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Date: Fri, 16 Dec 2011 08:31:03 +0000
From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] [Bug 899140] Re: Problem with Linux Kernel
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To: Vincent Autefage <899140@bugs.launchpad.net>
Cc: qemu-devel@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>

On Thu, Dec 15, 2011 at 04:48:13PM -0000, Vincent Autefage wrote:
> Here is the problem !
> 
> The Ubuntu version works only because it not uses an *Intel e1000* but a 
> *rtl8139*.
> Therefore, the problem about the e1000 is present in *all* version 
> (original or ubuntu ones).
> 
> Thus, the file *e1000.c* must contain some instructions which imply the 
> bad TC behavior.

You are right!  Looking back at your QEMU command-line you are not
explicitly specifying the NIC model so the default will take effect.

Now we're back to square one: e1000.c performs poorly when the tc
command you posted is used.  We don't know why yet.

Michael: Have you ever encountered unexpectedly low throughput when tc
is used inside the guest?

# tc qdisc add dev eth0 root tbf rate 20mbit burst 20k latency 50ms

The observed throughput from iperf is only 100kbit/s, not around
20mbit/s as expected.  When tc is not run inside the guest then the NIC
saturates 20mbit/s easily.

Stefan