Bursty data transfer with Dummynet

[Julian Elischer]

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
>