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Date: Fri, 06 Dec 2002 16:30:29 -0800
From: Andrew Morton <akpm@digeo.com>
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To: William Lee Irwin III <wli@holomorphy.com>
CC: Andrea Arcangeli <andrea@suse.de>, Norman Gaywood <norm@turing.une.edu.au>,
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Subject: Re: Maybe a VM bug in 2.4.18-18 from RH 8.0?
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William Lee Irwin III wrote:
> 
> > William Lee Irwin III wrote:
> > >
> > > ...
> > > A 16KB or 64KB kernel allocation unit would then annihilate
> >
> On Fri, Dec 06, 2002 at 04:01:24PM -0800, Andrew Morton wrote:
> > You want to be careful about this:
> >       CPU: L1 I cache: 16K, L1 D cache: 16K
> > Because instantiating a 16k page into user pagetables in
> > one hit means that it must all be zeroed.  With these large
> > pagesizes that means that the application is likely to get
> > 100% L1 misses against the new page, whereas it currently
> > gets 100% hits.
> 
> 16K is reasonable; after that one might as well go all the way.

16k will cause serious slowdowns.

> About the only way to cope is amortizing it by cacheing zeroed pages,
> and that has other downsides.

So will that.  You've seen the kernbench profiles...

You will need to find a way to clear the page just before it
is used.
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