Re: per backend WAL statistics - Mailing list pgsql-hackers
| From | Rahila Syed |
|---|---|
| Subject | Re: per backend WAL statistics |
| Date | |
| Msg-id | CAH2L28v9BwN8_y0k6FQ591=0g2Hj_esHLGj3bP38c9nmVykoiA@mail.gmail.com Whole thread Raw |
| Responses |
Re: per backend WAL statistics
|
| List | pgsql-hackers |
Hi,
Thank you for the patchset. Having per-backend WAL statistics,
in addition to cluster-wide ones, is useful.
Thank you,
Thank you for the patchset. Having per-backend WAL statistics,
in addition to cluster-wide ones, is useful.
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.
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?
+ <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?
+ /*
+ * 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.
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?
+ <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.
+ 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
+ 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,
This way, all code related to track_wal_io_timing will be grouped together,
closer to where the "end" variable is computed.
Thank you,
Rahila Syed
On Tue, Jan 21, 2025 at 12:50 PM Bertrand Drouvot <bertranddrouvot.pg@gmail.com> wrote:
Hi,
On Fri, Jan 17, 2025 at 08:43:57AM +0900, Michael Paquier wrote:
> On Thu, Jan 16, 2025 at 12:44:20PM -0500, Andres Freund wrote:
> > On 2025-01-16 17:11:09 +0000, Bertrand Drouvot wrote:
> >> So, do you think that the initial proposal that has been made here (See R1. in
> >> [2]) i.e make use of a new PendingBackendWalStats variable:
> >
> > Well, I think this first needs be fixed for for the IO stats change made in
> >
> > Once we have a pattern to model after, we can apply the same scheme here.
>
> Okay, thanks for the input. I was not sure what you intended
> originally with all this part of the backend code, and how much would
> be acceptable. The line is clear now.
>
> >> 0003 does not rely on pgstat_prep_backend_pending() for its pending statistics
> >> but on a new PendingBackendWalStats variable. The reason is that the pending wal
> >> statistics are incremented in a critical section (see XLogWrite(), and so
> >> a call to pgstat_prep_pending_entry() could trigger a failed assertion:
> >> MemoryContextAllocZero()->"CritSectionCount == 0 || (context)->allowInCritSection"
> >> "
> >>
> >> and implemented up to v4 is a viable approach?
> >
> > Yes-ish. I think it would be better to make it slightly more general than
> > that, handling this for all types of backend stats, not just for WAL.
>
> Agreed to use the same concept for all these parts of the backend
> stats kind rather than two of them. Will send a reply on the original
> backend I/O thread as well.
PFA v6 that now relies on the new PendingBackendStats variable introduced in
4feba03d8b9.
Remark: I moved PendingBackendStats back to pgstat.h because I think that the
"simple" pending stats increment that we are adding in xlog.c are not worth
an extra function call overhead (while it made more sense for the more complex IO
stats handling). So PendingBackendStats is now visible to the outside world like
PendingWalStats and friends.
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
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