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In-Reply-To: <e4e98e40-40d5-4d53-85de-6c456c2b301a@iki.fi>
From: Xuneng Zhou <xunengzhou@gmail.com>
Date: Mon, 6 Jul 2026 21:49:26 +0800
Message-ID: 
 <CABPTF7XV=rPe9iVTyR5O=CT0oMUYTPJpr+4+T35n0oKeaaNZHQ@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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Hi Heikki,

Thanks for looking into this!

On Mon, Jul 6, 2026 at 7:04=E2=80=AFPM Heikki Linnakangas <hlinnaka@iki.fi>=
 wrote:
                      /*
> >                        * Apply the record
> >                        */
> >                       ApplyWalRecord(xlogreader, record, &replayTLI);
> >
> >                       /*
> >                        * Wake up processes waiting for standby replay, =
write, or flush
> >                        * LSN to reach current replay position.  Replay =
implies that the
> >                        * WAL was already written and flushed to disk, s=
o write and flush
> >                        * waiters can be woken at the replay position to=
o.
> >                        */
> >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_REPLAY,
> >                                                 XLogRecoveryCtl->lastRe=
playedEndRecPtr);
> >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE,
> >                                                 XLogRecoveryCtl->lastRe=
playedEndRecPtr);
> >                       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH,
> >                                                 XLogRecoveryCtl->lastRe=
playedEndRecPtr);
>
> That's not wrong, but I've got some comments:
>
> 1. It's reading XLogRecoveryCtl->lastReplayedEndRecPtr without a lock or
> atomics. That's ok, no other process modifies lastReplayedEndRecPtr, but
> 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, XLo=
gRecord *record, TimeLineID *repl
> >         /*
> >          * Update lastReplayedEndRecPtr after this record has been succ=
essfully
> >          * replayed.
> >          */
> >         SpinLockAcquire(&XLogRecoveryCtl->info_lck);
> >         XLogRecoveryCtl->lastReplayedReadRecPtr =3D xlogreader->ReadRec=
Ptr;
> >         XLogRecoveryCtl->lastReplayedEndRecPtr =3D xlogreader->EndRecPt=
r;
> >         XLogRecoveryCtl->lastReplayedTLI =3D *replayTLI;
> >         SpinLockRelease(&XLogRecoveryCtl->info_lck);
> >
> > +       /*
> > +        * Wake up processes waiting for standby replay, write, or flus=
h LSN to
> > +        * reach current replay position.  Replay 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, xlogreader->EndRecP=
tr);
> > +       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE, xlogreader->EndRecPt=
r);
> > +       WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH, xlogreader->EndRecPt=
r);
> > +
> >         /* ------
> >          * Wakeup walsenders:
> >          *
> >          * On the standby, the WAL is flushed first (which will only wa=
ke up
> >          * physical walsenders) and then applied, which will only wake =
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
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.
--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.