Re: Performance bottleneck. High active sessions but postmaster kernel threads are in a sleep state, low CPU utilization

Hi,

You showed a lot of detail from the operating system perspective, but
nothing from pg_stat_activity or pg_locks during those contention times
where the cpu is low but lot of concurrent activity in the database.

I would guess that there is a lot of lightweight and/or spinlocks going
on. This activity usually occurs when there appears to be no heavy cpu
activity, no cpu io wait activity, but the db is being hit big time with
a bunch of concurrent active connections contending over the same
resources as Laurenz mentioned.

You can identify lightweight locks/spinlocks with the general wait type:
wait_event_type = LWLock. The wait_event column will drill down even
further.  You can also help yourself by installing and using
pg_wait_sampling extension. Because locks come and go so fast in
pg_stat_activity, this historical perspective from the sampling
extension may provide a better picture.
https://github.com/postgrespro/pg_wait_sampling

Regards,
Michael Vitale

Laurenz Albe wrote on 3/31/2021 5:47 AM:
> On Tue, 2021-03-30 at 12:22 -0600, Hotmail wrote:
>> We are trying to gain some insight into a performance bottleneck that we are hitting while load testing Postgres on 11.11.
>> [hundreds of active sessions, but CPU is not maxed out]
> With that many active sessions you are probably hitting some contention inside
> the database. Look at "wait_event" and "wait_event_type" in pg_stat_activity.
>
> Yours,
> Laurenz Albe