Re: Minimal logical decoding on standbys

On Mon, Apr 3, 2023 at 1:31 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>
> On Sun, 2023-04-02 at 10:11 +0200, Drouvot, Bertrand wrote:
> > I was thinking that, if a new LogicalWalSndWakeup() replaces
> > "ConditionVariableBroadcast(&XLogRecoveryCtl->replayedCV);"
> > in ApplyWalRecord() then, it could be possible that some walsender(s)
> > are requested to wake up while they are actually doing decoding (but
> > I might be wrong).
>
> I don't think that's a problem, right?
>

Agreed, I also don't see a problem because of the reason you mentioned
below that if the latch is already set, we won't do anything in
SetLatch.

> We are concerned about wakeups when they happen repeatedly when there's
> no work to do, or when the wakeup doesn't happen when it should (and we
> need to wait for a timeout).
>
> > >
> > > Currently, we wake up walsenders only after writing some WAL
> > > records
> > > at the time of flush, so won't it be better to wake up only after
> > > applying some WAL records rather than after applying each record?
> >
> > Yeah that would be better.
>
> Why? If the walsender is asleep, and there's work to be done, why not
> wake it up?
>

I think we can wake it up when there is work to be done even if the
work unit is smaller. The reason why I mentioned waking up the
walsender only after processing some records is to avoid the situation
where it may not need to wait again after decoding very few records.
But probably the logic in WalSndWaitForWal() will help us to exit
before starting to wait by checking the replay location.

--
With Regards,
Amit Kapila.