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.
(and after patch 3/3 also a page allocation/clear)
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