What is process priority and how do nice values work?

The "nice" value (−20 to +19, default 0) hints the scheduler how to share CPU: lower = more CPU priority, higher = "nicer" to others. Only root can go negative.

The counter-intuitive bit is the direction: a higher nice number means lower priority, because the process is being "nicer" and yielding CPU to others. −20 is the greediest, +19 the most self-effacing. Nice values only matter under contention — if the CPU is idle, even a +19 task runs full speed. Start a command niced with nice -n 19 cmd; change a running one with renice. Raising priority (negative values) needs root, since it lets one process starve others.

Nice values:

• Range: -20 (highest priority) to +19 (lowest priority)
• Default: 0
• Only root can set negative nice values

Nice value to priority mapping:

Nice: -20 ... 0 ... +19
      High    Normal  Low priority

Commands:

Examples:

# Run backup with low priority
nice -n 19 tar -czf backup.tar.gz /data

# Increase priority of process (root only)
sudo renice -n -5 -p 12345

# View process priorities
ps -eo pid,ni,comm | head

Real-time vs Normal scheduling:

• Normal: Uses nice values, fair sharing
• Real-time: Strict priority, can starve other processes

Tip: Use nice for CPU-intensive tasks that shouldn't slow down the system.

Go deeper:

• nice (Unix) — Wikipedia — the −20…+19 range, why higher = nicer = lower priority, and renice.