103.6. Modify process execution priorities

Weight: 2

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.(we use 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)

user1@ubuntu16-1:~$ sleep 11111 &
[1] 65043
user1@ubuntu16-1:~$ nice -n 10 sleep 22222 &
[2] 65046
user1@ubuntu16-1:~$ nice -n 19 sleep 33333 &
[3] 65047
user1@ubuntu16-1:~$ nice -n -10 sleep 44444 &
[4] 65048
user1@ubuntu16-1:~$ nice: cannot set niceness: Permission denied

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]

nice command example

Description

nice --20 application

highest priority

nice --15 application

very high

nice -10 application

medium low

nice -19 application

lowest

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