4. Operate Running Systems

📘Red Hat Certified System Administrator (RHCSA – EX200)

Objective of This Section

In the RHCSA (EX200) exam, under 4. Operate Running Systems, the sub-topic 4.5 Adjust process scheduling tests your ability to:

Change the priority of running processes
Start programs with a specific priority
Understand how Linux schedules CPU time
Use commands like nice, renice, and understand priority values

You must be comfortable working from the command line.

1. What Is Process Scheduling?

In Linux, many processes run at the same time. But the CPU can only execute one instruction at a time per core.

So the system uses a scheduler to decide:

Which process runs first
How long it runs
Which process gets more CPU time

This decision is based on priority.

2. Understanding Process Priority

In Linux (including Red Hat Enterprise Linux (RHEL)), each process has:

Priority value (PR) – Used internally by the kernel
Nice value (NI) – Value you can control

For the RHCSA exam, you mainly work with the nice value.

2.1 Nice Value (NI)

The nice value determines how “nice” a process is to other processes.

Range: -20 to 19
Default: 0

Important Rules:

Nice Value	Meaning
-20	Highest priority
0	Default priority
19	Lowest priority

Very Important for Exam:

Lower nice value = Higher priority
Higher nice value = Lower priority

2.2 Who Can Set Negative Nice Values?

Normal users → Can only set 0 to 19
Root user → Can set -20 to 19

This is a common exam test point.

3. Viewing Process Priority

To check running processes and their nice values:

ps -el

top

In top, look for:

PR → Priority
NI → Nice value

Example IT Scenario

On a server running:

A database service
A backup script
A log processing script

You may want:

Database → higher priority
Backup → lower priority

This ensures important services run smoothly.

4. Starting a Process with a Specific Nice Value

Use the nice command.

Syntax

nice -n value command

Example

Start a backup script with low priority:

nice -n 10 ./backup.sh

This means:

The process runs with nice value 10
Lower priority than default

Start with High Priority (Root Only)

nice -n -5 ./critical_script.sh

Only root can set negative values.

5. Changing Priority of a Running Process

Use the renice command.

Syntax

renice value -p PID

Step-by-Step Example

Find PID:

ps aux | grep backup

Change nice value:

renice 15 -p 1234

This changes process 1234 to nice value 15.

Important Notes for Exam

renice requires:
- Root → to increase priority (lower nice value)
- Normal user → can only increase nice value (lower priority)

6. Understanding Scheduler Behavior (Basic Level for Exam)

Linux uses a scheduler that:

Gives CPU time to all processes
Gives more CPU time to higher priority processes
Prevents one process from using 100% CPU permanently

You do NOT need deep kernel knowledge for RHCSA.

Focus on:

Nice values
Using nice
Using renice
Viewing process priority

7. Real IT Use Cases

These are realistic server scenarios:

1. Database Server

mysqld process must respond quickly
Log cleanup script is not urgent

Set:

Database → higher priority
Log script → lower priority

2. Web Server with Background Jobs

Apache or Nginx must serve users fast
Background report generation can be slower

Run report generator with:

nice -n 15 ./generate_report.sh

3. Large File Compression

Compressing large log files:

nice -n 19 tar -czf logs.tar.gz /var/log

This prevents CPU overload.

8. Important Commands Summary

Check processes

ps -el
top

Start with specific nice value

nice -n 5 command

Change running process priority

renice 10 -p PID

9. Exam Tips (Very Important)

For RHCSA:

You should be able to:

✔ Start a process with lower priority
✔ Change nice value of a running process
✔ Understand nice value range
✔ Know who can set negative values
✔ Use ps and top to verify

Common Mistakes

❌ Thinking 19 is highest priority (It is lowest)
❌ Forgetting only root can set negative values
❌ Mixing up priority and nice values

10. Practice Tasks for Exam Preparation

You should practice:

Start sleep 500 with nice value 10
Change its nice value to 15
Try setting nice value -5 as normal user (see error)
Switch to root and try again

Final Quick Revision Table

Task	Command
View processes	`ps -el`
Real-time process view	`top`
Start with priority	`nice -n value command`
Change running process	`renice value -p PID`
Nice range	-20 to 19
Default nice	0

What You Do NOT Need for RHCSA

Kernel tuning
Real-time scheduling classes
Advanced CPU affinity tools
CFS algorithm details

Final Words

For RHCSA EX200, adjusting process scheduling mainly means:

Understanding nice values
Using nice and renice correctly
Knowing how priority affects CPU usage