Re: per backend I/O statistics

On Sat, Jan 18, 2025 at 05:53:31PM +0900, Michael Paquier wrote:
> Hmm. Such special complexities in pgstat.c are annoying. There is
> a stupid thing I am wondering here. For the WAL stats, why couldn't
> we place some calls of pgstat_prep_backend_pending() in strategic
> places like XLogBeginInsert() to force all the allocation steps of the
> pending entry to happen before we would enter the critical sections
> when doing a WAL insertion? As far as I can see, there is a special
> case with 2PC where XLogBeginInsert() could be called in a critical
> section, but that seems to be the only one at quick glance.

I've looked first at this idea over the week-end with a quick hack,
and it did not finish well.

And then it struck me that with a bit of redesign of the callbacks, so
as the existing flush_fixed_cb and have_fixed_pending_cb are usable
with variable-numbered stats, we should be able to avoid the exception
your first version of the patch has introduced. Attached is a patch
to achieve what I have in mind, which is more generic than your
previous patch. I've built my stuff as 0002 on top of your 0001.

One key choice I have made is to hide PendingBackendStats within
pgstat_backend.c so as it is possible to enforce some sanity checks
there.
--
Michael