Re: [rfc][patch] store-free path walking

[Nick Piggin]

On Thu, Oct 08, 2009 at 03:12:08PM +0200, Denys Vlasenko wrote:
> On Wed, Oct 7, 2009 at 11:57 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > This, btw, is exactly the kind of thing we saw with some of the
> > non-temporal work, when we used nontemporal stores to copy pages on COW
> > faults, or when doing pre-zeroing of pages. You get rid of some of the
> > hot-spots in the kernel, and you then replace them with user space taking
> > the cache misses in random spots instead. The kernel profile looks better,
> > and system time may go down, but actual performace never went down - you
> > just moved your cache miss cost from one place to another.
> 
> A few years ago when K7s were not ancient yet, after hearing
> argument for and against non-temporal stores,
> I decided to finally figure it for myself.
> 
> I tested kernel build workload on two kernels with the only
> one difference - clear_page with and without non-temporal stores.
> 
> "Non-temporal stores" kernel was faster, not slower. Just a little bit,
> but reproducibly.

It is going to be highly dependent on architecture and workload and
exactly where you use the nontemporal stores of course. I would say
with non-temporal stores in clear_page (a case where we can often
expect the memory to be used again quickly because it is anonymous
process memory), then we are quite likely to cause _more_ activity
on the memory controller and dimms which cost far more power than
cache access.

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