Process Priorities: How nice and renice Work

Process Priorities: How nice and renice Work
Reading time: About 13 min Difficulty: Intermediate

What You Will Achieve

  • Explain the relationship between the nice value and the kernel priority (PRI)
  • Start a command at low priority with nice
  • Change a running process's priority with renice
  • Check priority accurately with top / ps
  • Answer the exam-frequent "regular users cannot raise priority" with reasoning

This is the core of LPIC-1 objective 103.6 "Modify process execution priorities". It is the technique to run CPU-heavy background work without disturbing interactive work.

Deciding the nice Value

The nice value is an integer from -20 (highest priority) to 19 (lowest priority). The smaller the value, the more CPU the process gets.

nice value Priority Typical use
-20 to -1 High (root only) Work needing real-time responsiveness
0 Standard (default) Normal commands
1 to 19 Low Heavy work like backup, batch, build

If "backup makes interactive work slow", start it at low priority with nice -n 19. A regular user can only change the nice value in the direction of raising it (larger number = lower priority). This is an exam-frequent point.

Steps

Step 1: Check the current nice value

ps -o pid,ni,comm -p $$
nice
  PID  NI COMMAND
 2451   0 bash
0

The NI column is the nice value. nice with no argument shows the current shell's nice value. The default is 0.

Step 2: Start a command at low priority with nice

nice -n 19 tar czf backup.tar.gz /var/www &
ps -o pid,ni,comm -C tar
  PID  NI COMMAND
 3120  19 tar

nice -n 19 cmd starts a command with nice value 19 (lowest priority). The work proceeds only when the CPU is idle, minimizing impact on interactive work.

Step 3: Change a running process with renice

renice -n 10 -p 3120
ps -o pid,ni,comm -p 3120
3120 (process ID) old priority 19, new priority 10
  PID  NI COMMAND
 3120  10 tar

renice -n 10 -p PID changes a running process's nice value. -u user changes all of a user's processes and -g group changes per group.

Step 4: Monitor priority with top

top -o NI
  PID USER      PR  NI    VIRT    RES  %CPU COMMAND
 3120 user       30  10  118000   4200   2.3 tar
 2451 user       20   0   12000   3800   0.1 bash

In top, the NI column is the nice value and PR is the kernel-internal priority, related by PR = 20 + NI (for normal processes). Pressing r inside top allows interactive renice.

Step 5: Confirm the regular-user constraint

renice -n -5 -p 3120
sudo renice -n -5 -p 3120
renice: failed to set priority for 3120 (process ID): Permission denied
3120 (process ID) old priority 10, new priority -5

A regular user cannot set a negative (high priority) nice value and gets Permission denied. Raising priority requires root privileges (sudo).

Why Regular Users Cannot Raise Priority

Making the nice value negative lets that process preferentially seize CPU over other users' processes. If anyone could freely raise priority, a malicious user or a buggy program could monopolize the entire system, collapsing fairness in a multi-user environment. So Linux adopts an asymmetric permission design: "lowering priority (yielding) is allowed for everyone, raising priority (seizing) is root-only".

The word nice comes from "being nice to other processes by yielding CPU". Raising the value from the default 0 means "yielding more nicely" = lower priority. Grasping this direction prevents confusion over the sign of the nice value. Note that nice is only a hint to the scheduler; if the CPU is idle, even a high-nice-value process proceeds.

Troubleshooting

Symptom: renice gives Permission denied

Cause: A regular user is trying to lower the nice value (negative direction), or change someone else's process

Check:

ps -o pid,user,ni -p PID

Fix: Use sudo renice to raise priority. Changing another user's process also requires root privileges.

Symptom: Work does not slow down even at nice -n 19

Cause: nice is a relative hint; with no competing processes it can use full CPU

Check:

top -o %CPU

Fix: It is normal behavior that nice takes effect only under CPU contention. To lower I/O priority, use ionice separately.

Symptom: A background build makes interactive work slow

Cause: The build process competes for CPU equally at the default nice value 0

Check:

ps -o pid,ni,comm -C make

Fix: Lower the running process to the lowest priority with renice -n 19 -p PID. From now on, start it with nice -n 19 make.

Completion Checklist

  • [ ] Checked the current nice value with ps -o pid,ni,comm
  • [ ] Started at low priority with nice -n 19
  • [ ] Changed a running process with renice -n -p PID
  • [ ] Monitored priority in the top NI column
  • [ ] Confirmed the constraint that regular users cannot raise priority

Summary

Scenario Command Purpose
At start nice -n 19 cmd Start heavy work at low priority
While running renice -n 10 -p PID Adjust a running process
Per user renice -n 5 -u user Change all processes at once
Check ps -el / top Monitor NI / PRI
Raise priority sudo renice -n -5 Root only

Process priority is the basis of CPU resource management. After covering the LPIC-1 process management area, combine it with shell environment and link mechanisms to complete your operational knowledge.

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