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

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Date:	Sun, 27 Nov 2011 19:01:20 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	ycheng@...gle.com, ncardwell@...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

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.
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