netdev - Re: small RPS cache for fragments?

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Message-ID: <1305667662.2691.7.camel@edumazet-laptop>
Date:	Tue, 17 May 2011 23:27:42 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>, therbert@...gle.com,
	netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

Le mardi 17 mai 2011 à 22:13 +0100, Ben Hutchings a écrit :
> 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.

Exactly

As is, RPS (based on our software rxhash computation) should be working
fine with frags, unless we receive different flows with same
(src_addr,dst_addr) pair.

This is why I asked David if real workloads could hit one cpu instead of
many ones.



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