Launching multiple nodes in ROS - Performance & Optimization

Performance: Launching multiple nodes

MEDIUM IMPACT

This affects the startup time and resource usage of a ROS-based system by controlling how many nodes launch simultaneously.

Starting multiple ROS nodes for a robot system

ROS

roslaunch my_robot staggered_nodes.launch  # nodes launched in small groups or with delays

Spreading node launches reduces CPU spikes and allows faster partial system readiness.

📈 Performance Gainstartup time reduced by 30-50%; smoother CPU usage during launch

Starting multiple ROS nodes for a robot system

ROS

roslaunch my_robot all_nodes.launch  # launches all nodes simultaneously

Launching all nodes at once causes high CPU and memory spikes, delaying system readiness and increasing startup time.

📉 Performance Costblocks startup for 2-5 seconds longer depending on node count; high CPU usage spikes

Performance Comparison

Pattern	CPU Usage	Memory Usage	Startup Time	Verdict
All nodes launched simultaneously	High spikes	High peak	Longer startup	[X] Bad
Nodes launched in batches or delayed	Moderate, steady	Lower peak	Faster startup	[OK] Good

Rendering Pipeline

Launching multiple nodes triggers system resource allocation and inter-process communication setup, impacting CPU scheduling and memory usage.

→Process Creation

→CPU Scheduling

→Memory Allocation

⚠️ BottleneckCPU Scheduling and Memory Allocation when many nodes start simultaneously

Optimization Tips

1Avoid launching all nodes simultaneously to prevent CPU and memory spikes.

2Use staggered or batched node launches to improve startup smoothness.

3Monitor system resources during launch to identify bottlenecks.

Performance Quiz - 3 Questions

What is a main performance issue when launching many ROS nodes simultaneously?

A Nodes fail to communicate

B Nodes consume no resources

C High CPU and memory spikes delaying startup

D Nodes start instantly without delay

DevTools: rosnode and top

How to check: Run 'rosnode list' to see active nodes and use 'top' or 'htop' to monitor CPU and memory during launch.

What to look for: Look for CPU usage spikes and memory peaks when nodes start; smoother resource usage indicates better performance.

Practice

1. What is the main purpose of a ROS launch file when working with multiple nodes?

easy

A. To debug a single ROS node

B. To write the source code of ROS nodes

C. To start several ROS nodes together with one command

D. To stop all running ROS nodes automatically

Solution

1. Step 1: Understand the role of launch files and compare options

   Launch files are used to start multiple ROS nodes at once, simplifying the process. Only "To start several ROS nodes together with one command" correctly describes starting multiple nodes together; others describe unrelated tasks.

2. Final Answer:

   To start several ROS nodes together with one command -> Option C

3. Quick Check:

   Launch file = start multiple nodes [OK]

Hint: Launch files group nodes to run with one command [OK]

Common Mistakes:

• Thinking launch files write node code
• Confusing launch files with debugging tools
• Assuming launch files stop nodes

2. Which of the following is the correct XML tag to define a node in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall ROS launch file syntax and check options

   The correct tag to start a node is <node> with attributes pkg, type, and name. Only <node pkg="package_name" type="node_type" name="node_name" /> uses the correct <node> tag; others use invalid tags like <launch>, <start>, <run>.

2. Final Answer:

   <node pkg="package_name" type="node_type" name="node_name" /> -> Option A

3. Quick Check:

   Node tag = <node> [OK]

Hint: Use <node> tag with pkg, type, and name attributes [OK]

Common Mistakes:

• Using <launch> or <start> instead of <node>
• Forgetting to close the tag properly
• Mixing up attribute names

3. Given this launch file snippet:

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim1" />
  <node pkg="turtlesim" type="turtlesim_node" name="sim2" />
</launch>

What happens when you run roslaunch with this file?

medium

A. The launch file causes a syntax error and no nodes start

B. Only one turtlesim node starts named sim1

C. Two nodes start but both have the same name sim1 causing a conflict

D. Two turtlesim nodes named sim1 and sim2 start simultaneously

Solution

1. Step 1: Analyze node definitions and ROS naming rules

   Two nodes are defined with different names: sim1 and sim2, both from turtlesim package. Nodes with unique names can run simultaneously without conflict.

2. Final Answer:

   Two turtlesim nodes named sim1 and sim2 start simultaneously -> Option D

3. Quick Check:

   Unique node names = both run [OK]

Hint: Different node names allow multiple nodes to run [OK]

Common Mistakes:

• Assuming same package means one node only
• Thinking duplicate names cause silent errors
• Believing launch files always cause syntax errors

4. You wrote this launch file snippet:

<launch>
  <node pkg="demo_pkg" type="demo_node" name="node1">
</launch>

What is the error and how to fix it?

medium

A. Missing closing tag for <node>, fix by adding </node>

B. Wrong attribute name 'type', fix by changing to 'exec'

C. Missing package name, fix by adding pkg attribute

D. Extra closing tag needed for <launch>, fix by adding </launch>

Solution

1. Step 1: Check XML tag completeness and verify attributes

   The <node> tag is opened but not closed properly; it needs a closing </node> tag. Attributes pkg and type are correct; <launch> tag is closed properly.

2. Final Answer:

   Missing closing tag for <node>, fix by adding </node> -> Option A

3. Quick Check:

   Unclosed node tag = add </node> [OK]

Hint: Every opened tag must be closed properly [OK]

Common Mistakes:

• Confusing 'type' with 'exec' attribute
• Ignoring missing closing tags
• Assuming launch tag is not closed

5. You want to launch three nodes from different packages in one launch file:

<launch>
  <node pkg="pkg1" type="node1" name="node1" />
  <node pkg="pkg2" type="node2" name="node2" />
  <node pkg="pkg3" type="node3" name="node3" />
</launch>

How can you modify this launch file to add a parameter use_sim_time set to true for all nodes?

hard

A. Add <param name="use_sim_time" value="true" /> inside each <node> tag

B. Add <param name="use_sim_time" value="true" /> directly inside <launch> tag

C. Add <arg name="use_sim_time" default="true" /> inside each <node> tag

D. Add <env name="use_sim_time" value="true" /> inside each <node> tag

Solution

1. Step 1: Understand parameter scope in launch files and compare options

   Parameters placed directly inside <launch> apply globally to all nodes. Adding <param name="use_sim_time" value="true" /> directly inside <launch> sets it once for all; inside each <node> repeats unnecessarily; options B and C use wrong tags (<env>, <arg>).

2. Final Answer:

   Add <param name="use_sim_time" value="true" /> directly inside <launch> tag -> Option B

3. Quick Check:

   Global param inside <launch> = all nodes get it [OK]

Hint: Put common params inside <launch> for all nodes [OK]

Common Mistakes:

• Adding param inside each node causing repetition
• Using <arg> or <env> instead of <param>
• Placing param outside launch tag