Re: [BUG]: the walsender does not update its IO statistics until it exits

Hi,

On Fri, Feb 28, 2025 at 02:41:34PM +0900, Michael Paquier wrote:
> On Wed, Feb 26, 2025 at 09:48:50AM +0000, Bertrand Drouvot wrote:
> > Yeah I think that makes sense, done that way in the attached.
> >
> > Speaking about physical walsender, I moved the test to 001_stream_rep.pl instead
> > (would also fail without the fix).
>
> Hmm. I was doing some more checks with this patch, and on closer look
> I am wondering if the location you have chosen for the stats reports
> is too aggressive: this requires a LWLock for the WAL sender backend
> type taken in exclusive mode, with each step of WalSndLoop() taken
> roughly each time a record or a batch of records is sent. A single
> installcheck with a primary/standby setup can lead to up to 50k stats
> report calls.

Yeah, what I can observe (installcheck with a primary/standby):

- extras flush are called about 70K times.

Among those 70K:

- about 575 are going after the "have_iostats" check in pgstat_io_flush_cb()
- about 575 are going after the PendingBackendStats pg_memory_is_all_zeros()
check in pgstat_flush_backend() (makes sense due to the above)
- about 575 are going after the PendingBackendStats.pending_io pg_memory_is_all_zeros()
check in pgstat_flush_backend_entry_io() (makes sense due to the above)

It means that only a very few of them are "really" flushing IO stats.

> With smaller records, the loop can become hotter, can't it? Also,
> there can be a high number of WAL senders on a single node, and I've
> heard of some customers with complex logical decoding deployments with
> dozens of logical WAL senders. Isn't there a risk of having this code
> path become a point of contention? It seems to me that we should
> benchmark this change more carefully, perhaps even reduce the
> frequency of the report calls.

That sounds a good idea to measure the impact of those extra calls and see
if we'd need to mitigate the impacts. I'll collect some data.

Regards,

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