netdev - Re: [PATCH net-next] fq_codel: report congestion notification at enqueue time

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Message-ID: <1340943539.13187.193.camel@edumazet-glaptop>
Date:	Fri, 29 Jun 2012 06:18:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Dave Taht <dave.taht@...il.com>,
	codel@...ts.bufferbloat.net, Tom Herbert <therbert@...gle.com>,
	Matt Mathis <mattmathis@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] fq_codel: report congestion notification at
 enqueue time

On Thu, 2012-06-28 at 16:52 -0700, Nandita Dukkipati wrote:
> As you know I really like this idea. My main concern is that the same
> packet could cause TCP to reduce cwnd twice within an RTT - first on
> enqueue and then if this packet is ECN marked on dequeue. I don't
> think this is the desired behavior. Can we avoid it?

I'll work on this.

In my experiences, I found that no drops (or ECN marks) were done at
dequeue time once one NET_XMIT_CN was returned, but its certainly
possible if other flows compete with this one.

Strangely, SFQ has the same behavior and nobody complained yet ;)

Thanks


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