netdev - Re: [RFC] csum experts, csum_replace2() is too expensive

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Message-ID: <20140321132820.GM22728@two.firstfloor.org>
Date:	Fri, 21 Mar 2014 14:28:20 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, "H. Peter Anvin" <hpa@...or.com>,
	Patrick McHardy <kaber@...sh.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Michael Dalton <mwdalton@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] csum experts, csum_replace2() is too expensive

On Fri, Mar 21, 2014 at 05:50:50AM -0700, Eric Dumazet wrote:
> On Thu, 2014-03-20 at 18:56 -0700, Andi Kleen wrote:
> > Eric Dumazet <eric.dumazet@...il.com> writes:
> > >
> > > I saw csum_partial() consuming 1% of cpu cycles in a GRO workload, that
> > > is insane...
> > 
> > 
> > Couldn't it just be the cache miss?
> 
> Or the fact that we mix 16 bit stores and 32bit loads ?

It should cause a small stall from not doing load-store
forwarding, but 1% of a serious workload would be surprising.

Are you sure it's not some skid effect?

-Andi
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