Re: Context switch storm

On Tue, 2006-11-14 at 09:17 -0500, Merlin Moncure wrote:
> On 11/14/06, Cosimo Streppone <cosimo(at)streppone(dot)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.

In 8.0 there is a performance issue such that bgwriter will cause a
performance problem with large shared_buffers setting. That in itself
could lead to some fairly poor measurements of the value of
shared_buffers.

In 7.4 and prior releases setting shared_buffers higher was counter
productive in many ways, so isn't highly recommended.

In general, setting shared_buffers higher works for some workloads and
doesn't for others. So any measurements anybody makes depend upon the
workload and the size of the database. The more uniformly/randomly you
access a large database, the more benefit you'll see from large
shared_buffers. 8.1 benefits from having a higher shared_buffers in some
cases because it reduces contention on the buffer lwlocks; 8.2 solves
this issue.

Even in 8.2 ISTM that a higher shared_buffers setting wastes memory with
many connected users since the PrivRefCount array uses memory that could
have been used as filesystem cache.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com