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

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Message-ID: <1271842773.7895.1692.camel@edumazet-laptop>
Date:	Wed, 21 Apr 2010 11:39:33 +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 11:29 +0200, Franco Fichtner a écrit :
> Tom Herbert wrote:
> >> I thought about this for some time...
> >>
> >> Do we really need the port numbers here at all? A simple
> >> addr1^addr2 can provide a good enough pointer for
> >> distribution amongst CPUs.
> >>
> > 
> > What about a server behind a TCP proxy?  Also, need to minimize
> > collisions for RPS to be effective
> 
> What about routers? What about loopback? This all boils down to
> the same issue of obscuring IP data by "magical" means and then
> reattaching functionality by reaching for upper layer information.
> It is necessary in some cases, but it can cripple performance
> for other cases.
> 
> The interesting thing is you don't need to deal with collisions
> while distributing amonst cpus at all. You just need to make sure
> the distribution algorithm keeps every single flow attached to
> the correct cpu.
> 
> All of the actual flow hashing, tracking and whatever else the
> traffic needs to go through can be done locally by cpu x which
> helps a lot with load distribution and cache issues in mind. It
> also helps locking because there is no global flow lookup table.
> Oh, and it also reduces collisions with every cpu you add for
> receiving.
> 
> I work with a lot of plain office and ISP traffic in mind daily,
> so please don't misunderstand my motivation here. I'd hate to
> see poor performance in scenarios in which there is a lot of
> potential improvement.
> 

I am a bit lost by this conversation.

Are you saying something is wrong with current schem ?

What are exactly your suggestions ?

Tom replied to you that a hash derived from (addr1 ^ addr2) would not
work in situations where all flows goes from machine A to machine B
(all hashes would be the same)

Current hash is probably more than enough to cover all situations.





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