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

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Message-ID: <AANLkTi=qi2GOprXEWCf6v=j=L98SQMdHb1OaHcdALepf@mail.gmail.com>
Date:	Mon, 14 Mar 2011 16:44:32 -0400
From:	John Heffner <johnwheffner@...il.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Carsten Wolff <carsten@...ffcarsten.de>,
	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 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.

  -John
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