netdev - Re: [PATCH net-next] net: optimize csum

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Message-Id: <20140324.002059.1347431123598236103.davem@davemloft.net>
Date:	Mon, 24 Mar 2014 00:20:59 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	herbert@...dor.apana.org.au, hkchu@...gle.com, mwdalton@...gle.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: optimize csum_replace2()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sun, 23 Mar 2014 19:51:36 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> When changing one 16bit value by another in IP header, we can adjust the
> IP checksum by doing a simple operation described in RFC 1624,
> as reminded by David.
> 
> csum_partial() is a complex function on x86_64, not really suited
> for small number of checksummed bytes.
> 
> I spotted csum_partial() being in the top 20 most consuming
> functions (more than 1 %) in a GRO workload, which was rather
> unexpected.
> 
> The caller was inet_gro_complete() doing a csum_replace2() when
> building the new IP header for the GRO packet.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks for following through on this Eric.

Would be nice to improve csum_replace4() similarly, since every
NAT packet uses that thing when we change the address in the
protocol header.
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