Re: Software prefetching considered harmful

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2011-05-19 at 12:05 -0700, Linus Torvalds wrote:
> On Thu, May 19, 2011 at 10:12 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Now, notice that right now I'm *only* talking about removing it for
> > the "hlist" cases (patch attached). I suspect we should do the same
> > thing for all the list helpers.
> 
> Actually, it's the "rcu" versions of the hlist helpers that need this
> most, since those are the performance-critical ones and the ones used
> in avc traversal. So the previous patch did nothing.
> 
> So here's the actual patch I think I should commit.
> 
> Added davem, benh and rmk explicitly - I think you're on linux-arch,
> but still..  You may have machines that like prefetch more, although I
> think the "pollute the L1 cache" issue means that even if  you don't
> have the NULL pointer microtrap issue you'll still find this actually
> performs better..

Asked our local performance god:

Anton Blanchard: yeah we found this 5 years ago, i thought intel were filtering null prefetches
Anton Blanchard: turns out they werent. funny

:-)

Cheers,
Ben.