Description: Candidates should should be able to manage process execution priorities.
Key Knowledge Areas:
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 and Utilities:
nice
ps
renice
top
Linux, like most modern operating systems, can run multiple processes. It does this by sharing the CPU and other resources among the processes. If one process can use 100 percent of the CPU, then other processes may become unresponsive. We’ll introduce you to the way Linux assigns priorities for tasks.(We have already talked about ps and top commands in previous section)
nice
In Linux we can set guidelines for the CPU to follow when it is looking at all the tasks it has to do. These guidelines are called niceness or nice value.(weuse ubuntu 16 here)
top - 03:15:57 up 3 days, 20:17, 1 user, load average: 0.03, 0.01, 0.00
Tasks: 235 total, 1 running, 171 sleeping, 0 stopped, 0 zombie
%Cpu(s): 2.1 us, 1.4 sy, 0.0 ni, 96.2 id, 0.3 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 985080 total, 116124 free, 436040 used, 432916 buff/cache
KiB Swap: 1045500 total, 374404 free, 671096 used. 335984 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
949 root 20 0 496000 30668 15864 S 2.0 3.1 11:17.57 Xorg
55222 user1 20 0 667544 18964 12132 S 1.3 1.9 0:22.76 gnome-term+
2112 user1 20 0 2346400 53212 24204 S 0.7 5.4 36:38.03 compiz
1916 user1 20 0 477868 5304 3464 S 0.3 0.5 0:07.53 ibus-ui-gt+
1948 user1 20 0 188388 1384 1108 S 0.3 0.1 0:18.18 ibus-engin+
55210 root 20 0 6536 600 572 S 0.3 0.1 0:19.60 ping
64405 root 20 0 41920 3760 3040 R 0.3 0.4 0:00.13 top
1 root 20 0 185244 3996 2500 S 0.0 0.4 0:08.10 systemd
NI column Represents a Nice Value of task. The Linux niceness scale goes from -20 to 19. The lower the number the more priority that task gets. If the niceness value is high number like 19 the task will be set to the lowest priority and the CPU will process it whenever it gets a chance. The default nice value is zero.
Different OS distributions can have different default values for new processes. The simplest method to determine the default value is to simply run the nice command with no arguments. By default nice will simply return the current niceness value
root@ubuntu16-1:~# nice
0
Determining the niceness value of a current process
The niceness value of current processes are also pretty simple to find as they are visible in the ps command’s full long format :
root@ubuntu16-1:~# sleep 11111 &
[1] 65128
root@ubuntu16-1:~# sleep 22222 &
[2] 65129
root@ubuntu16-1:~# ps -fl
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
4 S root 55643 55624 0 80 0 - 13594 wait Oct21 pts/17 00:00:00 su -
4 S root 55644 55643 0 80 0 - 5725 wait Oct21 pts/17 00:00:00 -su
4 S root 65014 55644 0 80 0 - 13594 wait 04:36 pts/17 00:00:00 su - user1
4 S root 65097 65015 0 80 0 - 13594 wait 04:45 pts/17 00:00:00 su - root
4 S root 65098 65097 0 80 0 - 5691 wait 04:45 pts/17 00:00:00 -su
0 S root 65128 65098 0 80 0 - 1822 hrtime 04:48 pts/17 00:00:00 sleep 11111
0 S root 65129 65098 0 80 0 - 1822 hrtime 04:48 pts/17 00:00:00 sleep 22222
0 R root 65133 65098 0 80 0 - 9341 - 04:49 pts/17 00:00:00 ps -fl
-f do full-format listing and -l is for Long format.
Changing the nice value of a new process
Changing the niceness value of a new process is fairly simple. The nice command itself will run the supplied command with the desired niceness value.(Please note we are logged in as a user here)
note: root user is the only person who can start an application with the high priority (lower than zero), but, any body can start an application with low priority (higher than zero).
if we try to run an application with high priority without root permissions, it would trough an error and starts application with priority zero.
user1@ubuntu16-1:~$ ps -fl
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
0 S user1 55624 55222 0 80 0 - 5669 wait Oct21 pts/17 00:00:00 bash
4 S user1 65015 65014 0 80 0 - 5677 wait 04:36 pts/17 00:00:00 -su
0 S user1 65043 65015 0 80 0 - 1822 hrtime 04:36 pts/17 00:00:00 sleep 11111
0 S user1 65046 65015 0 90 10 - 1822 hrtime 04:37 pts/17 00:00:00 sleep 22222
0 S user1 65047 65015 0 99 19 - 1822 hrtime 04:37 pts/17 00:00:00 sleep 33333
0 S user1 65048 65015 0 80 0 - 1822 hrtime 04:37 pts/17 00:00:00 sleep 44444
0 R user1 65085 65015 0 80 0 - 9341 - 04:40 pts/17 00:00:00 ps -fl
Now lets try the last command using root :
user1@ubuntu16-1:~$ su - root
Password:
root@ubuntu16-1:~# nice -n -20 sleep 55555 &
[1] 65112
root@ubuntu16-1:~# ps -fl
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
4 S root 55643 55624 0 80 0 - 13594 wait Oct21 pts/17 00:00:00 su -
4 S root 55644 55643 0 80 0 - 5725 wait Oct21 pts/17 00:00:00 -su
4 S root 65014 55644 0 80 0 - 13594 wait 04:36 pts/17 00:00:00 su - user1
4 S root 65097 65015 0 80 0 - 13594 wait 04:45 pts/17 00:00:00 su - root
4 S root 65098 65097 0 80 0 - 5691 wait 04:45 pts/17 00:00:00 -su
4 S root 65112 65098 0 60 -20 - 1822 hrtime 04:45 pts/17 00:00:00 sleep 55555
0 R root 65113 65098 0 80 0 - 9341 - 04:45 pts/17 00:00:00 ps -fl
it good to know that nice command has there different syntax:
$ nice -n niceness-value [command args]
OR
$ nice -niceness-value [command args] #it’s confusing for negative values
OR
$ nice --adjustment=niceness-value [command args]
Changing the nice value of a running process
renice
To change the niceness of a running process to a negative value we will use the renice command again.
renice value PID
It is important to note that changing a processes niceness value to a negative value requires root privileges. As the effects of giving a process a higher priority could have detrimental effects on a system.
user1@ubuntu16-1:~$ sleep 88888 &
[1] 67534
user1@ubuntu16-1:~$ ps -alf
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
4 S root 55643 55624 0 80 0 - 13594 - Oct22 pts/17 00:00:00 su -
4 S root 55644 55643 0 80 0 - 5725 - Oct22 pts/17 00:00:00 -su
4 S root 65014 55644 0 80 0 - 13594 - Oct22 pts/17 00:00:00 su - user1
4 S user1 65015 65014 0 80 0 - 5677 wait Oct22 pts/17 00:00:00 -su
0 S user1 67534 65015 0 80 0 - 1822 hrtime 05:59 pts/17 00:00:00 sleep 88888
0 R user1 67535 65015 0 80 0 - 9341 - 05:59 pts/17 00:00:00 ps -alf
user1@ubuntu16-1:~$ renice 10 67534
67534 (process ID) old priority 0, new priority 10
user1@ubuntu16-1:~$ ps -alf
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
4 S root 55643 55624 0 80 0 - 13594 - Oct22 pts/17 00:00:00 su -
4 S root 55644 55643 0 80 0 - 5725 - Oct22 pts/17 00:00:00 -su
4 S root 65014 55644 0 80 0 - 13594 - Oct22 pts/17 00:00:00 su - user1
4 S user1 65015 65014 0 80 0 - 5677 wait Oct22 pts/17 00:00:00 -su
0 S user1 67534 65015 0 90 10 - 1822 hrtime 05:59 pts/17 00:00:00 sleep 88888
0 R user1 67539 65015 0 80 0 - 9341 - 05:59 pts/17 00:00:00 ps -alf
user1@ubuntu16-1:~$ renice 5 67534
renice: failed to set priority for 67534 (process ID): Permission denied
user can only raise nice level.
user1@ubuntu16-1:~$ su - root
Password:
root@ubuntu16-1:~# renice -1 67534
67534 (process ID) old priority 10, new priority -1
root@ubuntu16-1:~# ps -alf
F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD
4 S root 55643 55624 0 80 0 - 13594 wait Oct22 pts/17 00:00:00 su -
4 S root 55644 55643 0 80 0 - 5725 wait Oct22 pts/17 00:00:00 -su
4 S root 65014 55644 0 80 0 - 13594 wait Oct22 pts/17 00:00:00 su - user1
4 S user1 65015 65014 0 80 0 - 5677 wait Oct22 pts/17 00:00:00 -su
0 S user1 67534 65015 0 79 -1 - 1822 hrtime 05:59 pts/17 00:00:00 sleep 88888
4 S root 67541 65015 0 80 0 - 13594 wait 06:00 pts/17 00:00:00 su - root
4 S root 67542 67541 0 80 0 - 5691 wait 06:00 pts/17 00:00:00 -su
4 R root 67558 67542 0 80 0 - 9341 - 06:00 pts/17 00:00:00 ps -alf
we can also use -p option before giving PID, but that is not necessary.
