netdev - Re: [RFC] TCP illinois max rtt aging

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Date:	Thu, 6 Dec 2007 19:27:21 -0800
From:	"Lachlan Andrew" <lachlan.andrew@...il.com>
To:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Cc:	Netdev <netdev@...r.kernel.org>,
	"Tom Quetchenbach" <quetchen@...tech.edu>
Subject: Re: [RFC] TCP illinois max rtt aging

Greetings Ilpo,

On 04/12/2007, Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:
> On Mon, 3 Dec 2007, Lachlan Andrew wrote:
> >
> > When SACK is active, the per-packet processing becomes more involved,
> > tracking the list of lost/SACKed packets.  This causes a CPU spike
> > just after a loss, which increases the RTTs, at least in my
> > experience.
>
> I suspect that as long as old code was able to use hint, it wasn't doing
> that bad. But it was seriously lacking ability to take advantage of sack
> processing hint when e.g., a new hole appeared, or cumulative ACK arrived.
>
> ...Code available in net-2.6.25 might cure those.

We had been using one of your earlier patches, and still had the
problem.  I think you've cured the problem with SACK itself, but there
still seems to be something taking a lot of CPU while recovering from
the loss.  It is possible that it was to do with  web100  which we
have also been running, but I cut out most of the statistics from that
and still had problems.

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
Ph: +1 (626) 395-8820    Fax: +1 (626) 568-3603
http://netlab.caltech.edu/~lachlan
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