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

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Date:	Fri, 15 Mar 2013 00:12:50 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Ben Hutchings <bhutchings@...arflare.com>
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 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)?

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