netdev - Re: tbench wrt. loopback TSO

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Message-Id: <20081026.185911.193696873.davem@davemloft.net>
Date:	Sun, 26 Oct 2008 18:59:11 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	zbr@...emap.net
Cc:	netdev@...r.kernel.org, efault@....de, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, herbert@...dor.apana.org.au
Subject: Re: tbench wrt. loopback TSO

From: Evgeniy Polyakov <zbr@...emap.net>
Date: Sun, 26 Oct 2008 15:34:42 +0300

> On Wed, Oct 15, 2008 at 05:14:08PM -0700, David Miller (davem@...emloft.net) wrote:
> > 
> > I got curious about this aspect of the investigation so I wanted
> > to see it first-hand :-)
> > 
> > To be honest, this reported effect of disabling TSO in the loopback
> > driver surprised me because:
> > 
> > 1) If the benchmark is doing small writes, TSO should have zero
> >    effect.  The TSO logic won't kick in.
> 
> But GSO will try to create a huge packet and that overhead will not be
> overweighted?
> 
> That's what I got with the current tree for 8 threads on a 4-way 32-bit
> Xeons (2 physical CPUs) and 8gb of ram:
> gso/tso off: 361.367
> tso/gso on:  354.635
> 
> Disabled/enabled via ethtools: -k tso off/on gso off/on

Yes, it might do this, in which case tcp_tso_should_defer() simply needs
some tweaking.
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