Re: prefetch on ppc64

[Antonio Vargas]

On Wed, 30 Mar 2005 13:55:25 +1000, Paul Mackerras <paulus@xxxxxxxxx> wrote:
> Serge E. Hallyn writes:
> 
> > While investigating the inordinate performance impact one of my patches
> > seemed to be having, we tracked it down to two hlist_for_each_entry
> > loops, and finally to the prefetch instruction in the loop.
> 
> I would be interested to know what results you get if you leave the
> loops using hlist_for_each_entry but change prefetch() and prefetchw()
> to do the dcbt or dcbtst instruction only if the address is non-zero,
> like this:
> 
> static inline void prefetch(const void *x)
> {
>         if (x)
>                 __asm__ __volatile__ ("dcbt 0,%0" : : "r" (x));
> }
> 
> static inline void prefetchw(const void *x)
> {
>         if (x)
>                 __asm__ __volatile__ ("dcbtst 0,%0" : : "r" (x));
> }
> 
> It seems that doing a prefetch on a NULL pointer, while it doesn't
> cause a fault, does waste time looking for a translation of the zero
> address.
> 
> Paul.

Don't know exactly about power5, but G5 processor is described on IBM
docs as doing automatic whole-page prefetch read-ahead when detecting
linear accesses.

-- 
Greetz, Antonio Vargas aka winden of network

http://wind.codepixel.com/

Las cosas no son lo que parecen, excepto cuando parecen lo que si son.
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