Re: Linux max on shared buffers?

On Thu, 11 Jul 2002 18:43:08 +1000
Martijn van Oosterhout <kleptog@svana.org> wrote:

> On Thu, Jul 11, 2002 at 05:07:29PM +0900, Curt Sampson wrote:
> > Let's walk through an example. I have four pages total for caching.
> > Let's look at a read scenario based on two for postgres and two for the
> > OS, and one for postgres and three for the OS. Pn is a postgres buffer
> > and OSn is an OS buffer; the numbers below those show which disk blocks
> > are in which caches. We'll use an LRU algorithm for both caches and read
> > the blocks in this order: 1 2 3 2 1 2 3.
>
> Hmm, what about OS's that swap shared memory to disk. Wouldn't that change
> things somewhat? Probably more in favour of giving more memory to the OS.

I don't know about Linux, but under FreeBSD you can tell the OS to lock shared
mem in core.  This DOES help out.

> The other possibility would be to use mmap instead. That way you avoid the
> double buffering altogether. Do you have any ideas about that?

Not all platforms have mmap.  This has been discussed before I belive.

> --
> Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> > There are 10 kinds of people in the world, those that can do binary
> > arithmetic and those that can't.

--
GB Clark II             | Roaming FreeBSD Admin
gclarkii@VSServices.COM | General Geek
           CTHULU for President - Why choose the lesser of two evils?