linux-kernel - Re: [PATCH v2 2/2] x86: add prefetching to do

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Message-ID: <87iow58eqf.fsf@tassilo.jf.intel.com>
Date:	Wed, 06 Nov 2013 12:19:52 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, sebastien.dugue@...l.net,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum

Neil Horman <nhorman@...driver.com> writes:

> do_csum was identified via perf recently as a hot spot when doing
> receive on ip over infiniband workloads.  After alot of testing and
> ideas, we found the best optimization available to us currently is to
> prefetch the entire data buffer prior to doing the checksum

On what CPU? Most modern CPUs should not have any trouble at all
prefetching a linear access.

Also for large buffers it is unlikely that all the prefetches
are actually executed, there is usually some limit.

As a minimum you would need:
- run it with a range of buffer sizes
- run this on a range of different CPUs and show no major regressions
- describe all of this actually in the description

But I find at least this patch very dubious.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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