netdev - Re: small RPS cache for fragments?

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Date:	Tue, 17 May 2011 15:42:48 -0700
From:	Rick Jones <rick.jones2@...com>
To:	David Miller <davem@...emloft.net>
Cc:	andi@...stfloor.org, netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

On Tue, 2011-05-17 at 17:50 -0400, David Miller wrote:
> From: Andi Kleen <andi@...stfloor.org>
> Date: Tue, 17 May 2011 14:48:28 -0700
> 
> > David Miller <davem@...emloft.net> writes:
> > 
> >> Guys we can't time out fragments if we are not the final
> >> destination.
> > 
> > If you're not the final destination you should never even
> > try to reassemble them?
> > 
> > I'm probably missing something...
> 
> We're discussing the idea to do the defragmentation first
> so we can choose the flow properly and steer the packet
> to the correct cpu.
> 
> This also would allos each fragmented packet to traverse the
> stack only once (one route lookup etc.) instead of once per
> fragment.
> 
> Please read the rest of this thread, we have discussed this
> and now I'm repeating information solely for your benefit.

Well, I should probably be beaten with that stick too because I wasn't
thinking about forwarding, only being the destination system when I
broached the suggestion of doing RFS after reassembly.  I can see where
one *might* be able to do limited RPS when forwarding, but I didn't know
that RFS had been extended to forwarding.

Now though I see why you were rightfully concerned about timeouts -
given all the concerns about added latency from bufferbloat, I wouldn't
think that an additional 10 or perhaps even 1ms timeout on a reassembly
attempt to get the layer four header when forwarding would sit well with
folks - they will expect the fragments to flow through without
additional delay.

rick jones

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