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Re: Potential memory usage issue

From: Bill Moran <wmoran(at)collaborativefusion(dot)com>
To: David Brain <dbrain(at)bandwidth(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Potential memory usage issue
Date: 2007-03-22 13:17:00
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
In response to David Brain <dbrain(at)bandwidth(dot)com>:
> Thanks for the response.
> Bill Moran wrote:
> > In response to David Brain <dbrain(at)bandwidth(dot)com>:
> >> I recently migrated one of our large (multi-hundred GB) dbs from an 
> >> Intel 32bit platform (Dell 1650 - running 8.1.3) to a 64bit platform 
> >> (Dell 1950 - running 8.1.5).  However I am not seeing the performance 
> >> gains I would expect
> > 
> > What were you expecting?  It's possible that your expectations are
> > unreasonable.
> Possibly - but there is a fair step up hardware performance wise from a 
> 1650 (Dual 1.4 Ghz PIII with U160 SCSI) to a 1950 (Dual, Dual Core 2.3 
> Ghz Xeons with SAS) - so I wasn't necessarily expecting much from the 
> 32->64 transition (except maybe the option to go > 4GB easily - although 
> currently we only have 4GB in the box), but was from the hardware 
> standpoint.

Ahh ... I didn't get that from your original message.

> I am curious as to why 'top' gives such different output on the two 
> systems - the datasets are large and so I know I benefit from having 
> high shared_buffers and effective_cache_size settings.

Have you done any actual queries on the new system?  PG won't use the
shm until it needs it -- and that doesn't occur until it gets a request
for data via a query.

Install the pg_bufferstats contrib module and take a look at how shared
memory is being use.  I like to use MRTG to graph shared buffer usage
over time, but you can just do a SELECT count(*) WHERE NOT NULL to see
how many buffers are actually in use.

> > Provide more information, for one thing.  I'm assuming from the top output
> > that this is some version of Linux, but more details on that are liable
> > to elicit more helpful feedback.
> > 
> Yes the OS is Linux - on the 1650 version 2.6.14, on the 1950 version 2.6.18

Bill Moran
Collaborative Fusion Inc.

Phone: 412-422-3463x4023

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