Re: large page patch (fwd) (fwd)

From: Eric W. Biederman (
Date: Sun Aug 04 2002 - 18:51:51 EST

Andi Kleen <> writes:

> Andrew Morton <> writes:
> > If we instead clear out 4 or 8 pages, we trash a ton of cache and
> > the chances of userspace _using_ pages 1-7 in the short-term are
> > lower. We could clear the pages with 7,6,5,4,3,2,1,0 ordering,
> > but the cache implications of faultahead are still there.
> What you could do on modern x86 and probably most other architectures as
> well is to clear the faulted page in cache and clear the other pages
> with a non temporal write. The non temporal write will go straight
> to main memory and not pollute any caches.

Plus a non temporal write is 3x faster than a write that lands in
the cache on x86 (tested on Athlons, P4, & P3).
> When the process accesses it later it has to fetch the zeroes from
> main memory. This is probably still faster than a page fault at least
> for the first few accesses. It could be more costly when walking the full
> page (then the added up cache miss costs could exceed the page fault cost),
> but then hopefully the CPU will help by doing hardware prefetch.
> It could help or not help, may be worth a try at least :-)


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