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From: Xuneng Zhou <xunengzhou@gmail.com>
Date: Mon, 6 Jul 2026 22:17:41 +0800
Message-ID: 
 <CABPTF7V1GqXRe9Kyiw=oyDoUU4Oss49nMBhco12V7xb72P6WvQ@mail.gmail.com>
Subject: Re: Implement waiting for wal lsn replay: reloaded
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Alexander Korotkov <aekorotkov@gmail.com>,
 Andres Freund <andres@anarazel.de>,
	Tom Lane <tgl@sss.pgh.pa.us>, Peter Eisentraut <peter@eisentraut.org>,
	Thomas Munro <thomas.munro@gmail.com>,
 =?UTF-8?Q?=C3=81lvaro_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>
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On Mon, Jul 6, 2026 at 9:49=E2=80=AFPM Xuneng Zhou <xunengzhou@gmail.com> w=
rote:
>
> Hi Heikki,
>
> Thanks for looking into this!
>
> On Mon, Jul 6, 2026 at 7:04=E2=80=AFPM Heikki Linnakangas <hlinnaka@iki.f=
i> wrote:
>                       /*
> > >                        * Apply the record
> > >                        */
> > >                       ApplyWalRecord(xlogreader, record, &replayTLI);
> > >
> > >                       /*
> > >                        * Wake up processes waiting for standby replay=
, write, or flush
> > >                        * LSN to reach current replay position.  Repla=
y implies that the
> > >                        * WAL was already written and flushed to disk,=
 so write and flush
> > >                        * waiters can be woken at the replay position =
too.
> > >                        */
> > >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_REPLAY,
> > >                                                 XLogRecoveryCtl->last=
ReplayedEndRecPtr);
> > >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE,
> > >                                                 XLogRecoveryCtl->last=
ReplayedEndRecPtr);
> > >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH,
> > >                                                 XLogRecoveryCtl->last=
ReplayedEndRecPtr);
> >
> > That's not wrong, but I've got some comments:
> >
> > 1. It's reading XLogRecoveryCtl->lastReplayedEndRecPtr without a lock o=
r
> > atomics. That's ok, no other process modifies lastReplayedEndRecPtr, bu=
t
> > it feels a little dirty.
> >
> > 2. We're now doing three extra function calls on every WAL record. This
> > is a very hot path, and most of the time, we'll just take the fast path
> > in WaitLSNWakeup to return without doing anything. Andres and others
> > assumed up-thread that it's negligible (we used to have pre-checks here
> > in the caller), but I wonder if you did any performance testing?
>
> Agreed, this is a hot path. The performance impact of these extra
> calls doing real work hasn't been measured yet. I'll do some testing.
>
> > 3. There are other "wakeup" calls inside ApplyWalRecord(), to wake up
> > walsenders and walreceivers. They could perhaps use the same wait-lsn
> > machinery now, but that's v20 material. However, I think these
> > WaitLSNWakeup() calls should also be moved inside ApplyWalRecord(), so
> > that we'd have all the wakeup actions in one place.
>
> + 1. This makes the code safer and more readable.
>
> > 4. Once you move those calls inside ApplyWalRecord(), like this:
> >
> > > @@ -1979,20 +1979,30 @@ ApplyWalRecord(XLogReaderState *xlogreader, X=
LogRecord *record, TimeLineID *repl
> > >         /*
> > >          * Update lastReplayedEndRecPtr after this record has been su=
ccessfully
> > >          * replayed.
> > >          */
> > >         SpinLockAcquire(&XLogRecoveryCtl->info_lck);
> > >         XLogRecoveryCtl->lastReplayedReadRecPtr =3D xlogreader->ReadR=
ecPtr;
> > >         XLogRecoveryCtl->lastReplayedEndRecPtr =3D xlogreader->EndRec=
Ptr;
> > >         XLogRecoveryCtl->lastReplayedTLI =3D *replayTLI;
> > >         SpinLockRelease(&XLogRecoveryCtl->info_lck);
> > >
> > > +       /*
> > > +        * Wake up processes waiting for standby replay, write, or fl=
ush LSN to
> > > +        * reach current replay position.  Replay implies that the WA=
L was already
> > > +        * written and flushed to disk, so write and flush waiters ca=
n be woken at
> > > +        * the replay position too.
> > > +        */
> > > +       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_REPLAY, xlogreader->EndRe=
cPtr);
> > > +       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE, xlogreader->EndRec=
Ptr);
> > > +       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH, xlogreader->EndRec=
Ptr);
> > > +
> > >         /* ------
> > >          * Wakeup walsenders:
> > >          *
> > >          * On the standby, the WAL is flushed first (which will only =
wake up
> > >          * physical walsenders) and then applied, which will only wak=
e up logical
> > >          * walsenders.
> >
> > It becomes clear that you don't actually need the memory barrier inside
> > WaitLSNWakeup(). Not sure if they're needed for other callers, but here
> > we have just released a spinlock, which acts as a memory barrier. It
> > might not be worth relaxing, but it does seem a little silly.
>
> If we made the move here, I think the memory barrier could be relaxed
> since other callers are guarded by either the spinlock or full-barrier
> atomic write already.  We might also want to make the contract of

OK, the 'if' here is redundant...

> WaitLSNWakeup() explicit: callers should not publish the LSN with an
> unsynchronized plain store and then immediately probe minWaitedLSN.
> Another motivation for doing this might be slightly better performance
> though untested.
>
> > If nothing else, I'd like to move those calls into ApplyWalRecord() for
> > clarity (point 3 above). What do you think?
>
> Personally + 1.


--=20
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.