Skip to main content

Chapter 4.4 - Priorities and Nice Values

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand process priorities
  • Use nice values
  • Change priorities with nice and renice
  • Understand the Linux scheduler

What is Priority?​

A process's priority determines its access to the CPU. The higher the priority, the more CPU time the process gets.

Nice Values​

Scale​

Nice values: -20 to +19

  • -20: Highest priority (root only)
  • 0: Default priority
  • +19: Lowest priority

Rule:

  • Lower = more priority
  • Higher = less priority

Viewing Priority​

# With ps
ps -o pid,ni,comm
ps aux | head

# Column NI = Nice value
# Column PRI = Actual priority (calculated)

# With top
top
# Column NI shows nice value

Changing Priority​

nice - At Startup​

# Start with low priority
nice -n 10 command
nice -n +10 command  # Same thing

# Start with high priority (root)
sudo nice -n -10 command

# Verify
ps -o pid,ni,comm | grep command

renice - Existing Process​

# Change process priority
renice 10 PID

# By user
sudo renice 10 -u user1

# By group
sudo renice 10 -g developers

# By process and children
sudo renice 10 -p PID

Examples:

# CPU-intensive process → low priority
renice 15 $(pidof backup_script)

# Important process → high priority (root)
sudo renice -10 $(pidof critical_app)

Linux Scheduler (CFS)​

Completely Fair Scheduler​

The Linux scheduler uses nice values to balance CPU time:

Operation:

  • Lower nice = more CPU time
  • Fairness among processes of the same priority
  • Considers time already used

Real-Time Processes​

chrt - Real-Time​

For processes requiring time guarantees:

# View policies
chrt -m

# Set FIFO policy
sudo chrt -f 50 command
# -f: FIFO
# 50: Priority (1-99)

# Set Round-Robin policy
sudo chrt -r 50 command

# Change existing process
sudo chrt -f -p 50 PID

Policies:

  • SCHED_FIFO: First In First Out
  • SCHED_RR: Round Robin
  • SCHED_OTHER: Normal (CFS)

Warning: Real-time processes can monopolize the CPU!

Practical Examples​

Scenario 1: Nightly Backup​

# Run backup with very low priority
nice -n 19 backup_script.sh &

# Will not disturb other processes

Scenario 2: Critical Process​

# Increase priority of an important process
sudo renice -10 $(pidof database_server)

# Will get more CPU time

Scenario 3: Balancing​

# Multiple processes, balance them
renice 5 $(pidof app1)
renice 5 $(pidof app2)
renice 5 $(pidof app3)

# All with the same priority

Monitoring Priorities​

top/htop​

# View nice values
top
# Column NI

# Sort by priority
# In top: press 'r' then enter PID

ps with Priority​

# View nice and priority
ps -eo pid,ni,pri,comm

# Sort by priority
ps -eo pid,ni,pri,comm --sort=-pri

Summary​

In this chapter, you learned:

Nice values: -20 (high) to +19 (low), 0 by default
Change: nice (startup), renice (existing)
Scheduler: CFS balances according to nice values
Real-time: chrt for real-time guarantees
Monitoring: ps, top to view priorities
Practice: Adjust according to process importance

Next Steps​

Chapter 4.5: Processes and Resources
Chapter 4.6: Jobs and Background Tasks

Chapter created on: December 2024