netdev - Re: [PATCH net-next-2.6] rps: consistent rxhash

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Date:	Wed, 21 Apr 2010 13:16:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Franco Fichtner <franco@...tsummer.de>
Cc:	Tom Herbert <therbert@...gle.com>, Changli Gao <xiaosuo@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] rps: consistent rxhash

Le mercredi 21 avril 2010 à 13:06 +0200, Franco Fichtner a écrit :
> > 
> 
> Hashing for cpu distribution should be as minimal as it could possibly
> be with the least number operations needed to compute a hash, which 
> normally involves touching one cold cache line (ip header). If you add 
> the ports to your mix you have the luxury of solving static ip mappings,
> but only for protocols that support it. Usage of the destination port
> may also prove to be more or less pointless with a lot of http traffic,
> because it's most likely static. And you add another potential cold
> cache line access. For a lot of traffic scenarios, we'll have a bunch of
> internal ips and the internet on the other side, so having a simple hash
> based on a flavor if internal/external ip is more than enough to work
> with for distribution. If the network card can provide a complete hash
> all the better. Then this part of my point is void.
> 

But we already have to bring into our cpu cache one cache line, needed
in eth_type_trans() : (12+2 bytes of ethernet header)

TCP/UDP tuples are included into this cache line (64 bytes on current
popular arches)

Cost of rxhash is absolute noise into the picture.
A device provided hash, to be effective, would also make
eth_type_trans() call not done.



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