cycleTime returning very incorrect times

asked 2017-03-08 13:53:22 -0500

jdenny
3 ●1 ●1 ●3

I am trying to look at the timing actually performed by ros::Rate::sleep.

ros::Rate loop_rate{1};
while(ros::ok()) {
    //publish to a topic
    ros::spinOnce();
    loop_rate.sleep();
    ROS_INFO("time: %f \n", loop_rate.cycleTime().toSec());
}

I am doing this on the turtlebot platform running ros indigo.

What I am seeing is that the expected cylce time of 1 second holds true in output, however I see outputs of the actual cycle time of 0.0005 seconds on average. However, through

rostopic hz <my topic>

I see a very accurate frequency of 1hz for publishing.

Any ideas on how to fix this/where I am going wrong?

answered 2017-03-08 13:59:14 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2017-03-08 14:03:45 -0500

I think what you are seeing is actually expected behaviour.

From the documentation of cycleTime() (here):

Get the actual run time of a cycle from start to sleep.

See also here for where this is calculated in rostime/rate.cpp:

set the actual amount of time the loop took in case the user wants to know

So apparently your while-loop body takes 0.0005 seconds to execute. loop_rate.sleep() will just suspend your task/thread for the remainder of the second.

In other words: cycle time != number of iterations per second, but total time per iteration.

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So to clarify, cycleTime() is approximately the time between calls to sleep, excluding the actual sleep-time. And by extension, what would be the best way to accurately time a sleep call? to get the frequency of publishing (actual time) in my cpp program -- expectedCycleTime() - cycleTime()?

jdenny ( 2017-03-08 14:03:43 -0500 )edit

You'll have to clarify what you mean with "time a sleep call". ros::Rate already takes time spend doing work into account when calculating the time to sleep so you hit / get near your desired loop rate.

gvdhoorn ( 2017-03-08 14:05:25 -0500 )edit

I guess what I am asking is how do you accurately time the loop including the call to sleep with ros? I get that the loop_rate variable above already is close, but I am wanting to see actual time, not expected.

jdenny ( 2017-03-08 14:07:15 -0500 )edit

Nevermind, I understand it now. I was confusing myself over nothing. Thank you for your time and patience!

jdenny ( 2017-03-08 14:10:31 -0500 )edit

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