Re: Add wait event for log emission?

Hi,

On 2023-06-13 20:55:19 +0200, Magnus Hagander wrote:
> On Tue, Jun 13, 2023 at 6:59 PM Andres Freund <andres@anarazel.de> wrote:
> >
> > Hi,
> >
> > I just helped somebody debug a postgres performance problem that turned out to
> > be not actually be postgres' fault.  It turned out to be because postgres'
> > stdout/stderr were piped to a program, and that program was slow. Whenever the
> > pipe buffer filled up, postgres stopped making progress.
> >
> > That's not postgres' fault. But we make it too hard to debug such an
> > issue. There's no way to figure this out from within postgres, one pretty much
> > needs to look at stack traces.
> >
> > I think we should add a few wait events for log emission. I think it'd be good
> > to have one wait event for each log destination.
> >
> > That's not perfect - we'd e.g. still not be able to debug where the logger
> > process is stuck, due it not being in pg_stat_activity. But other processes
> > reporting the wait event for writing to the logger process would be a pretty
> > good hint.
> 
> 
> +1.
> 
> Would it make sense to at the same time create a separate one for
> syslog, or just use the same?

I think it should be a separate one for each of the log paths in
send_message_to_server_log(). I don't think we gain anything by being stingy
here - and it's not like we add one every other day.

I do wonder if it'd be worth setting up a wait event around emit_log_hook -
it's somewhat of a misuse of wait events, but might be useful nonetheless?

Greetings,

Andres Freund