Understanding Priority Levels in Linux

Every OS has to divide the time it uses for each process. There are some priority processes such as an anti-virus that should be run asap and some other processes that run in background that is given time only if there is some time to spear. Now, how does this work and how can we give a process we launch a higher or lower priority?

If we go to a linux terminal and run

You will see a screen like this:

The first thing to see is the PR value, this means Priority. The lower the priority, the more time the OS will give to that process.

PR values range from [39 to -100] so -100 will be the most important process ever.

The rt value you can see in the image corresponds to a -100 value (that, because they use only 3 digits (including the negative sign) they decided to put rt and not -100. rt stands for Real Time.

Normally, and over all if you have no root/admin permission, you can only hope to achieve positive numbers, that means, assign a PR (priority) number between [39 to 0]. That is done using a value called Niceness (NI) which is a value between [19 to -20]. You can see it’s value on the 3rd column of the above image.

PR value then is computed as follows:

PR = 20 + NI

As you can see from the above expression, only possitive numbers can be achieved using this Niceness value.

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

The nice priority is actually used for user programs. Whereas the UNIX/LINUX overall priority (PR) has a range of 140 values, nice value enables the process to map to the last part of the range (from 0 to 39). This equation leaves the values from -100 to -1 unreachable. To be able to access to those values, the process should be stated as “real time”.

If we go again to the image, negative PR values gets a NI value of 0 and yet it’s PR remains different to 20 as one would expect from the last explanation. That is because for real time processes, the PR is computed differently.

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

Which will show the following list:

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

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

There are 3 names to remember:

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

PR is calculated as follows:

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