Messages in this thread | | | Date | Wed, 30 Mar 2005 08:33:55 -0600 | From | "Serge E. Hallyn" <> | Subject | Re: prefetch on ppc64 |
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Quoting Paul Mackerras (paulus@samba.org): > 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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