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Re: Need help with 8.4 Performance Testing

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Need help with 8.4 Performance Testing
Date: 2008-12-08 20:15:36
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Lists: pgsql-performance
On Sun, 7 Dec 2008, Scott Marlowe wrote:

> When I last used pgbench I wanted to test it with an extremely large
> dataset, but it maxes out at -s 4xxx or so, and that's only in the
> 40Gigabyte range.  Is the limit raised for the pgbench included in
> contrib in 8.4?  I'm guessing it's an arbitrary limit.

There's no artificial limit, just ones that result from things like 
integer overflow.  I don't think has been an issue so far because pgbench 
becomes seek limited and stops producing interesting results once the 
database exceeds the sum of all available caching, which means you'd need 
more than 32GB of RAM in the system running pgbench before this is an 
issue.  Which happens to be the largest size system I've ever ran it on...

I'd expect this statement around line 1060 of pgbench.c 
to overflow first:

     for (i = 0; i < naccounts * scale; i++)

Where i is an integer, naccounts=100,000 , and scale is what you input. 
That will overflow a signed 32-bit integer at a scale of 2147.  If you had 
tests that failed at twice that, I wonder if you were actually executing 
against a weird data set for scales of (2148..4294).

It's not completely trivial to fix (I just tried), the whole loop needs to 
be restructured to run against scale and 100,000 separately while keeping 
the current progress report intact.  I'll take a shot at fixing it 
correctly, this is a bug that should be corrected before 8.4 goes out.  I 
guarantee that version will be used on systems with 64GB of RAM where this 

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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