Re: Estimating seq_page_fetch and random_page_fetch

On Thu, Mar 08, 2007 at 05:35:03PM +0000, Gregory Stark wrote:
>
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
> > "Umar Farooq Minhas" <umarfm13(at)hotmail(dot)com> writes:
> >> How can we accrately estimate the "seq_page_fetch" and =
> >> "random_page_fetch" costs from outside the postgres using for example a =
> >> C routine.
> >
> > Use a test case larger than memory. Repeat many times to average out
> > noise. IIRC, when I did the experiments that led to the current
> > random_page_cost of 4.0, it took about a week before I had numbers I
> > trusted.
>
> When I was running tests I did it on a filesystem where nothing else was
> running. Between tests I unmounted and remounted it. As I understand it Linux
> associates the cache with the filesystem and not the block device and discards
> all pages from cache when the filesystem is unmounted.
>
> That doesn't contradict anything Tom said, it might be useful as an additional
> tool though.

Another trick I've used in the past is to just run the machine out of
memory, using the following:

/*
* $Id: clearmem.c,v 1.1 2003/06/29 20:41:33 decibel Exp $
*
* Utility to clear out a chunk of memory and zero it. Useful for flushing disk buffers
*/

int main(int argc, char *argv[]) {
if (!calloc(atoi(argv[1]), 1024*1024)) { printf("Error allocating memory.\n"); }
}

I'll monitor top while that's running to ensure that some stuff gets
swapped out to disk. I believe this might still leave some cached data
in other areas of the kernel, but it's probably not enough to worry
about.
--
Jim Nasby jim(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)