netdev - Re: Requeues and ECN marking

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Date:	Mon, 03 Feb 2014 09:28:23 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Greg Kuperman <gregk@....EDU>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Requeues and ECN marking

On Mon, 2014-02-03 at 09:50 -0500, Greg Kuperman wrote:
> Hi all,
> 
> I am testing a new congestion control protocol that relies on explicit
> congestion notifications (ECN) to notify the receiver of a congestion
> event. I have a rate limited link of 1 Mbps, and I am using the RED
> queuing discipline with ECN enabled. What I have noticed is that no
> matter how small I set my queue size, or how low I set my minimum
> marking level, the first ECN marked packet does not get sent out for
> about 10 seconds after the input rate exceeds the output rate. Further
> examination shows that ECN marking does not occur until the number or
> requeues hits 1000. Below are two queries of tc -s -d qdisc ls dev
> eth1.
> 
> qdisc red 8028: root refcnt 2 limit 10000000b min 1b max 0b ecn ewma
> 30 Plog 21 Scell_log 31
>  Sent 1307892 bytes 1247 pkt (dropped 0, overlimits 0 requeues 960)
>  backlog 1052118b 962p requeues 960
>   marked 0 early 0 pdrop 0 other 0
> 
> qdisc red 8028: root refcnt 2 limit 10000000b min 1b max 0b ecn ewma
> 30 Plog 21 Scell_log 31
>  Sent 1379262 bytes 1312 pkt (dropped 0, overlimits 72 requeues 1024)
>  backlog 1122468b 1027p requeues 1024
>   marked 72 early 0 pdrop 0 other 0
> 
> 
> The txqueuelen defaults to 1000 for the interface, so I figured that
> packets maybe buffering there, and then dequeuing, before any packets
> are marked. I set txqueuelen to lower values (all the way down to 1),
> but the exact same behavior occurs (no marked packets until number of
> dequeues hits 1000). In contrast, if I set txqueuele to something very
> high, I get no requeues, drops, or marked packets.
> 
> My goal is for packets to be marked as soon as the ingress rate
> exceeds the egress. Am I correct in thinking that the requeuing
> operation is the culprit? Can I eliminate requeues? Is there something
> else I can do to get the behavior I am looking for?
> 
> Thank you all for the help. And please cc me in your replies; I'm not
> 100% sure if I get all the messages from this mailing list.

requeues have nothing to do with ECN marking.

How is done your rate limiting ?

Post the whole setup, not part of it, it will help to spot the problem
in one go, instead of many mail exchanges.


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