Re: RFC: replace pg_stat_activity.waiting with something more descriptive - Mailing list pgsql-hackers
| From | Alexander Korotkov |
|---|---|
| Subject | Re: RFC: replace pg_stat_activity.waiting with something more descriptive |
| Date | |
| Msg-id | CAPpHfdsNJU40QUD2sqJs_iu+z-JM-b_F39fOC7eYDJVZa5e++A@mail.gmail.com Whole thread Raw |
| In response to | Re: RFC: replace pg_stat_activity.waiting with something more descriptive (Amit Kapila <amit.kapila16@gmail.com>) |
| Responses |
Re: RFC: replace pg_stat_activity.waiting with something
more descriptive
|
| List | pgsql-hackers |
On Sat, Sep 12, 2015 at 2:05 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
On Thu, Aug 6, 2015 at 3:31 PM, Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru> wrote:
>
> On 08/05/2015 09:33 PM, Robert Haas wrote:
>>
>>
>> You're missing the point. Those multi-byte fields have additional
>> synchronization requirements, as I explained in some detail in my
>> previous email. You can't just wave that away.
>
> I see that now. Thank you for the point.
>
> I've looked deeper and I found PgBackendStatus to be not a suitable
> place for keeping information about low level waits. Really, PgBackendStatus
> is used to track high level information about backend. This is why auxiliary
> processes don't have PgBackendStatus, because they don't have such information
> to expose. But when we come to the low level wait events then auxiliary
> processes are as useful for monitoring as backends are. WAL writer,
> checkpointer, bgwriter etc are using LWLocks as well. This is certainly unclear
> why they can't be monitored.
>I think the chances of background processes stuck in LWLock is quite lessas compare to backends as they do the activities periodically. As an exampleWALWriter will take WALWriteLock to write the WAL, but actually there will neverbe any much contention for WALWriter. In synchronous_commit = on, thebackends themselves write the WAL so WALWriter won't do much in thatcase and for synchronous_commit = off, backends won't write the WAL soWALWriter won't face any contention unless some buffers have to be writtenby bgwriter or checkpoint for which WAL is not flushed which I don't thinkwould lead to any contention.
Hmm, synchronous_commit is per session variable: some transactions could run with synchronous_commit = on, but some with synchronous_commit = off. This is very popular feature of PostgreSQL: achieve better performance by making non-critical transaction asynchronous while leaving critical transactions synchronous. Thus, contention for WALWriteLock between backends and WALWriter could be real.
I am not denying from the fact that there could be some contention in rarescenarios for background processes, but I think tracking them is not asimportant as tracking the LWLocks for backends.
I would be more careful in calling some of scenarios rare. As DBMS developers we should do our best to evade contention for LWLocks: any contention, not only between backends and background processes. One may assume that high LWLock contention is rare scenario in general. Once we're here we doesn't think so, though.
You claims that there couldn't be contention for WALWriteLock between backends and WALWriter. This is unclear for me: I think it could be. Nobody opposes tracking wait events for backends and tracking them for background processes. I think we need to track both in order to provide full picture to DBA.
Also as we are planning to track the wait_event information in pg_stat_activityalong with other backends information, it will not make sense to includeinformation about backend processes in this variable as pg_stat_activityjust displays information of backend processes.
I'm not objecting that we should track only backends information in pg_stat_activity. I think we should have also some other way of tracking wait events for background processes. We should think it out before extending pg_stat_activity to evade design issues later.
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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