Re: RFC: replace pg_stat_activity.waiting with something more descriptive

On Sat, Sep 12, 2015 at 2:05 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:

> On Thu, Aug 6, 2015 at 3:31 PM, Ildus Kurbangaliev <
> i(dot)kurbangaliev(at)postgrespro(dot)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 less
> as compare to backends as they do the activities periodically. As an
> example
> WALWriter will take WALWriteLock to write the WAL, but actually there will
> never
> be any much contention for WALWriter. In synchronous_commit = on, the
> backends themselves write the WAL so WALWriter won't do much in that
> case and for synchronous_commit = off, backends won't write the WAL so
> WALWriter won't face any contention unless some buffers have to be written
> by bgwriter or checkpoint for which WAL is not flushed which I don't think
> would 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 rare
> scenarios for background processes, but I think tracking them is not as
> important 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_activity
> along with other backends information, it will not make sense to include
> information about backend processes in this variable as pg_stat_activity
> just 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