netdev - Re: [PATCH] netem: fix rate extension and drop accounting

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Date:	Tue, 17 Jul 2012 19:39:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mark Gordon <msg@...gle.com>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Andreas Terzis <aterzis@...gle.com>
Subject: Re: [PATCH] netem: fix rate extension and drop accounting

Please Mark :

1) Dont top post on netdev

2) Dont write HTML mails on netdev (your mail never went to netdev,
   only to CCed people). Only text mails are allowed.

On Tue, 2012-07-17 at 10:20 -0700, Mark Gordon wrote:
> Even the static delay case seems wrong with the new patch.  Assume all
> packets have the same sched_time.  Then if you spam packets that get
> processed at the same time by netem they will all get scheduled with
> the same time_to_send because the first packet will get time_to_send
> of [1] = clock_time + sched_time.  Then packet n compute 'now' as
> [n-1] and delay as sched_time - (clock_time - [1]) = 0 so that [n] =
> [n-1].  Therefore every packet gets scheduled at the same time.
> 
> 
> The above modification seems to fix the issue when latency/jitter is 0
> but suffers from a missing non-linearity when delay is present.  Is
> there a technical reason I'm missing that prevents us from doing rate
> and latency here?  Why wouldn't the 'official' patch have correct
> rate?

Because delay is variable (jitter)

netem as is is not working correctly if you have both a rate limit and
delay.

Hagen is working on a solution, but there is no easy fix.

The right solution is to have :

1) A rate stage, using a child qdisc (that you can graft to install your
own qdisc hierarchy if needed, say if you want codel or fq_codel ;))

  Thats basically a TBF...

2) skb orphan

3) drops/reorders/corrupt/additional delay (variable delay)
   using an internal tfifo, to mimic real networks behavior.

Thats the reverse of how its currently done.

Alternatively, this could be implemented as a special network device,
like bonding, instead of a qdisc.



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