netdev - Re: tbench wrt. loopback TSO

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Message-ID: <Pine.LNX.4.64.0810270942430.1290@wrl-59.cs.helsinki.fi>
Date:	Mon, 27 Oct 2008 09:49:21 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	zbr@...emap.net, David Miller <davem@...emloft.net>
cc:	Netdev <netdev@...r.kernel.org>, efault@....de, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: tbench wrt. loopback TSO

On Sun, 26 Oct 2008, David Miller wrote:

> 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.

Good point, could you Evgeniy verify first if simple goto send_now; in 
beginning there recovers it all...

-- 
 i.
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