Bursty data transfer with Dummynet

[Julian Elischer]

On 11/13/13, 2:22 PM, Lee Dilkie wrote:
> seriously?
>
> you expect that kind of precise timing running in a VM?
the fact that it is in a vm only just came out... still I'd expect 
better than "several seconds"..

>
> -lee
>
> On 11/13/2013 16:29, Ahmed Hamza wrote:
>> I'm not sure if this would explain anything. But I am running DummyNet
>> on a VirtualBox VM on an Ubuntu host. I have also experienced the same
>> behaviour with both Frenzy (a FreeBSD based LiveCD) and Ubuntu guests.
>> In the case of Frenzy, the tick was set to 1000Hz. But the host tick
>> was 250Hz since it is an Ubuntu machine. Also, the delay between
>> bursts is in order of seconds! My assumption about DummyNet is that
>> while the queue is being drained new packets will be queued (i.e.
>> there is no waiting to fill the queues before transmitting them using
>> the specified bandwidth).
>>
>> On Tue, Nov 12, 2013 at 9:21 PM, Julian Elischer<julian at freebsd.org>  wrote:
>>> On 11/12/13, 9:06 PM, Ahmed Hamza wrote:
>>>> On Tue, Nov 12, 2013 at 8:50 PM, Julian Elischer<julian at freebsd.org>
>>>> wrote:
>>>>> On 11/12/13, 6:35 PM, Ahmed Hamza wrote:
>>>>>> Hi All,
>>>>>>
>>>>>> I'm trying to use Dummynet to test the behaviour of my video streaming
>>>>>> application in various network conditions. Dummynet was compiled and
>>>>>> installed on an Ubuntu 12.04 box with a 2.6 Linux kernel. I'm
>>>>>> experiencing a strange behaviour when I reduce the bandwidth of the
>>>>>> link/path.
>>>>>>
>>>>>> For some reason, instead of having a slow download speed. It seems as
>>>>>> if the download is happening in bursts! A portion of the data is
>>>>>> downloaded at a high speed, then the data transfer stops for a period
>>>>>> of time and then resumes again (and so on). Does anyone have an idea
>>>>>> what could be the cause? Or is this even an expected behaviour? If so,
>>>>>> why?
>>>>> I can't really speak for dummynet on Linux but the granularity of the
>>>>> queues
>>>>> is dependent on the timer granularity of the kernel you have and to some
>>>>> extent will rely on the correct integration of dummynet into the timer
>>>>> facility of the kernel you are running it in. On freeBSD with a 1kHz
>>>>> 'tick'
>>>>> I'd' expect to see packets being release from the queue each mSec or so.
>>>>> if you are getting second sized chunks then it probably is a bug. Either
>>>>> dummynet is not compatible with that kind of kernel, something else has
>>>>> gone
>>>>> wrong.  It COULD also be that you are catching the wrong packets.. I've
>>>>> seen
>>>>> similar behaviour when I was accidentally queuing all the acks instead of
>>>>> all the data going in the other direction, but I presume you have already
>>>>> checked to see what you are queuing.
>>>>>
>>>> Thanks Julian and Matthew for your replies. To clarify my settings,
>>>> below are the outputs from 'ipfw show' and 'ipfw pipe show'. I should
>>>> also mention that it seems the by default the Ubuntu kernel is
>>>> configured for 250Hz, not 1000Hz.
>>> I would expect that that would give a queue kick every 4 mSec or so so you
>>> should get a burst every 4mSec or so.
>>> but your queue is only doing 500kbit/sec or 1/2kbit /msec,  or about
>>> 2kbit/4mSec or 250bytes per tick which is less than a packet. I'm afraid
>>> you'll have to talk to Luigi to know what happens in such a case..
>>>
>>>
>>>> root at vm1:~/# ipfw pipe 1 config bw 500Kbit/s queue 500Kbyte
>>>>
>>>> root at nsl-vm1:~/# ipfw show
>>>> 00100  202247  105063701 pipe 1 ip from 192.168.56.4 to any
>>>> 65535 2245577 2648958386 allow ip from any to any
>>>>
>>>> root at nsl-vm1:~/# ipfw pipe show
>>>> 00001: 500.000 Kbit/s    0 ms  500 KB 1 queues (1 buckets) droptail
>>>>       mask: 0x00 0x00000000/0x0000 -> 0x00000000/0x0000
>>>> BKT Prot ___Source IP/port____ ____Dest. IP/port____ Tot_pkt/bytes
>>>> Pkt/Byte Drp
>>>>     0 tcp     192.168.56.1/33547    192.168.56.4/80    238083 134515909
>>>> 0    0 623
>>>>
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>