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Message-ID: <4E3C5F03.8090100@linux.intel.com> Date: Fri, 05 Aug 2011 14:22:11 -0700 From: "H. Peter Anvin" <hpa@...ux.intel.com> To: Christoph Lameter <cl@...ux.com> CC: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com> Subject: Re: [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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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