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 09:11:50 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.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

Eric Dumazet wrote:
> Le lundi 19 juillet 2010 à 16:59 -0700, Jeff Kirsher a écrit :
>> From: Alexander Duyck <alexander.h.duyck@...el.com>
>>
>> This change removes UDP from the supported protocols for RSS hashing.  The
>> reason for removing this protocol is because IP fragmentation was causing a
>> network flow to be broken into two streams, one for fragmented, and one for
>> non-fragmented and this in turn was causing out-of-order issues.
>>
> 
> Jeff, does it mean all UDP packets are going to be delivered to a single
> queue ?
> 
> This would be a serious regression.
> 
> Many UDP applications try hard to not use fragments. 
> 
> They are going to pay the price because some application :
> - Use big segments, fragmented.
> - Is subject to OOO artifacts.
> 
> We would like some clarifications please :)
> 
> 
> 

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.

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.

Thanks,

Alex
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