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SubjectRe: [x86] Fix prefetch instruction
On 08/05/2011 02:10 PM, Christoph Lameter wrote:
>>
>> Have you done any performance analysis on this versus the null case? I
>> know there are some workloads where it helps, but if it hurts as many as
>> it helps...
>
> No I have not. prefetch IMHO means that the cacheline is fetched early so
> that the cacheline is fully available like any other to the code.
> prefetchnta does fetch the cacheline too but its not treated like the other cacheline but
> preferably thrown out again. Its a "streamfetch" designed for apps that
> scan over large amounts of memory and want to avoid cache pollution.
>
> This is surprising to the end user as far as I can tell.
>

Right. However, Linus has brought up the hypothesis that prefetch might
actually be a net loss on x86, because current x86 processors are
generally doing a good job with prefetching in hardware. Directed
prefetches can thus be a net minus.

-hpa


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