Re: Implement waiting for wal lsn replay: reloaded

On Mon, Jul 6, 2026 at 9:49 PM Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
>
> Hi Heikki,
>
> Thanks for looking into this!
>
> On Mon, Jul 6, 2026 at 7:04 PM Heikki Linnakangas <hlinnaka(at)iki(dot)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, so write and flush
> > > * waiters can be woken at the replay position too.
> > > */
> > > WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_REPLAY,
> > > XLogRecoveryCtl->lastReplayedEndRecPtr);
> > > WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE,
> > > XLogRecoveryCtl->lastReplayedEndRecPtr);
> > > WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH,
> > > XLogRecoveryCtl->lastReplayedEndRecPtr);
> >
> > 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, XLogRecord *record, TimeLineID *repl
> > > /*
> > > * Update lastReplayedEndRecPtr after this record has been successfully
> > > * replayed.
> > > */
> > > SpinLockAcquire(&XLogRecoveryCtl->info_lck);
> > > XLogRecoveryCtl->lastReplayedReadRecPtr = xlogreader->ReadRecPtr;
> > > XLogRecoveryCtl->lastReplayedEndRecPtr = xlogreader->EndRecPtr;
> > > XLogRecoveryCtl->lastReplayedTLI = *replayTLI;
> > > SpinLockRelease(&XLogRecoveryCtl->info_lck);
> > >
> > > + /*
> > > + * 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, so write and flush waiters can be woken at
> > > + * the replay position too.
> > > + */
> > > + WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_REPLAY, xlogreader->EndRecPtr);
> > > + WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_WRITE, xlogreader->EndRecPtr);
> > > + WaitLSNWakeup(WAIT_LSN_TYPE_STANDBY_FLUSH, xlogreader->EndRecPtr);
> > > +
> > > /* ------
> > > * Wakeup walsenders:
> > > *
> > > * On the standby, the WAL is flushed first (which will only wake 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

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.

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