On Tue, 26 Aug 2008 22:41:53 +1000
Herbert Xu [off-list ref] wrote:

(Adding Jamal to the cc)

On Tue, Aug 26, 2008 at 08:24:55AM -0400, Stephen Hemminger wrote:

quoted

The problem with netem as child of TBF is that TBF counts the number
of packets in the queue to determine the rate. Therefore TBF gets confused
about the rate because of the large number of packets that are held in
netem when delaying. 

OK I'm probably missing something.  I can't find any code in TBF
that looks at the number of packets held in the queue.  All it
does is look at the dequeued packet and whether we have enough
tokens to send it right now.

In any case, looking at the number of packets in the queue sounds
broken for TBF as the packets could be held in upper-level queues
which are invisible.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} [off-list ref]
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Last time I tried TBF(100kbit) { netem(+100ms) } it gave different answers
than netem(+100ms) { TBF(100kbit) }.  

I would prefer a peek() to the current dequeue/requeue.
An alternative would be to have netem keep a parallel data structure with
the time to send for all packets, but that would be assuming the underlying
qdisc's were work conserving.