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Date: Tue, 25 Feb 2003 23:16:02 +0100
From: Andrea Arcangeli <andrea@suse.de>
To: "Martin J. Bligh" <mbligh@aracnet.com>
Cc: William Lee Irwin III <wli@holomorphy.com>, Andrew Morton <akpm@digeo.com>,
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Subject: Re: Minutes from Feb 21 LSE Call
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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
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