Re: performance issues

On Thu, 14 Jun 2001, Bruce Momjian wrote:

> > 2. If you are on a platform where the kernel thinks it can swap out
> > parts of shared memory (which is most platforms, these days), it's
> > counterproductive to make your shared mem large enough that it's not
> > all being used heavily. If the kernel decides to swap out some buffer
> > space, you have a net loss in performance, because you just wasted
> > disk I/O (especially if the page in the buffer is dirty --- it'll
> > eventually have to be read in again, then written out again, to no
> > purpose).
>
> FYI, FreeBSD had the ability to disable shared memory swapping with a
> sysctl setting, while BSD/OS does not swap out shared memory.
>
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I was going to mention this but had to get to my database machine to find
it first. The command is:

sysctl -w kern.ipc.shm_use_phys=1

This is supposed to make shared memory use physical memory, but I don't
think it is a an absolute rule. If you use too much memory, it will still
swap I believe.

- brian