Re: Minutes from Feb 21 LSE Call

Andrea Arcangeli
2003-02-25T22:04:43Z

On Tue, Feb 25, 2003 at 01:12:55PM -0800, Martin J. Bligh wrote:
> >> Because you don't set up and tear down the rmap pte-chains for every
> >> fault in / delete of any page ... it just works off the vmas.
> >
> > so basically it uses the rmap that we always had since at least 2.2 for
> > everything but anon mappings, right? this is what DaveM did a few years
> > back too. This makes lots of sense to me, so at least we avoid the
> > duplication of rmap information, even if it won't fix the anonymous page
> > overhead, but clearly it's much lower cost for everything but anonymous
> > pages.
>
> Right ... and anonymous chains are about 95% single-reference (at least for
> the case I looked at), so they're direct mapped from the struct page with
> no chain at all. Cuts out something like 95% of the space overhead of
> pte-chains, and 65% of the time (for kernel compile -j256 on 16x system).
> However, it's going to be a little more expensive to *use* the mappings,
> so we need to measure that carefully.

Sure, it is more expensive to use them, but all we care about is
complexity, and they solve the complexity problem just fine, so I
definitely prefer it. Cpu utilization during heavy swapping isn't a big
deal IMHO

Andrea