Re: CLOG contention

On Thu, Jan 5, 2012 at 2:57 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I would be in favor of that, or perhaps some other formula (eg, maybe
> the minimum should be less than 8 for when you've got very little shmem).

I have some results that show that, under the right set of
circumstances, 8->32 is a win, and I can quantify by how much it wins.
I don't have any data at all to quantify the cost of dropping the
minimum from 8->6, or from 8->4, and therefore I'm reluctant to do it.
My guess is that it's a bad idea, anyway. Even on a system where
shared_buffers is just 8MB, we have 1024 regular buffers and 8 CLOG
buffers. If we reduce the number of CLOG buffers from 8 to 4 (i.e. by
50%), we can increase the number of regular buffers from 1024 to 1028
(i.e. by <0.5%). Maybe you can find a case where that comes out to a
win, but you might have to look pretty hard.

--
Robert Haas
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