Re: Context switch storm

On 11/14/06, Jim C. Nasby <jim@nasby.net> wrote:
> On Tue, Nov 14, 2006 at 09:17:08AM -0500, Merlin Moncure wrote:
> > On 11/14/06, Cosimo Streppone <cosimo@streppone.it> 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?

merlin