Nice value is a user-space and priority PR is the process's actual priority that use by Linux kernel. In linux system priorities are 0 to 139 in which 0 to 99 for real time and 100 to 139 for users. nice value range is -20 to +19 where -20 is highest, 0 default and +19 is lowest. relation between nice value and priority is :

PR = 20 + NI

so , the value of PR = 20 + (-20 to +19) is 0 to 39 that maps 100 to 139.

According to top manual:

PR -- Priority The scheduling priority of the task. If you see 'rt' in this field, it means the task is running under 'real time' scheduling priority.

NI is nice value of task.

NI -- Nice Value The nice value of the task. A negative nice value means higher priority, whereas a positive nice value means lower priority.Zero in this field simply means priority will not be adjusted in determining a task's dispatch-ability

Edit: By default when a program is launched in Linux, it gets launched with the priority of '0'. However you can change the priority of your programs by either of the following methods.

1. You can launch a program with your required priority using

   nice -n nice_value program_name
   

2. you can also change the priority of an already running process using

   renice -n nice_value -p process_id
   

What is Priority and Why Should I Care?

When talking about processes priority is all about managing processor time. The Processor or CPU is like a human juggling multiple tasks at the same time. Sometimes we can have enough room to take on multiple projects. Sometimes we can only focus on one thing at a time. Other times something important pops up and we want to devote all of our energy into solving that problem while putting less important tasks on the back burner.

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. 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.

By using this scale we can allocate our CPU resources more appropriately. Lower priority programs that are not important can be set to a higher nice value, while high priority programs like daemons and services can be set to receive more of the CPU’s focus. You can even give a specific user a lower nice value for all of his/her processes so you can limit their ability to slow down the computer’s core services.

_Source

Set the priority for new processes with nice, eg

nice -n 10 firefox

for existing processes

renice 10 -p $(pgrep firefox)

To set the priority <0 you need sudo, eg:

renice -1 -p $(pgrep firefox)
renice: failed to set priority for 2769 (process ID): Permission denied

but not for a priority >=0

Example

% ps -o pid,comm,pri,nice -p $(pgrep firefox)
  PID COMMAND         PRI  NI
 2769 firefox          19   0

% renice 10 -p 2769     # note, we don't need sudo here
2769 (process ID) old priority 0, new priority 10

% ps -o pid,comm,pri,nice -p $(pgrep firefox)
  PID COMMAND         PRI  NI
 2769 firefox           9  10

% sudo renice -19 -p 2769                    
 2769 (process ID) old priority 10, new priority -19

% ps -o pid,comm,pri,nice -p $(pgrep firefox)
  PID COMMAND         PRI  NI
 2769 firefox          38 -19

Other example

To renice all running processes for a specific user

renice 20 -u user_name

Short Answer

PR is the priority level. The lower the PR, the higher the priority of the process will be.

PR is calculated as follows:

• for normal processes: PR = 20 + NI (NI is nice and ranges from -20 to 19)
• for real time processes: PR = - 1 - real_time_priority (real_time_priority ranges from 1 to 99)

Long Answer

There are 2 types of processes, the normal ones and the real time.

For the normal ones (and only for those), nice is applied as follows:

Nice

The "niceness" scale goes from -20 to 19, whereas -20 is the highest priority and 19 the lowest priority. The priority level is calculated as follows:

PR = 20 + NI

Where NI is the nice level and PR is the priority level. So as we can see, the -20 actually maps to 0, while the 19 maps to 39.

By default, a program nice value is 0, but it is possible for the root user to launch programs with a specified nice value by using the following command:

nice -n <nice_value> ./myProgram 

Real Time

We could go even further. The nice priority is actually used for user programs. Whereas the UNIX/LINUX overall priority has a range of 140 values, nice value enables the process to map to the last part of the range (from 100 to 139). This equation leaves the values from 0 to 99 unreachable which will correspond to a negative PR level (from -100 to -1). To be able to access to those values, the process should be stated as "real time".

There are 5 scheduling policies in a LINUX environment that can be displayed with the following command:

chrt -m 

Which will show the following list:

1. SCHED_OTHER   the standard round-robin time-sharing policy
2. SCHED_BATCH   for "batch" style execution of processes
3. SCHED_IDLE    for running very low priority background jobs.
4. SCHED_FIFO    a first-in, first-out policy
5. SCHED_RR      a round-robin policy

The scheduling processes could be divided into 2 groups, the normal scheduling policies (1 to 3) and the real time scheduling policies (4 and 5). The real time processes will always have priority over normal processes. A real time process could be called using the following command (The example is how to declare a SCHED_RR policy):

chrt --rr <priority between 1-99> ./myProgram

To obtain the PR value for a real time process the following equation is applied:

PR = -1 - rt_prior

Where rt_prior corresponds to the priority between 1 and 99. For that reason the process which will have the higher priority over other processes will be the one called with the number 99.

It is important to note that for real time processes, the nice value is not used.

To see the current "niceness" and PR value of a process the following command can be executed:

top

Which shows the following output:

In the figure the PR and NI values are displayed. It is good to note the process with PR value -51 that corresponds to a real time value. There are also some processes whose PR value is stated as "rt". This value actually corresponds to a PR value of -100.