netdev - Re: small RPS cache for fragments?

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Date:	Tue, 17 May 2011 22:11:43 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, therbert@...gle.com,
	netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

On Tue, 2011-05-17 at 23:00 +0200, Eric Dumazet wrote:
> 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.

But then each datagram lands on a different CPU, and reordering is
liable to happen far more often than it does now.

> At least all frags of a given datagram should be reassembled on same
> cpu, so we get RPS (but not RFS)

You could still do RPS with just IP addresses (same as RSS using
Toeplitz hashes).

Ben.

-- 
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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