netdev - Re: [PATCH v2 net-next] tcp: fix ABC in tcp_slow

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Date:	Fri, 20 Jul 2012 09:03:45 -0700
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	John Heffner <johnwheffner@...il.com>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: fix ABC in tcp_slow_start()

On Fri, Jul 20, 2012 at 8:07 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> On Fri, Jul 20, 2012 at 8:02 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>         tp->snd_cwnd_cnt += cnt;
>>         while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {

Nice catch, Eric.

One thing that's always bothered me about the tp->snd_cwnd_cnt code is
that the slow start and congestion avoidance use different criteria
for incrementing snd_cwnd_cnt. tcp_slow_start() increments
snd_cwnd_cnt by snd_cwnd for each ACKed packet, and congestion
avoidance increases snd_cwnd_cnt by just 1 for each packet.

This means that if we exit slow start and enter congestion avoidance,
then we think we can have a "credit" for a bunch of ACKs that never
happened (up to snd_cwnd-1), so we can conceivably do our first
additive increase in congestion avoidance up to almost 1RTT too
early. Can we just get rid of the use of snd_cwnd_cnt in slow start,
and just use local variables in tcp_slow_start() rather than trying to
carry state between ACKs?

neal
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