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Re: Analyse without locking?

From: Grzegorz Jaśkiewicz <gryzman(at)gmail(dot)com>
To: Richard Neill <rn214(at)cam(dot)ac(dot)uk>
Cc: PostgreSQL Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Analyse without locking?
Date: 2009-11-26 16:26:30
Message-ID: 2f4958ff0911260826y1b297ee6y9aa7511c3e6faac8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, Nov 26, 2009 at 4:20 PM, Richard Neill <rn214(at)cam(dot)ac(dot)uk> 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
>

those are basically thresholds. So in essence you are forcing your
autovacuum to be active pretty often,

And from what I can read here, you are looking for completely opposite
behaviour. Unless you think statistical image of your table will be
completely invalid, after 20 modifications to it, which I am sure is not
true.




-- 
GJ

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