On Nov 14, 2006, at 1:11 PM, Merlin Moncure wrote:
On 11/14/06, Jim C. Nasby <jim@xxxxxxxxx> wrote:
On Tue, Nov 14, 2006 at 09:17:08AM -0500, Merlin Moncure wrote:
> On 11/14/06, Cosimo Streppone <cosimo@xxxxxxxxxxxx> wrote:
> >I must say I lowered "shared_buffers" to 8192, as it was before.
> >I tried raising it to 16384, but I can't seem to find a
relationship
> >between shared_buffers and performance level for this server.
>
> My findings are pretty much the same here. I don't see any link
> between shared buffers and performance. I'm still looking for hard
> evidence to rebut this point. Lower shared buffers leaves more
> memory for what really matters, which is sorting.
It depends on your workload. If you're really sort-heavy, then having
memory available for that will be hard to beat. Otherwise, having a
large shared_buffers setting can really help cut down on switching
back
and forth between the kernel and PostgreSQL.
BTW, shared_buffers of 16384 is pretty low by today's standards,
so that
could be why you're not seeing much difference between that and 8192.
Try upping it to 1/4 - 1/2 of memory and see if that changes things.
Can you think of a good way to construct a test case that would
demonstrate the difference? What would be the 'best case' where a
high shared buffers would be favored over a low setting?
Something that's read-heavy will benefit the most from a large
shared_buffers setting, since it means less trips to the kernel.
Write-heavy apps won't benefit that much because you'll end up double-
buffering written data.
--
Jim Nasby jim@xxxxxxxxx
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
