On Mon, 5 Mar 2007, Mark Kirkwood wrote:
> To add a little to this - forgetting the scan resistant point for the
> moment... cranking down shared_buffers to be smaller than the L2 cache
> seems to help *any* sequential scan immensely, even on quite modest HW:
>
> e.g: PIII 1.26Ghz 512Kb L2 cache, 2G ram,
>
> SELECT count(*) FROM lineitem (which is about 11GB) performance:
>
> Shared_buffers Elapsed
> -------------- -------
> 400MB 101 s
> 128KB 74 s
>
> When I've profiled this activity, I've seen a lot of time spent
> searching for/allocating a new buffer for each page being fetched.
> Obviously having less of them to search through will help, but having
> less than the L2 cache-size worth of 'em seems to help a whole lot!
Could you demonstrate that point by showing us timings for shared_buffers
sizes from 512K up to, say, 2 MB? The two numbers you give there might
just have to do with managing a large buffer.
Thanks,
Gavin