Re: large page patch (fwd) (fwd)

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <ak@suse.de> writes:

> Andrew Morton <akpm@zip.com.au> 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 :-)

Certainly.

Eric