Quoting Paul Mackerras ([email protected]):
> 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.
Hi,
Olof Johansson had suggested that earlier, except that his patch used
if (unlikely(!x))
return;
Performance was quite good, but not as good as having prefetch completely
disabled. I got
# elements: 50, mean 851.263680, variance 24.561146, std dev 4.955920
compared to 860.823880 stdev 6.896914 with prefetch disabled.
thanks,
-serge
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