netdev - RE: [PATCH] ipv6: Limit mtu to 65572 bytes

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6F5DBD@AcuExch.aculab.com>
Date:	Fri, 11 Apr 2014 08:40:42 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
CC:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>,
	David Miller <davem@...emloft.net>,
	Francois WELLENREITER <f.wellenreiter@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] ipv6: Limit mtu to 65572 bytes

From: Eric Dumazet
> Yes, recent Intel cpus have really fast mem copy, even with different
> alignments for source/dest.

Yes, they've finally thrown a small amount of logic at it.
Probably a cache line sized barrel shifter.
'rep movsb' has also been optimised for short transfers (but is
slower for longer ones).

At least the zero length 'rep movsb' at the end of the inlined
memcpy will not take 40+ clocks on those cpus.

	David