Re: MultiXact\SLRU buffers configuration

On 11/10/20 7:16 AM, Andrey Borodin wrote:
> 
> 
>> 10 нояб. 2020 г., в 05:13, Tomas Vondra <tomas.vondra@enterprisedb.com> написал(а):
>> After the issue reported in [1] got fixed, I've restarted the multi-xact
>> stress test, hoping to reproduce the issue. But so far no luck :-(
> 
> 
> Tomas, many thanks for looking into this. I figured out that to make multixact sets bigger transactions must hang for a while and lock large set of tuples. But not continuous range to avoid locking on buffer_content.
> I did not manage to implement this via pgbench, that's why I was trying to hack on separate go program. But, essentially, no luck either.
> I was observing something resemblant though
> 
>                                     пятница,  8 мая 2020 г. 15:08:37 (every 1s)
> 
>   pid  |         wait_event         | wait_event_type | state  |                       query                        
> -------+----------------------------+-----------------+--------+----------------------------------------------------
>  41344 | ClientRead                 | Client          | idle   | insert into t1 select generate_series(1,1000000,1)
>  41375 | MultiXactOffsetControlLock | LWLock          | active | select * from t1 where i = ANY ($1) for share
>  41377 | MultiXactOffsetControlLock | LWLock          | active | select * from t1 where i = ANY ($1) for share
>  41378 |                            |                 | active | select * from t1 where i = ANY ($1) for share
>  41379 | MultiXactOffsetControlLock | LWLock          | active | select * from t1 where i = ANY ($1) for share
>  41381 |                            |                 | active | select * from t1 where i = ANY ($1) for share
>  41383 | MultiXactOffsetControlLock | LWLock          | active | select * from t1 where i = ANY ($1) for share
>  41385 | MultiXactOffsetControlLock | LWLock          | active | select * from t1 where i = ANY ($1) for share
> (8 rows)
> 
> but this picture was not stable.
> 

Seems we haven't made much progress in reproducing the issue :-( I guess
we'll need to know more about the machine where this happens. Is there
anything special about the hardware/config? Are you monitoring size of
the pg_multixact directory?

> How do you collect wait events for aggregation? just insert into some table with cron?
> 

No, I have a simple shell script (attached) sampling data from
pg_stat_activity regularly. Then I load it into a table and aggregate to
get a summary.


regards

-- 
Tomas Vondra
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