Re: Need help with 8.4 Performance Testing

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.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD