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Re: [Qemu-devel] [Bug 899140] Re: Problem with Linux Kernel Traffic Cont

From: Vincent Autefage
Subject: Re: [Qemu-devel] [Bug 899140] Re: Problem with Linux Kernel Traffic Control
Date: Sun, 04 Dec 2011 15:54:12 -0000

The result without TC is about 120 Mbit/s.
I check the bandwidth with lot of programs (not only with Iperf) and the 
result is also the same....

However, if I use the same raw image and the same TC configuration with 
the version 0.14.0 of QEMU or with some real physical hosts, the result 
with TC is about 19.2 Mbit/s what is the desired result...


Le 03/12/2011 19:48, Stefan Hajnoczi a écrit :
> On Fri, Dec 2, 2011 at 2:42 PM, Vincent Autefage
> <address@hidden>  wrote:
>> address@hidden tc qdisc add dev eth0 root tbf rate 20mbit burst 20480 latency
>> 50ms*
>> address@hidden **ifconfig eth0*
>> Then if we check with /Iperf/, the real rate will be about 100kbit/s :
> What is the iperf result without tc?  It's worth checking what rate
> the unlimited interface saturates at before applying tc.  Perhaps this
> setup is just performing very poorly and it has nothing to do with tc.
> Stefan

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  Problem with Linux Kernel Traffic Control

Status in QEMU:

Bug description:

  The last main versions of QEMU (0.14.1, 0.15 and 1.0) have an important 
problem when running on a Linux distribution which running itself a Traffic 
Control (TC) instance.
  Indeed, when TC is configured with a Token Bucket Filter (TBF) with a 
particular rate, the effective rate is very slower than the desired one.

  For instance, lets consider the following configuration :

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

  The effective rate will be about 100kbit/s ! (verified with iperf)
  I've encountered this problem on versions 0.14.1, 0.15 and 1.0 but not with 
the 0.14.0...
  In the 0.14.0, we have a rate of 19.2 mbit/s which is quiet normal.

  I've done the experimentation on several hosts :

  - Debian 32bit core i7, 4GB RAM
  - Debian 64bit core i7, 8GB RAM
  - 3 different high performance servers : Ubuntu 64 bits, 48 AMD Opteron, 
128GB of RAM

  The problem is always the same... The problem is also seen with a
  Class Based Queuing (CBQ) in TC.


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