>>
> >>>> Thanks for updating the patches! I started reading 0001 patch.
> >>>
> >>> Thank you for reviewing this patch.
> >>>
> >>>>
> >>>> -                       /*
> >>>> -                        * Report via ps if we have been waiting for more than 500 msec
> >>>> -                        * (should that be configurable?)
> >>>> -                        */
> >>>> -                       if (update_process_title && new_status == NULL &&
> >>>> -                               TimestampDifferenceExceeds(waitStart, GetCurrentTimestamp(),
> >>>> -                                                                                  500))
> >>>>
> >>>> The patch changes ResolveRecoveryConflictWithSnapshot() and
> >>>> ResolveRecoveryConflictWithTablespace() so that they always add
> >>>> "waiting" into the PS display, whether wait is really necessary or not.
> >>>> But isn't it better to display "waiting" in PS basically when wait is
> >>>> necessary, like originally ResolveRecoveryConflictWithVirtualXIDs()
> >>>> does as the above?
> >>>
> >>> You're right. Will fix it.
> >>>
> >>>>
> >>>>     ResolveRecoveryConflictWithDatabase(Oid dbid)
> >>>>     {
> >>>> +       char            *new_status = NULL;
> >>>> +
> >>>> +       /* Report via ps we are waiting */
> >>>> +       new_status = set_process_title_waiting();
> >>>>
> >>>> In  ResolveRecoveryConflictWithDatabase(), there seems no need to
> >>>> display "waiting" in PS because no wait occurs when recovery conflict
> >>>> with database happens.
> >>>
> >>> Isn't the startup process waiting for other backend to terminate?
> >>
> >> Yeah, you're right. I agree that "waiting" should be reported in this case.
> >>
> >> Currently ResolveRecoveryConflictWithLock() and
> >> ResolveRecoveryConflictWithBufferPin() don't call
> >> ResolveRecoveryConflictWithVirtualXIDs and don't report "waiting"
> >> in PS display. You changed them so that they report "waiting". I agree
> >> to have this change. But this change is an improvement rather than
> >> a bug fix, i.e., we should apply this change only in v13?
> >>
> >
> > Did you mean ResolveRecoveryConflictWithDatabase and
> > ResolveRecoveryConflictWithBufferPin?
>
> Yes! Sorry for my typo.
>
> > In the current code as far as I
> > researched there are two cases where we don't add "waiting" and one
> > case where we doubly add "waiting".
> >
> > ResolveRecoveryConflictWithDatabase and
> > ResolveRecoveryConflictWithBufferPin don't update the ps title.
> > Although the path where GetCurrentTimestamp() >= ltime is false in
> > ResolveRecoveryConflictWithLock also doesn't update the ps title, it's
> > already updated in WaitOnLock. On the other hand, the path where
> > GetCurrentTimestamp() >= ltime is true in
> > ResolveRecoveryConflictWithLock updates the ps title but it's wrong as
> > I reported.
> >
> > I've split the patch into two patches: 0001 patch fixes the issue
> > about doubly updating ps title, 0002 patch makes the recovery conflict
> > resolution on database and buffer pin update the ps title.
>
> Thanks for splitting the patches. I think that 0001 patch can be back-patched.
>
> -                       /*
> -                        * Report via ps if we have been waiting for more than 500 msec
> -                        * (should that be configurable?)
> -                        */
> -                       if (update_process_title && new_status == NULL &&
> -                               TimestampDifferenceExceeds(waitStart, GetCurrentTimestamp(),
> -                                                                                  500))
>
> Originally, "waiting" is reported in PS if we've been waiting for more than
> 500 msec, as the above does. But you got rid of those codes in the patch.
> Did you confirm that it's safe to do that? If not, isn't it better to apply
> the attached patch?

In WaitOnLock() we update the ps title regardless of waiting time. So
I thought we can change it to make these behavior consistent. But
considering back-patch, your patch looks better than mine.

Regards,

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