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From: Daniel Phillips <phillips@bonn-fries.net>
To: Rik van Riel <riel@conectiva.com.br>, Oliver Xymoron <oxymoron@waste.org>
Subject: Re: Note describing poor dcache utilization under high memory pressure
Date: Tue, 29 Jan 2002 21:48:55 +0100
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Cc: Linus Torvalds <torvalds@transmeta.com>,
        Josh MacDonald <jmacd@CS.Berkeley.EDU>,
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On January 29, 2002 06:25 pm, Rik van Riel wrote:
> On Tue, 29 Jan 2002, Oliver Xymoron wrote:
> 
> > Daniel's approach seems to be workable (once he's spelled out all the
> > details) but it misses the big performance win for fork/exec, which is
> > surely the common case. Given that exec will be throwing away all these
> > mappings, we can safely assume that we will not be inheriting many shared
> > mappings from parents of parents so Daniel's approach also still ends up
> > marking most of the pages RO still.
> 
> It gets worse.  His approach also needs to adjust the reference
> counts on all pages (and swap pages).

Well, Rik, time to present your algorithm.  I assume it won't reference 
counts on pages, and will do some kind of traversal of the mm tree.  Note 
however, that I did investigate the class of algorithm you are interested in, 
and found only nasty, complex solutions there, with challenging locking 
problems.  (I also looked at a number of possible improvements to virtual 
scanning, as you know, and likewise only found ugly or inadequate solutions.)

Before you sink a lot of time into it though, you might add up the actual 
overhead you're worried about above, and see if it moves the needle in a real 
system.

-- 
Daniel
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