netdev - Re: [PATCH 5/5] tcp: skip cwnd moderation in TCP_CA_Open in tcp_try_to

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Date:	Mon, 28 Nov 2011 12:35:01 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	ilpo.jarvinen@...sinki.fi, ycheng@...gle.com,
	netdev@...r.kernel.org, nanditad@...gle.com, therbert@...gle.com
Subject: Re: [PATCH 5/5] tcp: skip cwnd moderation in TCP_CA_Open in tcp_try_to_open

On Sun, Nov 27, 2011 at 7:01 PM, David Miller <davem@...emloft.net> wrote:
> From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
> Date: Tue, 22 Nov 2011 13:47:13 +0200 (EET)
>
>> On Mon, 21 Nov 2011, Yuchung Cheng wrote:
>>
>>> AFAIK cwnd moderation is to lower bursty drops not to discourage
>>> (dsack) cheating. I believe the reason is the same in
>>> tcp_try_to_open(). We are in common cases, e.g., loss-recovery, this
>>> logic hurts performance.
>>
>> Quote from the patch description: "Senders were overriding cwnd values
>> picked during an undo by calling tcp_moderate_cwnd()" ...so after all it
>> has to do with undo being limited. IMHO only up to orig_cwnd/2+IW is safe
>> due to cheating opportunities. Also FRTO uses orig_cwnd/2 due to same
>> reason (it could do the +IW too but IIRC it is only /2 currently). What
>> would be the safeguard there after this one is removed? I kind of see your
>> point about this particular line being related to burst mitigation but on
>> the same time the end result of removal is that undo becomes potentially
>> much more aggressive.
>
> I'm apply this patch to net-next now, but Neil and Yucheng are on the hook
> to fully look into the issues Ilpo has raised.

Thanks! We will spend some time looking into these issues.

neal
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