linux-kernel - Re: [x86] Fix prefetch instruction

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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
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