Re: Minutes from Feb 21 LSE Call

Martin J. Bligh
2003-02-25T21:12:37Z

>> 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.

M.