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Re: Analyse without locking?

From: Andres Freund <andres(at)anarazel(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Cc: Richard Neill <rn214(at)cam(dot)ac(dot)uk>
Subject: Re: Analyse without locking?
Date: 2009-11-26 17:26:11
Message-ID: 200911261826.11185.andres@anarazel.de (view raw or flat)
Thread:
Lists: pgsql-performance
On Thursday 26 November 2009 17:20:35 Richard Neill wrote:
> Dear All,
> 
> I'm wondering whether Vacuum/analyse (notably by the autovaccuum daemon)
> is responsible for some deadlocks/dropouts I'm seeing.
> 
> One particular table gets hit about 5 times a second (for single row
> updates and inserts) + associated index changes. This is a very light
> load for the hardware; we have 7 CPU cores idling, and very little disk
> activity. The query normally runs in about 20 ms.
> 
> However, the query must always respond within 200ms, or userspace gets
> nasty errors.  [we're routing books on a sorter machine, and the book
> misses its exit opportunity]. Although this is a low load, it's a bit
> like a heartbeat.
> 
> The question is, could the autovacuum daemon (running either in vacuum
> or in analyse mode) be taking out locks on this table that sometimes
> cause the query response time to go way up (exceeding 10 seconds)?
> 
> I think I've set up autovacuum to do "little and often", using
>    autovacuum_vacuum_cost_delay = 20ms
>    autovacuum_vacuum_cost_limit = 20
> but I'm not sure this is doing exactly what I think it is. In
> particular, the system-wide I/O (and CPU) limit of autovacuum is
> negligible, but it's possible that queries may be waiting on locks.
> 
> In particular, I want to make sure that the autovacuum daemon never
> holds any lock for more than about 50ms at a time. (or will release it
> immediately if something else wants it)
> 
> Or am I barking up the wrong tree entirely?
I would suggest enabling log_log_wait and setting deadlock_timeout to a low 
value - should give you more information.

Andres

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