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Re: memory allocation ; postgresql-8.0

From: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>
To: "Kavan, Dan (IMS)" <KavanD(at)imsweb(dot)com>
Cc: postgres <pgsql-admin(at)postgresql(dot)org>
Subject: Re: memory allocation ; postgresql-8.0
Date: 2005-05-12 15:20:53
Message-ID: 1115911253.3868.262.camel@state.g2switchworks.com (view raw or flat)
Thread:
Lists: pgsql-admin
On Thu, 2005-05-12 at 10:10, Kavan, Dan (IMS) wrote:
> Hi Scott,
> 
> Thanks again for all your tips.
> 
> If I knock the buffer size down to 65,536 (still higher than what you
> are recommending)  then my shmmax becomes:
> 256,000 + 550,292,685 (65536*8396.8) + 1,454,100 = 552,002,785  
> 
> That will leave me with 3.5 GB of free memory for the system & work
> memory to use.
> Will those free system resources ever get used with a 10 million record,
> 10 GB database?

Certainly.  As you access the data the kernel will cache all the data
sent through it.  Once the machine's been up and processing for a while
you should see a top output that shows "free" memory at a few megs (8 to
30 meg is typical) and all the rest of the memory being used as kernel
cache.

> If I go with 65,536 as my buffer size, Would having the SHMMAX set to 1
> GB on my sysctl.conf system parameters allow me to run two seperate
> instances of postgresql on 2 seperate ports?

Yes, but you may want to set it just a tad higher for things like fsm
and whatnot.

Definitely benchmark both the 64k setting of shared_buffers and lower
settings, looking for a knee with your data set.  It may well be that
the best performance happens at a lower number, and doesn't really
increase as you bump up the shared_buffers.  Be sure to test things as
realistically as possible, i.e. the right amount of parallel users and
all that.

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