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Re: Need help with 8.4 Performance Testing

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: "Greg Smith" <gsmith(at)gregsmith(dot)com>
Cc: "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>, "Josh Berkus" <josh(at)agliodbs(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Need help with 8.4 Performance Testing
Date: 2008-12-09 01:23:16
Message-ID: b42b73150812081723qdf7f203tcba3c3660023890f@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, Dec 8, 2008 at 5:52 PM, Greg Smith <gsmith(at)gregsmith(dot)com> wrote:
> On Mon, 8 Dec 2008, Scott Marlowe wrote:
>
>> Well, I have 32 Gig of ram and wanted to test it against a database
>> that was at least twice as big as memory.  I'm not sure why you'd
>> consider the results uninteresting though, I'd think knowing how the
>> db will perform with a very large transactional store that is twice or
>> more the size of memory would be when it starts getting interesting.
>
> If you refer back to the picture associated with the link Josh suggested:
>
> http://www.westnet.com/~gsmith/gregsmith/content/postgresql/scaling.png
>
> You'll see that pgbench results hit a big drop once you clear the amount of
> memory being used to cache the accounts table.  This curve isn't unique to
> what I did; I've seen the same basic shape traced out by multiple other
> testers on different hardware, independent of me.  It just expands to the
> right based on the amount of RAM available.
>
> All I was trying to suggest was that even if you've got 32GB of RAM, you may
> already be into the more flat right section of that curve even with a 40GB
> database.  That was a little system with 1GB of RAM+256MB of disk cache, and
> it was already toast at 750MB of database.  Once you've gotten a database
> big enough to reach that point, results degrade to something related to
> database seeks/second rather than anything else, and further increases don't
> give you that much more info.  This is why I'm not sure if the current limit
> really matters with 32GB of RAM, but it sure will be important if you want
> any sort of useful pgbench results at 64GB.


I wonder if shared_buffers has any effect on how far you can go before
you hit the 'tipping point'.

merlin

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