Re: Context switch storm

Merlin wrote:

> On 11/14/06, Jim C. Nasby <jim(at)nasby(dot)net> wrote:
>
>> On Tue, Nov 14, 2006 at 09:17:08AM -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.
>> > [...]
>>
>> BTW, shared_buffers of 16384 is pretty low by today's standards
>
> Can you think of a good way to construct a test case that would
> demonstrate the difference?

Not sure of actual relevance, but some time ago I performed
(with 8.0) several pg_bench tests with 1,5,10,20 concurrent
clients with same pg configuration except one parameter for
every run.

In one of these tests I run pgbench with shared_buffers starting
at 1024 and doubling it to 2048, ..., until 16384.
I found the best performance in terms of transactions per second
around 4096/8192.

That said, I don't know if pgbench stresses the database
like my typical oltp application does.

And also, I suspect that shared_buffers should not be
evaluated as an absolute number, but rather as a number relative to
maximum main memory (say 1/2 the total ram, 1/3, 2/3, ...).

--
Cosimo