netdev - Re: [PATCH net-next] netem: fix classful handling

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Message-ID: <20111229165249.GB2948@hell>
Date:	Thu, 29 Dec 2011 17:52:49 +0100
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Dave Taht <dave.taht@...il.com>,
	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] netem: fix classful handling

* Eric Dumazet | 2011-12-29 10:12:02 [+0100]:

>> Also, the whole tfifo idea is only to support the wierd idea that
>> if doing random delay that packets should get reordered based on the
>> results of the random value; it was an behavior some users wanted
>> because that is what NISTnet did.
>
>tfifo supports a time ordered queuing, wich mimics some jitter in the
>network. This seems quite useful.
>
>I see what you suggest : adding 'time_to_send' in the generic qdisc cb.
>
>But it makes no sense if we attach a reordering qdisc, like SFQ :
>A 'high prio' packet will block the whole netem because we'll have to
>throttle since this packet time_to_send will be in the future, while
>many other elligible packets are in queue.

In other words netem jitter and a qdisc !tfifo will not work. Correct? The
rate extension also peak the last packet to get the reference time (assuming a
strict ordering):

[...]
now = netem_skb_cb(skb_peek_tail(list))->time_to_send;
[...]


We should avoid a different (unseeable) behavior depending on the queue
(tfifo, SFQ).  Another point: operate netem and qdisc on the same computer can
lead to timing abnormalities. In our test setups we operate qdisc/tcp/whatever
setups and netem on more then on computer.

Hagen
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