netdev - Re: small RPS cache for fragments?

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Date:	Tue, 17 May 2011 22:13:42 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, therbert@...gle.com, netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

On Tue, 2011-05-17 at 17:10 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 17 May 2011 23:00:50 +0200
> 
> > Le mardi 17 mai 2011 à 16:49 -0400, David Miller a écrit :
> >> From: Tom Herbert <therbert@...gle.com>
> >> Date: Tue, 17 May 2011 13:02:25 -0700
> >> 
> >> > I like it!  And this sounds like the sort of algorithm that NICs might
> >> > be able to implement to solve the UDP/RSS unpleasantness, so even
> >> > better.
> >> 
> >> Actually, I think it won't work.  Even Linux emits fragments last to
> >> first, so we won't see the UDP header until the last packet where it's
> >> no longer useful.
> >> 
> >> Back to the drawing board. :-/
> > 
> > Well, we could just use the iph->id in the rxhash computation for frags.
> > 
> > At least all frags of a given datagram should be reassembled on same
> > cpu, so we get RPS (but not RFS)
> 
> That's true, but one could also argue that in the existing code at least
> one of the packets (the one with the UDP header) would make it to the
> proper flow cpu.

No, we ignore the layer-4 header when either MF or OFFSET is non-zero.

Ben.

> That could be as much as half of the packets.
> 
> So I don't yet see it as a clear win.
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-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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