netdev - Re: tbench wrt. loopback TSO

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Message-Id: <20081031.011411.238617852.davem@davemloft.net>
Date:	Fri, 31 Oct 2008 01:14:11 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	zbr@...emap.net, 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: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Tue, 28 Oct 2008 00:17:00 +0200 (EET)

> On Mon, 27 Oct 2008, Evgeniy Polyakov wrote:
> 
> > That's why when I see modulo something screams inside me...
> > I used following patch (without goto in the tcp_tso_should_defer():
> > 
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index e4c5ac9..8ee7597 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -1075,7 +1075,6 @@ unsigned int tcp_current_mss(struct sock *sk, int large_allowed)
> >  				  tp->tcp_header_len);
> >  
> >  		xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
> > -		xmit_size_goal -= (xmit_size_goal % mss_now);
> >  	}
> >  	tp->xmit_size_goal = xmit_size_goal;
> >  
> > 
> > tso/gso  on: 361.866 362.662
> > tso/gso off: 370.038 368.768
> > 
> > So this is another improvement hidden and not accounted in the tso/gso stuff.
> > Another modulo sits in tcp_mss_split_point().
> 
> I know it's there but it should occur not that often.

It happens every sendmsg() call, and all tbench does is small send,
small recv, repeat.

So with TSO on, a small increase in performance is no surprise, as
we will save some cycles often enough.

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