What You Need to Know
- What priority a process gets by default.
- How to start a program with a higher or lower priority.
- How to change the priority of an already-running process.
The Linux Scheduler
Operating systems that run multiple processes simultaneously are called multitasking. True parallelism only exists with multiple CPU cores, but even single-processor systems simulate it by rapidly switching between tasks.
At any given moment the CPU handles exactly one process. Most process operations — reading files, network access, screen output — are system calls: the process hands control to the kernel, and while the kernel handles the request the CPU can switch to another process.
The Linux scheduler divides processes into two classes:
- Real-time processes. Get static priorities from 0 to 99. They always preempt normal processes.
- Normal processes (normal scheduling). Get static priorities from 100 to 139. A lower value means higher priority.
Static real-time priorities are sometimes called static scheduling priorities to distinguish them from dynamic priorities that depend on the nice value.
In preemptive multitasking a higher-priority process interrupts a lower-priority one as soon as it enters the run queue.
Reading Priorities
Priorities in /proc
The actual static priority of a process is stored in /proc/<PID>/sched:
$ grep ^prio /proc/1/sched
prio : 120
The default priority of a normal process is 120. The range for normal processes is 100 (highest) to 139 (lowest).
Priorities in ps
ps -Al or ps -el show priorities in the PRI column. Due to historical quirks, ps displays values in the range -40 to 99. To get the real priority, add 40:
PRI (ps) + 40 = real priority
80 + 40 = 120 (default)
The NI column shows the process’s nice number.
Sample output:
$ ps -el
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
4 S 0 1 0 0 80 0 - 9292 - ? 00:00:00 systemd
1 S 0 3 2 0 60 -20 - 0 - ? 00:00:00 rcu_gp
Sort by CPU usage (ascending):
$ ps -el --sort=pcpu
Extended format with column selection:
$ ps -e -o user,uid,comm,tty,pid,ppid,pri,pmem,pcpu --sort=-pcpu | head
Priorities in top
top displays priorities differently: it subtracts 100 from the real value. This makes real-time priorities negative and easy to spot. Normal processes appear in the range 0 to 39. Real-time processes show rt or a negative number.
Correspondence table:
| Real priority | ps PRI | top PR |
|---|---|---|
| 120 (default) | 80 | 20 |
| 100 (highest normal) | 60 | 0 |
| 139 (lowest normal) | 99 | 39 |
Niceness: Process Politeness
Niceness lets you influence the priority of normal processes. The logic is simple: the “nicer” a process is, the more willingly it yields to others.
- Nice value range: from -20 (less polite, high priority) to 19 (more polite, low priority).
- By default all processes start with nice = 0, corresponding to real priority 120.
- Only
rootcan decrease nice below zero (raise priority). - A regular user can only increase nice (lower priority).
- Linux allows different nice values for individual threads of the same process.
Starting with a Changed Priority: nice
The nice command launches a program with a specified nice value. Without flags, nice sets niceness to 10.
# Run with niceness 15
$ nice -n 15 tar czf home_backup.tar.gz /home
# Run with niceness -5 (root only)
# nice -n -5 ./critical_task
Changing the Priority of a Running Process: renice
renice changes the nice value of an already-running process.
# By PID (root only for negative values)
# renice -10 -p 2164
2164 (process ID) old priority 0, new priority -10
# By process group
$ renice +5 -g users
# By user
$ renice +3 -u john
-p specifies a PID, -g applies the change to all processes in a group, -u to all processes of a user.
Changing Priority via top
In interactive top, press r. A prompt appears: PID to renice [default pid = ...]. Enter the PID and press Enter, then enter the new nice value.
Quick Reference
# View the real priority of a process
grep ^prio /proc/<PID>/sched
# List processes with priorities
ps -el
ps -Al
# Start with a non-default priority
nice -n <value> <command>
# Change the priority of a running process by PID
renice <new_value> -p <PID>
# Change the priority for all processes of a user
renice <value> -u <user>
# Change priority via top
# In top press: r -> enter PID -> enter new nice value
# Sort ps by CPU usage (ascending)
ps -el --sort=pcpu
# schedtool: show scheduling parameters
schedtool <PID>
# schedtool: set real-time scheduling with priority
schedtool -R -p <priority> <PID>
Exam Tips
What nice number does a process get by default? 0 (real priority 120).
What is the nice value range? From -20 to 19.
Who can set negative nice values? Only root.
How do you determine the real priority from ps?
Add 40 to the PRI column value.
How does top mark real-time processes?
It shows rt or a negative number in the PR column.
In ps -el a process has PRI = -40. What does that mean?
It’s a real-time process: real priority = -40 + 40 = 0 (RT priority range).
How do you start backup.sh with low priority (niceness 10)?
nice backup.sh or nice -n 10 backup.sh.
What does renice +5 -g users do?
Increases the nice value (lowers priority) by 5 for all processes in the users group.
Exercises
Guided Exercises
Exercise 1
In a preemptive multitasking system, a low-priority process holds the CPU. A higher-priority process enters the queue. What happens?
Answer
The low-priority process is suspended and the higher-priority process begins executing in its place.
Exercise 2
Look at this top output:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 ... ... ... S 0,0 0,1 9:59.15 systemd
2 root 20 0 ... 0 0 S 0,0 0,0 0:02.76 kthreadd
3 root 0 -20 ... 0 0 I 0,0 0,0 0:00.00 rcu_gp
4 root 0 -20 ... 0 0 I 0,0 0,0 0:00.00 rcu_par_gp
8 root 0 -20 ... 0 0 I 0,0 0,0 0:00.00 mm_percpu_wq
9 root 20 0 ... 0 0 S 0,0 0,0 0:49.06 ksoftirqd/0
10 root 20 0 ... 0 0 I 0,0 0,0 18:24.20 rcu_sched
11 root 20 0 ... 0 0 I 0,0 0,0 0:00.00 rcu_bh
12 root rt 0 ... 0 0 S 0,0 0,0 0:08.17 migration/0
14 root 20 0 ... 0 0 S 0,0 0,0 0:00.00 cpuhp/0
15 root 20 0 ... 0 0 S 0,0 0,0 0:00.00 cpuhp/1
16 root rt 0 ... 0 0 S 0,0 0,0 0:11.79 migration/1
17 root 20 0 ... 0 0 S 0,0 0,0 0:26.01 ksoftirqd/1
Which PIDs have real-time priority?
Answer
PID 12 and PID 16. Both show rt in the PR column.
Exercise 3
Look at this ps -el output:
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
4 S 0 1 0 0 80 0 - 42855 - ? 00:09:59 systemd
1 S 0 2 0 0 80 0 - 0 - ? 00:00:02 kthreadd
1 I 0 3 2 0 60 -20 - 0 - ? 00:00:00 rcu_gp
1 S 0 9 2 0 80 0 - 0 - ? 00:00:49 ksoftirqd/0
1 I 0 10 2 0 80 0 - 0 - ? 00:18:26 rcu_sched
1 I 0 11 2 0 80 0 - 0 - ? 00:00:00 rcu_bh
1 S 0 12 2 0 -40 - - 0 - ? 00:00:08 migration/0
1 S 0 14 2 0 80 0 - 0 - ? 00:00:00 cpuhp/0
5 S 0 15 2 0 80 0 - 0 - ? 00:00:00 cpuhp/1
Which PID has the highest priority?
Answer
PID 12.
PRI = -40 in ps output corresponds to real priority -40 + 40 = 0. This falls in the real-time range (0–99), so this process preempts all normal processes.
Exercise 4
A renice attempt produces an error:
$ renice -10 21704
renice: failed to set priority for 21704 (process ID): Permission denied
What is the likely cause?
Answer
Decreasing the nice value below zero (i.e., raising priority above the default) requires root. The command was run as a regular user.
Explorational Exercises
Exercise 1
Changing priorities is often needed when a process consumes too much CPU. Using ps with standard options for a long listing of all system processes, which --sort flag sorts them by CPU usage in ascending order?
Answer
$ ps -el --sort=pcpu
Exercise 2
The schedtool command lets you view and set CPU scheduling parameters for processes. How do you display the scheduling parameters of process 1750? And how do you switch process 1750 to real-time mode with priority -90 (in top notation)?
Answer
# Show scheduling parameters
$ schedtool 1750
# Switch to real-time with priority -90 (top) = 89 (schedtool)
$ schedtool -R -p 89 1750
top shows real-time priority as real_priority - 100. So -90 in top corresponds to real priority 10. schedtool takes the value 0–99 directly. The -R flag sets the SCHED_RR (round-robin real-time) policy; -p sets the numeric priority.
Topic 103: GNU and Unix Commands