netdev - Re: RFC crap-patch [PATCH] net: Per CPU separate frag mem accounting

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Date:	Thu, 14 Mar 2013 23:39:44 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
CC:	Jesper Dangaard Brouer <jbrouer@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	<netdev@...r.kernel.org>, <yoshfuji@...ux-ipv6.org>
Subject: Re: RFC crap-patch [PATCH] net: Per CPU separate frag mem accounting

On Fri, 2013-03-15 at 00:12 +0100, Hannes Frederic Sowa wrote:
> On Thu, Mar 14, 2013 at 08:59:03PM +0000, Ben Hutchings wrote:
> > On Thu, 2013-03-14 at 09:59 +0100, Jesper Dangaard Brouer wrote:
> > > On Thu, 2013-03-14 at 08:25 +0100, Jesper Dangaard Brouer wrote:
> > > > This is NOT the patch I just mentioned in the other thread, of removing
> > > > the LRU list.  This patch does real per cpu mem acct, and LRU per CPU.
> > > > 
> > > > I get really good performance number with this patch, but I still think
> > > > this might not be the correct solution.
> > > 
> > > The reason is this depend on fragments entering the same HW queue, some
> > > NICs might not put the first fragment (which have the full header
> > > tuples) and the remaining fragments on the same queue. In which case
> > > this patch will loose its performance gain.
> > [...]
> > 
> > The Microsoft RSS spec only includes port numbers in the flow hash for
> > TCP, presumably because TCP avoids IP fragmentation whereas datagram
> > protocols cannot.  Some Linux drivers allow UDP ports to be included in
> > the flow hash but I don't think this is the default for any of them.
> > 
> > In Solarflare hardware the IPv4 MF bit inhibits layer 4 flow steering,
> > so all fragments will be unsteered.  I don't know whether everyone else
> > got that right though. :-)
> 
> Shouldn't they be steered by the IPv4 2-tuple then (if ipv4 hashing is enabled
> on the card)?

IP fragments should get a flow hash based on the 2-tuple, yes.

To make myself clear, my working definitions are:
- Flow steering maps specified flows to specified RX queues
- Flow hashing maps all flows to RX queues
- Flow steering rules generally override flow hashing

Ben.

-- 
Ben Hutchings, Staff 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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