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

[Henrique Rodrigues]

Vincent,

Have you tried to change the mtu of the tbf qdisc? The traffic control should 
work well if you set it to 65kb.
I believe that this is happening due to the napi gro functionality. I'm still 
not sure, but the problem seems to be related to that.

Henrique

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https://bugs.launchpad.net/bugs/899140

Title:
  Problem with Linux Kernel Traffic Control

Status in QEMU:
  New

Bug description:
  Hi,

  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.

  Thanks

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