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Re: Monitoring buffercache...

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>, Kevin Kempter <kevin(at)consistentstate(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Monitoring buffercache...
Date: 2008-11-25 04:40:52
Message-ID: Pine.GSO.4.64.0811242335040.1084@westnet.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, 24 Nov 2008, Scott Marlowe wrote:

> My guess is that the period of time for which pg_buffercache takes locks 
> on the buffer map are short enough that it isn't a real big deal on a 
> fast enough server.

As the server involved gets faster, the amount of time the locks are 
typically held for drops.

As your shared_buffers allocation increases, that amount of time goes up.

So how painful the overhead is depends on how fast your CPU is relative to 
how much memory is in it.  Since faster systems tend to have more RAM in 
them, too, it's hard to say whether the impact will be noticable.

Also, noting that the average case isn't impacted much isn't the concern 
here.  The problem is how much having all partition locks held will 
increase impact worst-case latency.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD

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