Tech Tutorial: Operate Running Systems #

Introduction #

In this tutorial, we will delve into an essential aspect of system administration for Red Hat Enterprise Linux (RHEL)—adjusting process scheduling. This topic is significant for the Red Hat Certified System Administrator (RHCSA) exam and for anyone looking to optimize the performance of their RHEL systems.

Understanding and adjusting process scheduling allows system administrators to fine-tune the system’s responsiveness and efficiency, particularly in environments with high load or specific performance requirements.

Step-by-Step Guide #

Understanding Process Priority and Niceness #

In Linux, each process is assigned a priority level, and within RHEL, you can adjust this priority using the nice and renice commands. The “niceness” ranges from -20 (highest priority) to 19 (lowest priority).

Viewing Current Process Priorities
To view the priorities of running processes, you can use the top or ps command. The NI column shows the niceness level.
```
top
```
Or:
```
ps -el | head -1; ps -el | grep [process_name]
```
Replace [process_name] with the actual name of the process to filter the output.
Changing Process Priority
To change the priority of a running process, use the renice command.
```
sudo renice -n 10 -p [PID]
```
Replace [PID] with the Process ID of the process you want to re-prioritize. This command sets the niceness of the process to 10.

Setting Real-Time Priorities with `chrt` #

For processes that require real-time execution characteristics, chrt (change real-time attributes) is used.

Viewing Real-Time Attributes
To see the real-time attributes of a process, use:
```
chrt -p [PID]
```
Replace [PID] with the Process ID of the process.
Setting Real-Time Scheduling Policy
There are different scheduling policies available: SCHED_FIFO, SCHED_RR, and SCHED_OTHER. To set a process to real-time with the FIFO policy:
```
sudo chrt -f -p 99 [PID]
```
This sets the process with PID [PID] to a FIFO real-time scheduling policy with the highest priority (99).

Detailed Code Examples #

Let’s consider a scenario where we have a Java application that needs more priority over other processes.

Identifying the Process ID
Suppose the application is named MyJavaApp.
```
ps -ef | grep MyJavaApp
```
Assume the output gives us a PID of 1234.
Increasing Priority
Increase the priority by setting a lower niceness.
```
sudo renice -n -5 -p 1234
```
Check the priority change:
```
ps -el | grep 1234
```
Setting to Real-Time
If the application needs real-time execution:
```
sudo chrt -r -p 80 1234
```
Validate the change:
```
chrt -p 1234
```

Conclusion #

Adjusting process scheduling in RHEL can significantly impact the performance and behavior of applications and processes. By mastering the use of nice, renice, and chrt, you gain finer control over how CPU time is distributed among processes, which is crucial for optimizing system performance. Remember that improper use of these tools, especially with real-time settings, can lead to system instability if not managed carefully. Always test changes in a controlled environment before applying them in a production setting.