[RFC SKBUFF]: Keep track of writable header len of headerless clones

[Patrick McHardy]

David Miller wrote:
> From: Patrick McHardy <kaber at trash.net>
> Date: Sun, 24 Jun 2007 14:53:36 +0200
>
>   
>> - sendmsg eth0, no NAT:		sys     0m2.508s
>> - sendmsg eth0, NAT:		sys     0m2.539s
>> - sendmsg eth0, NAT + patch:	sys     0m2.445s	(no change)
>>     
>
> This is probably because we're touching all the data anyways
> and if the resident set size is small enough (as a gigabit
> or 100MB ethernet TCP stream window would be) then it all fits
> mostly in the L2 caches of the cpu.

Yeah, thats what I suspected too. On loopback it seems to exceed
the cache size, which is a bit strange since its 1MB. Or maybe
there is some loopback optimization interfering.