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Re: Caching of Queries

From: Gaetano Mendola <mendola(at)bigfoot(dot)com>
To: Jason Coene <jcoene(at)gotfrag(dot)com>
Subject: Re: Caching of Queries
Date: 2004-09-23 17:12:16
Message-ID: 415303F0.3050409@bigfoot.com (view raw or flat)
Thread:
Lists: pgsql-performance
Jason Coene wrote:
> I'm not an expert, but I've been hunting down a killer performance problem
> for a while now.  It seems this may be the cause.
> 
> At peak load, our database slows to a trickle.  The CPU and disk utilization
> are normal - 20-30% used CPU and disk performance good.

For a peak load 20-30% used CPU this mean you reached your IO bottleneck.

> All of our "postgres" processes end up in the "semwai" state - seemingly
> waiting on other queries to complete.  If the system isn't taxed in CPU or
> disk, I have a good feeling that this may be the cause.  I didn't know that
> planning queries could create such a gridlock, but based on Mr Pink's
> explanation, it sounds like a very real possibility.
> 
> We're running on SELECT's, and the number of locks on our "high traffic"
> tables grows to the hundreds.  If it's not the SELECT locking (and we don't
> get that many INSERT/UPDATE on these tables), could the planner be doing it?
> 
> At peak load (~ 1000 queries/sec on highest traffic table, all very
> similar), the serialized queries pile up and essentially create a DoS on our
> service - requiring a restart of the PG daemon.  Upon stop & start, it's
> back to normal.

Give us informations on this queries, a explain analyze could be a good start
point.

> I've looked at PREPARE, but apparently it only lasts per-session - that's
> worthless in our case (web based service, one connection per data-requiring
> connection).

Trust me the PREPARE is not doing miracle in shenarios like yours . If you use postgres
in a web service environment what you can use is a connection pool ( look for pgpoll IIRC ),
if you use a CMS then try to enable the cache in order to avoid to hit the DB for each
request.




Regards
Gaetano Mendola

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Subject: Re: Caching of Queries
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