Re: per backend WAL statistics

Hi,

On Wed, Jan 29, 2025 at 01:14:09PM +0530, Rahila Syed wrote:
> Hi,
>
> Thank you for the patchset. Having per-backend WAL statistics,
> in addition to cluster-wide ones, is useful.

Thanks for looking at it!

> I had a few comments while looking at v6-0003-* patch.
>
> + /*
> + * This could be an auxiliary process but these do not report backend
> + * statistics due to pgstat_tracks_backend_bktype(), so there is no need
> + * for an extra call to AuxiliaryPidGetProc().
> + */
> + if (!proc)
> + PG_RETURN_NULL();
>
> Maybe an explicit call to AuxiliaryPidGetProc() followed by a check
> for pgstat_tracks_backend_bktype() would be more maintainable.
> Since the processes tracked by AuxiliaryPidGetProc and
> pgstat_tracks_backend_bktype might diverge in future.

I think that could make sense but that might need a separate thread as this
is not only related to this patch (already done that way in pg_stat_reset_backend_stats()
and pg_stat_get_backend_io()).

> On that note, it is not clear to me why the WAL writer statistics are not
> included in per backend
> wal statistics? I understand the same limitation currently exists in
> pgstats_track_io_bktype(), but why does that need to be extended to
> WAL statistics?

WAL writer might be fine but that would not add that much value here because
it's going to appear anyway in pg_stat_io once Nazir's patch [1] gets in.

> + <primary>pg_stat_get_backend_wal</primary>
> + </indexterm>
> + <function>pg_stat_get_backend_wal</function> (
> <type>integer</type> )
> + <returnvalue>record</returnvalue>
> + </para>
> Should the naming describe what is being returned more clearly?
> Something like pg_stat_get_backend_wal_activity()? Currently it
> suggests that it returns a backend's WAL, which is not the case.

Not sure. It aligns with pg_stat_get_backend_io() and the "stat" in its name
suggests this is related to stats.

> + if (pgstat_tracks_backend_bktype(MyBackendType))
> + {
> + PendingBackendStats.pending_wal.wal_write++;
> +
> + if (track_wal_io_timing)
> + INSTR_TIME_ACCUM_DIFF(PendingBackendStats.pending_wal.wal_write_time,
> + end, start);
> + }
> At the risk of nitpicking, may I suggest moving the above code, which is
> under the
> track_wal_io_timing check, to the existing check before this added chunk?
> This way, all code related to track_wal_io_timing will be grouped together,
> closer to where the "end" variable is computed.

I think we're waiting [1] to be in before moving forward with this patch. I think
that [1] also touches this part of the code. I'll keep your remark in mind and
see if it still makes sense once [1] gets in.

[1]: https://www.postgresql.org/message-id/flat/CAN55FZ3AiQ%2BZMxUuXnBpd0Rrh1YhwJ5FudkHg%3DJU0P%2B-W8T4Vg%40mail.gmail.com

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com