what is the difference among three priorities used in Linux kernel?

4
votes

I am new to Linux kernel and right now i am studying about Process Scheduling in Linux kernel . There are three types of priorities in Linux :

  1. Static priority
  2. Dynamic priority
  3. Real time priority

Now what i have understood is that :

  • Static priority and Dynamic priority are defined only for Conventional processes and they can take value from 100 to 139 only.
  • Static priority is used to determine base time slice of a process

  • Dynamic priority is used to select a process to be executed next.

  • Real time priorities are defined only for Real time processes and it's value can range from 0 to 99

Now my questions are :

  1. Correct me if i am wrong and please also tell me why we are using three types of priorities in Linux and what are the differences among these priorities?
  2. Are the processes are differentiated as Real time or Conventional on the basis of priorities that is if priority is between 100 to 139 then processes are Conventional processes otherwise Real time processes?
  3. How the priorities are changed in Linux , i mean , we know that priority of a process does not remain constant through out the execution ?
1
linuxlinux-kernel

1 Answers

3
votes

Disclaimer: Following is true for scheduling in linux (I am not sure about windows or other OS). Thread and process has been used interchangeably here, althogh there is a difference in between them.

Priorities & differences

1.Static priority: These are the default priorities (value 0 for conventional processes aka non-real time processes ie when real time scheduling is not used) set while creating a new thread. You can change them using:

`pthread_setschedparam(pthread_t thread, int policy, const struct sched_param *param);`

where, sched_param contains the priority:

struct sched_param 
{
    int sched_priority;     /* Scheduling priority */
};

2 Dynamic priority: When threads start to starve because higher priority threads being scheduled all the time, there becomes a need to raise the priority of such a thread using various mechanisms. This raised/lowered (yes, this happens too) priority is known as the dynamic priority because it keeps on changing. In Linux, even the fat kid gets to play.

3.Real time priority: This comes into picture only when threads (processes) are scheduled under one of the real-time policies (SCHED_FIFO, SCHED_RR) and have a sched_priority value in the range 1 (low) to 99 (high). This is the highest in comparison to the static/dynamic priorities of non real time processes.

More information: http://man7.org/linux/man-pages/man3/pthread_getschedparam.3.html

Now, to your questions:

Correct me if i am wrong and please also tell me why we are using three types of priorities in Linux and what are the differences among these priorities?

So, for non-real time scheduling policies, every process has some static priorities, a higher priority gives the thread a kick-start, and later to avoid any injustice, the priority is boosted/lowered which becomes the dynamic priority.

Are the processes are differentiated as Real time or Conventional on the basis of priorities that is if priority is between 100 to 139 then processes are Conventional processes otherwise Real time processes?

Not really, it depends upon the scheduling mechanism in place.

How the priorities are changed in Linux , i mean , we know that priority of a process does not remain constant through out the execution ?

That's when the dynamicness comes into picture. Read about the "nice value" in the given links.