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Re: Implement waiting for wal lsn replay: reloaded

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Heikki Linnakangas <hlinnaka@iki.fi>, Peter Eisentraut <peter@eisentraut.org>, Thomas Munro <thomas.munro@gmail.com>, Álvaro Herrera <alvherre@kurilemu.de>, Chao Li <li.evan.chao@gmail.com>, pgsql-hackers <pgsql-hackers@lists.postgresql.org>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@gmail.com>, Tomas Vondra <tomas@vondra.me>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2026-04-07T23:30:44Z
Lists: pgsql-hackers 
On Tue, Apr 7, 2026 at 6:55 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
>
> On Tue, Apr 7, 2026 at 9:46 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> >
> > On Tue, Apr 7, 2026, 16:18 Andres Freund <andres@anarazel.de> wrote:
> >>
> >> Hi,
> >>
> >> On 2026-04-07 21:05:40 +0800, Xuneng Zhou wrote:
> >> > I’ve posted two patches. The first fixes the duplication issue
> >> > reported by Andres and is fairly straightforward. The second turned
> >> > out to be more complex than expected, and I’m still working through
> >> > possible solutions. Feedback or alternative approaches would be very
> >> > helpful.
> >> > I also spent some time drafting a patch to address the memory ordering
> >> > issue and will post it later.
> >>
> >> I propose quickly applying a minimal patch like the attached, to get the test
> >> performance back to normal.
> >>
> >> Will do so unless somebody protests within in one CI cycle and one coffee.
> >
> >
> > I would be able to review them only after several hours. But +1 for applying now to get rid of buildfarm slowdown.
>
> Here is a patch addressing the memory order issue reported earlier.

I agree to change in WaitLSNWakeup(), memory barrier looks necessary there.
Regarding GetCurrentLSNForWaitType(), I don't think barrier is needed
here, nor think it makes things clearer.  I think it would be enough
to comment that LWLock operations in addLSNWaiter()/deleteLSNWaiter()
provide necessary barriers.

------
Regards,
Alexander Korotkov
Supabase

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Clean up 019_replslot_limit.pl comments

  2. Stabilize 019_replslot_limit.pl: wait on slot restart_lsn

  3. Fix memory ordering in WAIT FOR LSN wakeup mechanism

  4. Improve WAIT FOR LSN test coverage

  5. Remove redundant WAIT FOR LSN caller-side pre-checks

  6. Use barrier semantics when reading/writing writtenUpto

  7. Use replay position as floor for WAIT FOR LSN standby_(write|flush)

  8. Wake standby_write/standby_flush waiters from the WAL replay loop

  9. Minimal fix for WAIT FOR ... MODE 'standby_flush'

  10. Avoid syscache lookup while building a WAIT FOR tuple descriptor

  11. Document that WAIT FOR may be interrupted by recovery conflicts

  12. Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()

  13. Wake LSN waiters before recovery target stop

  14. Remove redundant pg_unreachable() after elog(ERROR) from ExecWaitStmt()

  15. Revert "Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()"

  16. Fix variable usage in wakeupWaiters()

  17. Add tab completion for the WAIT FOR LSN MODE option

  18. Add the MODE option to the WAIT FOR LSN command

  19. Extend xlogwait infrastructure with write and flush wait types

  20. Unify error messages

  21. Optimize shared memory usage for WaitLSNProcInfo

  22. Fix WaitLSNWakeup() fast-path check for InvalidXLogRecPtr

  23. Fix incorrect function name in comments

  24. Add infrastructure for efficient LSN waiting

  25. Add pairingheap_initialize() for shared memory usage

  26. Implement WAIT FOR command