netdev - Re: [PATCH] tcp: avoid cwnd moderation in undo

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Date:	Tue, 15 Mar 2011 11:20:54 +0100
From:	Carsten Wolff <carsten@...ffcarsten.de>
To:	John Heffner <johnwheffner@...il.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>,
	David Miller <davem@...emloft.net>,
	Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>,
	Nandita Dukkipati <nanditad@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: avoid cwnd moderation in undo

On Monday 14 March 2011, John Heffner wrote:
> On Mon, Mar 14, 2011 at 3:10 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
> > On Mon, Mar 14, 2011 at 3:06 AM, Carsten Wolff <carsten@...ffcarsten.de> 
wrote:
> >> The moderation is in place to avoid gigantic segment bursts, which could
> >> cause unnecessary pressure on buffers. In my eyes it's already
> >> suboptimal that the moderation is weakened in the presence of
> >> (detected) reordering, let alone removing it completely.
> > 
> > In the presence of reordering, cwnd is already moderated in Disorder
> > state before
> >  entering the (false) recovery.
> 
> I've always been somewhat skeptical of the usefulness of cwnd
> moderation.  First, I don't know that its behavior is well defined.
> When *should* tcp_moderate_cwnd() actually be called, and why?
> 
> Second, I've never liked the idea in general.  Reducing cwnd has an
> effect lasting many RTTs, so reducing it in response to a transient
> event like reordering seems dubious.  And it does not address many
> causes of bursts, such as ack compression or stretch acks.

I think the undo operations are a special case where it absolutely makes 
sense, because the cwnd _already_ has been reduced (down to 1 segment by RTO 
recovery, or, with its special mixture of ratehalving and newreno, Linux 
sometimes reduces cwnd far below half of it even in fast recovery.). The undo 
re-opens the window all at once, which may allow _really huge_ bursts. I don't 
know enough about the other causes of TCPs burstiness, I'm just concerned 
about allowing this special cause.

Carsten
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