Setting parameters from launch file in ROS - Performance & Optimization

Performance: Setting parameters from launch file

MEDIUM IMPACT

This affects the startup time and configuration loading speed of ROS nodes during launch.

Configuring node parameters at launch

ROS

<launch>
  <param name="param1" value="precomputed_value"/>
  <node pkg="my_pkg" type="my_node" name="node1"/>
</launch>

Precompute values outside launch file and set simple static parameters to speed up launch parsing.

📈 Performance GainLaunch parsing is near-instant, reducing startup delay by 90%+

Configuring node parameters at launch

ROS

<launch>
  <node pkg="my_pkg" type="my_node" name="node1">
    <param name="param1" value="$(eval expensive_function())"/>
  </node>
</launch>

Using expensive function calls inside parameter values causes launch file parsing delays and blocks node startup.

📉 Performance CostBlocks launch for 100+ ms depending on function complexity

Performance Comparison

Pattern	Launch Parsing Time	Parameter Server Calls	Node Startup Delay	Verdict
Dynamic eval in param value	High (100+ ms)	Multiple	High delay	[X] Bad
Static param set before node	Low (<10 ms)	Single	Minimal delay	[OK] Good

Rendering Pipeline

ROS launch files are parsed before node startup. Parameter setting affects the configuration phase before nodes initialize.

→Launch Parsing

→Parameter Server Setup

→Node Initialization

⚠️ BottleneckLaunch Parsing when complex expressions or external calls are used in parameters

Optimization Tips

1Avoid complex function calls inside launch file parameters.

2Set static or precomputed parameter values in launch files.

3Measure launch parsing time to identify parameter setting bottlenecks.

Performance Quiz - 3 Questions

What is a performance downside of using complex expressions inside ROS launch file parameters?

A It improves node runtime performance

B It increases launch file parsing time and delays node startup

C It reduces network bandwidth usage

D It decreases parameter server memory usage

DevTools: roslaunch logging and rosparam CLI

How to check: Run roslaunch with verbose logging; use 'rosparam get' to verify parameters after launch; measure node startup time with timestamps.

What to look for: Look for long delays in launch logs and multiple parameter server calls indicating inefficient parameter setting.

Practice

1. What is the main purpose of using <param> tags in a ROS launch file?

easy

A. To start multiple nodes at the same time

B. To set parameters that nodes can use when they start

C. To define the order in which nodes run

D. To stop nodes after they finish

Solution

1. Step 1: Understand the role of <param> in launch files

   The <param> tag is used to set parameters that nodes read when they start running.

2. Step 2: Differentiate from other launch file tags

   Tags like <node> start nodes, but <param> specifically sets values nodes use.

3. Final Answer:

   To set parameters that nodes can use when they start -> Option B

4. Quick Check:

   Setting parameters = A [OK]

Hint: Remember: <param> sets values, <node> starts nodes [OK]

Common Mistakes:

• Confusing <param> with <node> tag
• Thinking <param> controls node start/stop
• Assuming <param> sets node execution order

2. Which of the following is the correct syntax to set a string parameter named robot_name to "R2D2" in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Check the correct attribute names

   The <param> tag requires 'name' for the parameter name and 'value' for its value.

2. Step 2: Verify proper quoting of string values

   String values must be inside quotes, so value="R2D2" is correct.

3. Final Answer:

   <param name="robot_name" value="R2D2" /> -> Option C

4. Quick Check:

   Correct attributes and quotes = B [OK]

Hint: Use name="" and value="" with quotes for strings [OK]

Common Mistakes:

• Missing quotes around string values
• Swapping name and value attributes
• Using incorrect attribute names

3. Given this launch file snippet:

<launch>
  <param name="speed" value="2.5" />
  <node pkg="robot_pkg" type="move_node" name="mover" />
</launch>

What will be the value of the speed parameter inside the move_node when it starts?

medium

A. 2.5 (as a float)

B. 0, the default value

C. Undefined, because parameters must be set inside the node tag

D. "2.5" (as a string)

Solution

1. Step 1: Understand parameter value types in launch files

   Values in <param> tags are treated as strings by default unless explicitly converted in code.

2. Step 2: Check the launch file snippet

   The value is "2.5" in quotes, so the parameter is set as a string "2.5".

3. Final Answer:

   "2.5" (as a string) -> Option D

4. Quick Check:

   Parameter values are strings by default = D [OK]

Hint: Parameter values are strings unless converted in code [OK]

Common Mistakes:

• Assuming numeric values are auto-converted to float
• Thinking parameters must be inside <node> tag
• Believing default values apply if parameter is set

4. Identify the error in this launch file snippet:

<launch>
  <param name="use_sim" value=true />
  <node pkg="sim_pkg" type="sim_node" name="simulator" />
</launch>

medium

A. The value true should be in quotes: value="true"

B. The param tag should be inside the node tag

C. The node tag is missing the required args attribute

D. The launch tag is missing a closing slash

Solution

1. Step 1: Check the value attribute format

   Boolean values in <param> tags must be strings, so true should be "true" in quotes.

2. Step 2: Verify tag placement and syntax

   The <param> tag can be outside the <node> tag, and the <launch> tag is correctly closed.

3. Final Answer:

   The value true should be in quotes: value="true" -> Option A

4. Quick Check:

   Boolean values need quotes in <param> = C [OK]

Hint: Always put boolean values in quotes in <param> tags [OK]

Common Mistakes:

• Leaving boolean values unquoted
• Placing <param> tags only inside <node>
• Misunderstanding launch tag syntax

5. You want to set a parameter max_retries to 5 only if it is not already set by another launch file. Which launch file tag should you use to achieve this?

hard

A.

B. 5

C.

D.

Solution

1. Step 1: Understand conditional parameter setting

   The <param> tag supports the 'unless' attribute to set a parameter only if it is not already set.

2. Step 2: Check the options for correct usage

   <param unless="max_retries" name="max_retries" value="5" /> uses 'unless="max_retries"' with name and value, which is the correct syntax.

3. Final Answer:

   <param unless="max_retries" name="max_retries" value="5" /> -> Option A

4. Quick Check:

   Use 'unless' attribute to avoid overwriting = A [OK]

Hint: Use 'unless' attribute to set param only if unset [OK]

Common Mistakes:

• Omitting the 'name' attribute when using 'unless'
• Using <rosparam> without condition
• Misplacing 'unless' attribute