Populating ros messages is slow

 I noticed that populating ROS messages with values is quite slow. Filling 16 fields of a message takes in average around 2.2 ms. While this is not that much on its own, it quickly adds up in a control loop. The message in question is a control command which has to be sent at around 10 Hz to at least 10 different robots. That means at least a fifth of the control interval is spent on "creating" the messages. Together with the issue that rostime("now") is slow too (https://www.mathworks.com/matlabcentral/answers/658878-rostime-call-is-slow?s_tid=srchtitle), this leaves almost no time for the actual computation of the control commands.

 

I wonder if I am doing something fundamentally wrong or if the Matlab ROS API is simply that slow?

 

Script to time population of ROS message:

cmdFullStateMsg = rosmessage("crazyflie_driver/FullState");

pos = ones(3, 1);
vel = ones(3, 1);
acc = ones(3, 1);
yaw = 1;
omega = ones(3, 1);
quat = eul2quat([0, 0, yaw], 'xyz');

tic
for i = 1:1000  
    cmdFullStateMsg.Pose.Position.X    = pos(1);
    cmdFullStateMsg.Pose.Position.Y    = pos(2);
    cmdFullStateMsg.Pose.Position.Z    = pos(3);
    cmdFullStateMsg.Twist.Linear.X     = vel(1);
    cmdFullStateMsg.Twist.Linear.Y     = vel(2);
    cmdFullStateMsg.Twist.Linear.Z     = vel(3);
    cmdFullStateMsg.Acc.X              = acc(1);
    cmdFullStateMsg.Acc.Y              = acc(2);
    cmdFullStateMsg.Acc.Z              = acc(3);
    cmdFullStateMsg.Pose.Orientation.W = quat(1);
    cmdFullStateMsg.Pose.Orientation.X = quat(2);
    cmdFullStateMsg.Pose.Orientation.Y = quat(3);
    cmdFullStateMsg.Pose.Orientation.Z = quat(4);
    cmdFullStateMsg.Twist.Angular.X    = omega(1);
    cmdFullStateMsg.Twist.Angular.Y    = omega(2);
    cmdFullStateMsg.Twist.Angular.Z    = omega(3);   
end
cmd_full_state_msg_time = toc / 1000

ANSWER

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Cause of Performance Issues

 

Accessing Nested Messages (pre-R2020b)

 

Based on your other question, I assume you are using R2019a. All releases up through R2020a use rosjava under the hood, and the messages contain Java message objects. This means that as you use fields of deeply-nested messages, for either setting or getting data,  MATLAB has to do several conversions of message objects.

 

For example, when you call :

 

 

cmdFullStateMsg.Pose.Position.X = pos(1)

then MATLAB:

• Accesses the geometry_msgs/Pose Java message object from the "Pose" property
• Converts that to the MATLAB robotics.ros.msggen.geometry_msgs.Pose object
• Accesses geometry_msgs/Point Java message object from the "Position" property
• Converts that to the MATLAB robotics.ros.msggen.geometry_msgs.Point object
• Accesses the "X" property of the underlying geometry_msgs/Point Java message and assigns it to the provided value
• Assigns the appropriate properties of the various objects all the way up the nested tree

Multiple Calls to rosmessage

 

Additionally, any call to rosmessage requires the construction of the object and its underlying Java message. Limiting the number of calls is advantageous when trying to improve performance.

 

Solutions

Disclaimer

 

The performance improvement is just what I'm seeing through casual measurement (tic-toc) on my machine in particular. You may not see the same improvement depending on your machine, resources, environment, code structure or other factors.

 

Extract Nested Messages Before Setting Values

 

When setting multiple fields in a single message that is deeply nested, it can help a lot to pull out the message manually and set the fields. Since mesage objects in MATLAB are handles, you don't need to reassign it to the nested field either.

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