Re: Logical replication timeout problem

On Thu, Mar 17, 2022 at 12:27 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Mar 16, 2022 at 7:38 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > After more thought, can we check only wal_sender_timeout without
> > skip-count? That is, in WalSndUpdateProgress(), if we have received
> > any reply from the subscriber in last (wal_sender_timeout / 2), we
> > don't need to do anything in terms of keep-alive. If not, we do
> > ProcessRepliesIfAny() (and probably WalSndCheckTimeOut()?) then
> > WalSndKeepalivesIfNecessary(). That way, we can send keep-alive
> > messages every (wal_sender_timeout / 2). And since we don't call them
> > for every change, we would not need to worry about the overhead much.
> >
>
> But won't that lead to a call to GetCurrentTimestamp() for each change
> we skip? IIUC from previous replies that lead to a slight slowdown in
> previous tests of Wang-San.
>

If the above is true then I think we can use a lower skip_count say 10
along with a timeout mechanism to send keepalive message. This will
help us to alleviate the overhead Wang-San has shown.

BTW, I think there could be one other advantage of using
ProcessRepliesIfAny() (as you are suggesting) is that it can help to
release sync waiters if there are any. I feel that would be the case
for the skip_empty_transactions patch [1] which uses
WalSndUpdateProgress to send keepalive messages after skipping empty
transactions.

[1] - https://www.postgresql.org/message-id/CAFPTHDYvRSyT5ppYSPsH4Ozs0_W62-nffu0%3DmY1%2BsVipF%3DUN-g%40mail.gmail.com

--
With Regards,
Amit Kapila.