Weight: 2
Candidates should be able to manage process execution priorities.
Objectives
- Know the default priority of a job that is created.
- Run a program with higher or lower priority than the default.
- Change the priority of a running process.
Terms
- nice
- ps
- renice
- top
On a Linux machine, you might have 100s of processes running at the same time and competing for more CPU & RAM. The good news is that you can give some of the processes higher or lower priority (or nice-ness) in this competition. Let's have a look at a sample top output:
$ top
Tasks: 169 total, 1 running, 168 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 3.0 sy, 0.0 ni, 97.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 5948.9 total, 4115.2 free, 438.2 used, 1395.5 buff/cache
MiB Swap: 975.0 total, 975.0 free, 0.0 used. 5210.3 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6496 jadi 20 0 10112 3704 3212 R 6.2 0.1 0:00.01 top
1 root 20 0 165172 10576 7780 S 0.0 0.2 0:02.80 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.09 kthreadd
3 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_gp
4 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_par_gp
6 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/0:0H-events_highpri
8 root 0 -20 0 0 0 I 0.0 0.0 0:12.71 kworker/0:1H-events_highpri
9 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 mm_percpu_wq
10 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_rude_
11 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_trace
The NI column shows how nice this process is. The nicer the process, the less CPU it asks. The nice values can be from -20 to 19. To interpret this value, look at it like this: a process with nice = -20 is ANGRY and gets more priority for CPU and RAM while a process with nice = 19 is SUPER NICE and lets other processes use the resources before her).
The default value for nice is normally set to 0; this can be checked with:
$ nice
0
You can also check the nice-ness using the ps command:
$ ps -l
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
0 S 1000 15044 15035 0 80 0 - 7453 wait pts/29 00:00:00 bash
0 S 1000 15052 15044 0 60 -20 - 3976 hrtime pts/29 00:00:00 sleep
0 R 1000 15080 15044 0 80 0 - 4680 - pts/29 00:00:00 ps
Setting nice-ness
If you need to change the niceness level of a program you can run it with nice command and -n switch (for nice):
$ nice -n 19 echo "I am running!"
I am running!
$ nice -n -20 echo "I am running!"
nice: cannot set niceness: Permission denied
I am running!
$ sudo nice -n -20 echo "I am running!"
I am running!
As you can in the above example, only the root can issue high-priority niceness (below 0).
If you run a command with
nicewithout-n, the default will be-n 10
$ nice xeyes &
[1] 15217
$ ps -l
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
0 S 1000 15044 15035 0 80 0 - 7455 wait pts/29 00:00:00 bash
0 S 1000 15217 15044 0 90 10 - 12522 poll_s pts/29 00:00:00 xeyes
0 R 1000 15218 15044 0 80 0 - 4680 - pts/29 00:00:00 ps
Changing priorities
The renice command can be used to change the niceness of your running processes (or others if you are root):
$ ps -ef | grep firefox
jadi 13605 11226 30 08:28 ? 00:10:13 /usr/lib/firefox/firefox
jadi 15192 15044 0 09:01 pts/29 00:00:00 grep firefox
$ sudo renice -n -10 13605
13605 (process ID) old priority 5, new priority -10
You can also press
rintopcommand to renice a process
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