What language?  Actual code is always helpful as well.

Josh

On Wed, Aug 4, 2010 at 12:46 PM, Peter Müller <grandmastermt2000@yahoo.de>wrote:

> Hi,
>
> I observed some behaviour, which seemed strange to me. I have written a
> simple publisher and a simple subscriber. The job of the subscriber is to
> receive the data packet of the publisher, and save the timestamp of the data
> packet and the time of arrival in a vector for later analysis.
> The publisher simply generates the timestamp and publishes the packet at a
> rate of 20hz.
>
> Now I made 2 test runs with a volume of 10.000 messages each run. I
> computed the latency (t_arrival - t_sent) and plotted it using matlab.
> Everything looked good.
> However when I change the setup slightly, the data gets distorted heavily.
> The only thing I changed was the way the publisher works.
> The publisher now sends 2 packets at a rate of 20hz. This was achieved by a
> simple for-loop.
> Pseudocode:
> repeat 10000 times:
> for (i = 1 until 2)
>   generate timestamp for packet
>   node.publish(packet)
> end
> sleep
>
>
> Here is a plot of the example, which seemed to work (one packet at 20hz)
> http://img580.imageshack.us/img580/4402/matlabros1.png
>
> Now when I do a burst of 2 messages at 20hz I get this:
> http://img38.imageshack.us/img38/70/matlabros2.png
>
> I further looked at the data to try to find out what happens. Since I save
> every timestamp I plotted them with switching between red/blue as a color.
> This is a plot of the first 50 messages/timestamps with red being the entry
> i and blue being the entry i+1 in the vector (i being the index going from 1
> to 50 in this plot).
> To clarify here is a snippet of the matlab code:
> for i = 1:2:50
>    plot(i, ros_burst(i), '*', 'Color', 'r');
>    plot(i+1, ros_burst(i+1), '*', 'Color', 'b');
> end
> This means the red star should be the first message of the burst.
> Here is the corresponding plot:
> http://img704.imageshack.us/img704/994/matlabros3.png
>
> I also plotted the whole data set with this color scheme:
> http://img827.imageshack.us/img827/3715/matlabros4.png
>
>
> I did not expect something like this to happen and unfortunately I can't
> explain this on my own.
> Is anyone familiar with this? The ros code I have written is extremely
> simple and if requested I can post the relevant parts.
>
>
>
>
> _______________________________________________
> ros-users mailing list
> ros-users@code.ros.org
> https://code.ros.org/mailman/listinfo/ros-users
>