netdev - Re: [net-next-2.6 PATCH 2/5] ixgbe: drop support for UDP in RSS hash generation

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Date:	Tue, 20 Jul 2010 18:39:37 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"bphilips@...ell.com" <bphilips@...ell.com>,
	"Skidmore, Donald C" <donald.c.skidmore@...el.com>
Subject: Re: [net-next-2.6 PATCH 2/5] ixgbe: drop support for UDP in RSS
 hash generation

Le mardi 20 juillet 2010 à 09:11 -0700, Alexander Duyck a écrit :

> The packets will still be hashed on source and destination IPv4/IPv6 
> addresses.  The change just drops reading the UDP source/destination 
> ports since in the case of fragmented packets they are not available and 
> as such were being parsed as IPv4/IPv6 packets.  By making this change 
> the queue selection is consistent between all packets in the UDP stream.
> 

Excellent, this is perfect IMHO.

> The only regression I would expect to see would be in testing between 
> two fixed systems since the IP addresses of the two systems would be 
> fixed and so running multiple flows between the two would yield the same 
>   RSS hash for multiple UDP streams.  As long as multiple ip addresses 
> are used  you should see multiple RSS hashes generated and as such the 
> load should still be distributed.
> 

Ack. Fortunately, one can still use RPS to spread load onto multiple
cpus in this case.

This until ixgpe fills skb->rxhash with a non null value.
If it happens one day, we shall remind _not_ filling it for UDP packets.

BTW, this reminds me a netdev discussion we had for bnx2x

http://www.kerneltrap.com/mailarchive/linux-netdev/2010/4/23/6275415/thread

And now, I understand why Toepliz hash doesnt use src port/dst port,
since this is not available on fragments, obviously...

Thanks !


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