Re: shared-memory based stats collector

On 07/10/2018 02:07 PM, Kyotaro HORIGUCHI wrote:
> Hello. Thanks for the opinions.
> 
> At Fri, 6 Jul 2018 13:10:36 -0700, Andres Freund <andres@anarazel.de> wrote in
<20180706201036.awheoi6tk556x6aj@alap3.anarazel.de>
>> Hi,
>>
>> On 2018-07-06 22:03:12 +0200, Magnus Hagander wrote:
>>> *If* we can provide the snapshots view of them without too much overhead I
>>> think it's worth looking into that while *also* proviiding a lower overhead
>>> interface for those that don't care about it.
>>
>> I don't see how that's possible without adding significant amounts of
>> complexity and probably memory / cpu overhead. The current stats already
>> are quite inconsistent (often outdated, partially updated, messages
>> dropped when busy) - I don't see what we really gain by building
>> something MVCC like in the "new" stats subsystem.
>>
>>
>>> If it ends up that keeping the snapshots become too much overhead in either
>>> in performance or code-maintenance, then I agree can probably drop that.
>>> But we should at least properly investigate the cost.
>>
>> I don't think it's worthwhile to more than think a bit about it. There's
>> fairly obvious tradeoffs in complexity here. Trying to get there seems
>> like a good way to make the feature too big.
> 
> Agreed.
> 
> Well, if we allow to lose consistency in some extent for improved
> performance and smaller footprint, relaxing the consistency of
> database stats can reduce footprint further especially on a
> cluster with so many databases. Backends are interested only in
> the residing database and vacuum doesn't cache stats at all. A
> possible problem is vacuum and stats collector can go into a race
> condition. I'm not sure but I suppose it is not worse than being
> involved in an IO congestion.
> 

As someone who regularly analyzes stats collected from user systems, I 
think there's certainly some value with keeping the snapshots reasonably 
consistent. But I agree it doesn't need to be perfect, and some level of 
inconsistency is acceptable (and the amount of complexity/overhead 
needed to maintain perfect consistency seems rather excessive here).

There's one more reason why attempts to keep stats snapshots "perfectly" 
consistent are likely doomed to fail - the messages are sent over UDP, 
which does not guarantee delivery etc. So there's always some level of 
possible inconsistency even with "perfectly consistent" snapshots.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services