Re: Logical replication timeout problem

On Thu, Jan 22, 2022 at 7:12 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> Now, one idea to solve this problem could be that whenever we skip
> sending any change we do try to update the plugin progress via
> OutputPluginUpdateProgress(for walsender, it will invoke
> WalSndUpdateProgress), and there it tries to process replies and send
> keep_alive if necessary as we do when we send some data via
> OutputPluginWrite(for walsender, it will invoke WalSndWriteData). I
> don't know whether it is a good idea to invoke such a mechanism for
> every change we skip to send or we should do it after we skip sending
> some threshold of continuous changes. I think later would be
> preferred. Also, we might want to introduce a new parameter
> send_keep_alive to this API so that there is flexibility to invoke
> this mechanism as we don't need to invoke it while we are actually
> sending data and before that, we just update the progress via this
> API.

I tried out the patch according to your advice.
I found if I invoke ProcessRepliesIfAny and WalSndKeepaliveIfNecessary in
function OutputPluginUpdateProgress, the running time of the newly added
function OutputPluginUpdateProgress invoked in pgoutput_change brings notable
overhead:
--11.34%--pgoutput_change
          |         
          |--8.94%--OutputPluginUpdateProgress
          |          |         
          |           --8.70%--WalSndUpdateProgress
          |                     |         
          |                     |--7.44%--ProcessRepliesIfAny

So I tried another way of sending keepalive message to the standby machine
based on the timeout without asking for a reply(see attachment), the running
time of the newly added function OutputPluginUpdateProgress invoked in
pgoutput_change also brings slight overhead:
--3.63%--pgoutput_change
          |         
          |--1.40%--get_rel_sync_entry
          |          |         
          |           --1.14%--hash_search
          |         
           --1.08%--OutputPluginUpdateProgress
                     |         
                      --0.85%--WalSndUpdateProgress

Based on above, I think the second idea that sending some threshold of
continuous changes might be better, I will do some research about this
approach.

Regards,
Wang wei