Scaling Number of agents on a Jenkins Node - TerrenceMcGuinness-NOAA/global-workflow GitHub Wiki

The scalability of Jenkins nodes (agents) on a single Jenkins controller can be influenced by several factors, including the hardware resources of the controller, the network bandwidth, and the configuration of the Jenkins instance. While there is no hard limit on the number of agents that can be configured, practical limits are often encountered due to resource constraints.

Factors Affecting Scalability

Hardware Resources:
- CPU: The Jenkins controller needs sufficient CPU resources to manage multiple agents. Each agent connection consumes CPU cycles.
- Memory: The Jenkins controller requires adequate memory to handle the connections and manage the build queues.
- Disk I/O: High disk I/O can become a bottleneck if the controller is managing a large number of agents and builds.
Network Bandwidth:
- The network bandwidth between the Jenkins controller and the agents can impact the scalability. High latency or low bandwidth can limit the number of agents that can be effectively managed.
Jenkins Configuration:
- Java Heap Size: The Java heap size allocated to the Jenkins controller can limit the number of agents. Increasing the heap size can help manage more agents but also requires more memory.
- Executor Configuration: The number of executors configured on each agent can impact the overall load on the controller.
Agent Connection Type:
- JNLP (Java Web Start): JNLP agents connect to the controller using a Java Web Start protocol. This is a common method for connecting agents.
- SSH: SSH agents connect to the controller using SSH. This method can be more resource-intensive compared to JNLP.

Practical Limits

While there is no fixed limit, practical experience suggests that a single Jenkins controller can typically manage between 100 to 200 agents, depending on the factors mentioned above. For larger setups, it is common to use Jenkins Operations Center (JOC) or Jenkins Enterprise to manage multiple controllers and distribute the load.

Recommendations for Scaling

Monitor Resource Usage:
- Continuously monitor the CPU, memory, and disk I/O usage on the Jenkins controller. Use tools like top, htop, or monitoring solutions like Prometheus and Grafana.
Optimize Java Heap Size:
- Adjust the Java heap size for the Jenkins controller. This can be done by modifying the -Xmx and -Xms parameters in the Jenkins startup script.
```
java -Xms2g -Xmx4g -jar jenkins.war
```
Use Distributed Builds:
- Distribute the build load across multiple Jenkins controllers using Jenkins Operations Center (JOC) or Jenkins Enterprise.
Optimize Network Configuration:
- Ensure that the network bandwidth between the controller and agents is sufficient. Use network monitoring tools to identify and resolve bottlenecks.
Use Cloud-Based Agents:
- Consider using cloud-based agents that can be dynamically provisioned and de-provisioned based on the load. Plugins like the Kubernetes plugin or Amazon EC2 plugin can help with this.
Limit Executors per Agent:
- Configure a reasonable number of executors per agent to avoid overloading individual agents and the controller.

Example Configuration for Increasing Java Heap Size

To increase the Java heap size for the Jenkins controller, you can modify the Jenkins startup script or the service configuration file. Here’s an example for a systemd service:

Edit the Jenkins service file:
```
sudo nano /etc/default/jenkins
```
Add or modify the JAVA_ARGS line:
```
JAVA_ARGS="-Xms2g -Xmx4g"
```
Restart the Jenkins service:
```
sudo systemctl restart jenkins
```

Summary

While there is no fixed limit on the number of Jenkins agents that can be managed by a single controller, practical limits are often encountered due to resource constraints. By monitoring resource usage, optimizing Java heap size, using distributed builds, and leveraging cloud-based agents, you can scale your Jenkins setup to manage a larger number of agents effectively.