From: Andi Kleen <ak@suse.de>
To: Arjan van de Ven <arjan@intel.linux.com>
Subject: Re: [Patch 1/3] prefetch the mmap_sem in the fault path
Date: Thu, 23 Feb 2006 11:14:52 +0100
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On Thursday 23 February 2006 10:47, Arjan van de Ven wrote:
> On Thu, 2006-02-23 at 10:39 +0100, Andi Kleen wrote:
> > On Thursday 23 February 2006 10:30, Arjan van de Ven wrote:
> > > In a micro-benchmark that stresses the pagefault path, the down_read_trylock
> > > on the mmap_sem showed up quite high on the profile. Turns out this lock is
> > > bouncing between cpus quite a bit and thus is cache-cold a lot. This patch
> > > prefetches the lock (for write) as early as possible (and before some other
> > > somewhat expensive operations). With this patch, the down_read_trylock
> > > basically fell out of the top of profile.
> > 
> > It is hard to believe because you effectively didn't do the prefetch
> > very early
> 
> all you need is a few dozen cycles though; there's a cr2 move and the
> entire notifier inbetween.... neither of those is really cheap.

Ok. I added that patch.

-Andi
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