Re: Performance bottleneck. High active sessions but postmaster kernel threads are in a sleep state, low CPU utilization
We actually keep track of the wait events in another grafana graph. We use the following query to generate the graph. (We assume that an active session that has a NULL wait_event is "ON CPU”. Not sure if our assuming an active session with a null wait being on cpu is valid. We sample pg_stat_activity every 30 seconds.
SELECT
coalesce(wait_event, 'ON CPU') AS type,
count(*)
FROM
pg_stat_activity
WHERE
state = 'active'
AND usename != 'repmgr'
GROUP BY
TYPE
ORDER BY
count(*) DESC ";
Here’s a sample from our wait_event graph during times when we see high active sessions but a sleeping postgres OS process. Is it possible we could be hitting an un-instremented wait event?
SELECT
coalesce(wait_event, 'ON CPU') AS type,
count(*)
FROM
pg_stat_activity
WHERE
state = 'active'
AND usename != 'repmgr'
GROUP BY
TYPE
ORDER BY
count(*) DESC ";
Here’s a sample from our wait_event graph during times when we see high active sessions but a sleeping postgres OS process. Is it possible we could be hitting an un-instremented wait event?
Craig
On Mar 31, 2021, 10:00 AM -0600, Hotmail <crajac66@hotmail.com>, wrote:
Unfortunately, the wait_event and wait_event_type columns are all NULL for these active sessions.CraigOn Mar 31, 2021, 3:47 AM -0600, Laurenz Albe <laurenz.albe@cybertec.at>, wrote: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
--
Cybertec | https://www.cybertec-postgresql.com
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