If you want to change the priority of any process there are two things which you need to consider. There are two terms which will be used in this article i.e. NICE and PRIORITY.
In case you haven't notice when you run top command you get two different values for any process as I have marked in different color below
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2899 root 20 0 2704 1128 868 R 0.3 0.1 0:02.26 top
1 root 20 0 2892 1420 1200 S 0.0 0.1 0:01.29 initHere PR denoted PRIORITY and NI denotes NICE value where the PRIORITY range varies from 0 to 39 for any process in Linux and NICE value varies from -20 to 19.
`nice' prints or modifies a process's "niceness", a parameter that affects whether the process is scheduled favorably.
Syntax
# nice [OPTION] [COMMAND [ARG]...]
Example:
The below command will give a nice value of -20 to 2342 PID
# nice --20 2342NOTE: Use (--) to give a negative nice value and (-) to give a positive nice value
The below command will give a nice value of 20 to 2342 PID
# nice -20 2342Below comes the complicated part so please bear with me.
How do I understand from the NICE value about the priority of the process?
If you consider to look after NICE value to determine the priority of the process then as I said above its value ranges from -20 to 19 where
- -20 (process has high priority and gets more resources, thus slowing down other processes)
- 19 (process has lower priority and runs slowly itself, but has less impact on the speed of other running processes)
So in case you want any process to be given high priority (considering the fact that other processes might get slow) you can change their priority to any negative value upto -20 which will decrease the execution time of the process and the process will complete faster comparatively.
Let us see some real time examples
Run a process with nice value as -20
# time nice --20 seq 4234567 > file.txt
real 0m2.572s
user 0m2.519s
sys 0m0.047sDeleted the file and ran the same process with nice value of +20
# time nice -20 seq 4234567 > file.txt
real 0m2.693s
user 0m2.626s
sys 0m0.059sAs you can see the former command executed faster with a negative nice value.
| Value | Description |
| real | It represents time taken by command to execute since its initiation to its termination |
| user | It represents the amount of time that command/program took to execute its own code |
| sys | It represents time taken by Unix to fire the command |
How do I understand from the PRIORITY value about the priority of the process?
Again in case you consider to look at PR value for understanding the priority of the process the value ranges from 0 to 39 where
- 0 (process has high priority and gets more resources, thus slowing down other processes)
- 39 (process has lower priority and runs slowly itself, but has less impact on the speed of other running processes)
Let us see some real time examples
Run a process with positive nice value
# time nice -20 seq 42345671 > file.txt
real 0m27.548s
user 0m26.091s
sys 0m1.004sAs you see we are running a process with nice value +20 for which the NI appears 19 and PR as 39 which means the process will have the least priority and it will give priority to other process with higher nice value to use the system resources for their execution.
# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3192 root 39 19 4084 568 512 R 99.8 0.1 0:03.29 seq
1 root 20 0 2892 1420 1200 S 0.0 0.1 0:01.29 initSimilarly for a negative nice value
# time nice --20 seq 42345671 > file.txt
real 0m27.397s
user 0m26.555s
sys 0m0.600sAs you see the NI value is changed to -20 for PR value of 0.
# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3205 root 0 -20 4084 568 512 R 75.0 0.1 0:02.26 seq
1 root 20 0 2892 1420 1200 S 0.0 0.1 0:01.29 init
What would happen if I give a nice value out of range i.e. -20 to 19 to any process?
It can happen but your system won't understand any value other than -20 to 19 and will take the default value of -20 for high priority and 19 for least priority.
Let us see some real time examples
Assigning a nice value of -40
# time nice --40 seq 42345671 > file.txtBut still as you see the system is taking the nice value as -20 which is the highest recognizable value
# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3226 root 0 -20 4084 568 512 R 7.6 0.1 0:00.23 seq
1600 root 20 0 38616 3976 3284 S 0.3 0.4 0:01.68 vmtoolsdAssigning a nice value of 40
# time nice -40 seq 42345671 > file.txtNaah, it didn't worked either as top shows the process is using 19 as the nice value.
# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3235 root 39 19 4084 568 512 R 62.0 0.1 0:01.87 seq
2899 root 20 0 2704 1128 868 R 0.7 0.1 0:08.66 topSo I guess I made my point.
If you want to manually check the nice value along with the CPU and memory usage bu any process then use this command
# ps -o pid,pcpu,pmem,ni -p 88
PID %CPU %MEM NI
88 0.0 0.0 -10This will show you PID, %CPU,%MEM and nice value along with the process ID where 88 is the process id.
How to change the nice value of a running process?
In the above examples I started the process with a pre-defined nice value but what if the process is running and you want to change its nice value. For this we have another command i.e. renice
Syntax
# renice [-n] priority [[-p] pid ...]Some Examples:
# seq 4234567112 > file.txt# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3354 root 20 0 4084 568 512 R 95.4 0.1 0:07.19 seqChanging the nice value to -5
# renice -n -5 -p 3354
3354: old priority 0, new priority -5# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3354 root 15 -5 4084 568 512 R 99.7 0.1 0:30.26 seqChanging the nice value to 10
# renice -n 10 -p 3354
3354: old priority -5, new priority 10# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3354 root 30 10 4084 568 512 R 99.1 0.1 0:51.16 seqChanging the nice value to -15
# renice -n -15 -p 3354
3354: old priority 10, new priority -15# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3354 root 5 -15 4084 568 512 R 98.0 0.1 1:12.04 seq
How to change the nice value of any user?
Suppose you do not want a particular user to use much of your system resource, I thoses cases you can assign low nice value so that every process started by that user will use less system resources.
# renice -n 5 -u deepak
500: old priority 0, new priority 5Execute a process by user "deepak"
[deepak@test ~]$ seq 12345678 > file.txtVerify the NI value for "deepak"
# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4414 deepak 25 5 4084 588 532 R 97.3 0.1 0:05.54 seq
How to change the nice value of any user?
The same magic can be done for any particular group as well using the below command
# renice -n 5 -g workThe above command will change the default nice value of "work" group to 5 for any process running under their ownership.
What is the default NI value for any process?
The default NI value is 0 and PR value is 20 for any process running under Linux.
How to change the nice value for any user or group permanently?
The above shown examples are terminal based hence temporary. As soon as you reboot your machine the default nice value would be applicable for the defined user.
To make these changes permanent follow the below steps
NOTE: You can either user PR value or NI value to set the priority. I would suggest to use NICE value
# vi /etc/security/limits.conf
deepak hard priority 5This will set hard priority for user deepak as "5"
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