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

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Date:	Thu, 28 Jun 2012 15:56:46 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Dave Taht <dave.taht@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, codel@...ts.bufferbloat.net,
	Tom Herbert <therbert@...gle.com>,
	Matt Mathis <mattmathis@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] fq_codel: report congestion notification at
 enqueue time

On Thu, Jun 28, 2012 at 11:12 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2012-06-28 at 10:51 -0700, Dave Taht wrote:
>
>> clever idea. A problem is there are other forms of network traffic on
>> a link, and this is punishing a single tcp
Dave: it won't just punish a single TCP, all protocols that react to
XMIT_CN will share similar fate.

>> stream that may not be the source of the problem in the first place,
>> and basically putting it into double jeopardy.
>>
>
> Why ? In fact this patch helps the tcp session being signaled (as it
> will be anyway) at enqueue time, instead of having to react to packet
> losses indications given (after RTT) by receiver.
>
> Avoiding losses help receiver to consume data without having to buffer
> it into Out Of Order queue.
>
> So its not jeopardy, but early congestion notification without RTT
> delay.
>
> NET_XMIT_CN is a soft signal, far more disruptive than a DROP.
I don't read here: you mean far "less" disruptive in terms of performance?

>
>> I am curious as to how often an enqueue is actually dropping in the
>> codel/fq_codel case, the hope was that there would be plenty of
>> headroom under far more circumstances on this qdisc.
>>
>
> "tc -s qdisc show dev eth0" can show you all the counts.
>
> We never drop a packet at enqueue time, unless you hit the emergency
> limit (10240 packets for fq_codel). When you reach this limit, you are
> under trouble.

Another nice feature is to resume TCP xmit operation at when the skb
hits the wire.
My hutch is that Eric is working on this too.
>
>
>
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