Re: per backend I/O statistics

Hi,

On Wed, Jan 15, 2025 at 06:27:22PM +0900, Michael Paquier wrote:
> On Wed, Jan 15, 2025 at 08:30:20AM +0000, Bertrand Drouvot wrote:
> > On Wed, Jan 15, 2025 at 11:03:54AM +0300, Nazir Bilal Yavuz wrote:
> >> With this commit it may not be possible to count IOs in the critical
> >> sections. I think the problem happens only if the local
> >> PgStat_BackendPending entry is being created for the first time for
> >> this backend in the critical section.
> > 
> > Yeah, I encountered the exact same thing and mentioned it in [1] (see R1.).
> > 
> > In [1] I did propose to use a new PendingBackendWalStats variable to "bypass" the
> > pgstat_prep_backend_pending() usage.
> > 
> > Michael mentioned in [2] that is not really consistent with the rest (what I
> > agree with) and that "we should rethink a bit the way pending entries are
> > retrieved". I did not think about it yet but that might be the way to
> > go, thoughts? 
> > 
> > [1]: https://www.postgresql.org/message-id/Z3zqc4o09dM/Ezyz%40ip-10-97-1-34.eu-west-3.compute.internal
> > [2]: https://www.postgresql.org/message-id/Z4dRlNuhSQ3hPPv2%40paquier.xyz
> 
> My problem is that this is not only related to backend stats, but to
> all variable-numbered stats kinds that require this behavior.

Yeah, agree that it's better to have a "generic" solution.

> A generic solution to the problem would be to allow
> pgStatPendingContext to have MemoryContextAllowInCriticalSection() set
> temporarily for the allocation of the pending data.
> Perhaps not for all stats kinds, just for these where we're OK with
> the potential memory footprint so we could have a flag in
> PgStat_KindInfo.  Giving to all stats kinds the responsibility to
> allocate a pending entry beforehand outside a critical section is
> another option, but that means going through different tweaks for all
> stats kinds that require them.
> 
> Andres, do you have any thoughts about that?  Not sure what would be 
> your view on this matter.  MemoryContextAllowInCriticalSection() for
> stats kinds that allow the case would be OK for me I guess,

Same here, that looks ok to me too.

Regards,

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