Priority Assignment to Tasks in Operating System

The practice of giving each task or process in an operating system a priority level is known as a priority assignment. A priority level is a number that represents the relative urgency or significance of an activity or process in comparison to other tasks.

When there are numerous tasks that are ready to run, the operating system utilizes the priority level to decide which job should be done next. Priority-level tasks are carried out before those with lower priority levels. This guarantees that the system runs effectively and that the most important activities are finished first.

The assignment of priorities may be static or dynamic. Static priority assignment assigns a priority level to each task at the moment of creation, and this priority level stays constant for the duration of the job. The priority level of a job can be changed dynamically at runtime depending on the task's relative relevance, execution duration, and resource utilization.

In order to guarantee that the operating system responds fast to significant events and that resources are allocated effectively, priority assignment is a crucial component of operating system architecture. Priority inversion, where a lower-priority task prevents the execution of a higher-priority task, and priority starvation, where a task with a lower priority level is never executed because higher-priority tasks are constantly running, are examples of problems that can arise from improper priority assignment.

Types of Priorities

A process or task's importance in the system is determined by its priority. A procedure with a higher priority will be given more resources and will be carried out before those with a lower priority. Ordinarily, priorities are ranked from 0 to 127, with 127 being the highest priority and 0 being the lowest.

In operating systems, many forms of priority are utilized. As follows −

Static Priorities

In this style of assigning priorities, a process's or a task's priority is set and does not alter as it is being executed. Typically, the system administrator or the developer determines the priority when a process is created. Static priorities are simple to design, but if they are not allocated properly, they may result in ineffective resource use.

Dynamic Priorities

With this sort of priority assignment, a process's or task's priority can change while it runs, depending on things like how the process behaves, how much CPU it is using, and how much system load there is. Dynamic priorities make it possible to employ resources more effectively and guarantee that the most crucial activities are completed first.

Priority Inversion

When a lower priority job prevents or delays a higher priority task, it is known as priority inversion. This may occur when a low-priority process is using a resource that a high-priority task is awaiting. In this case, a low-priority task may hog a resource for a considerable amount of time, delaying the execution of a high-priority task.

Priority inversion can be avoided using a number of tactics, including priority inheritance, priority ceilings, and priority inheritance combined with ceilings. Priority inheritance momentarily increases the low-priority activity's priority to that of the high-priority task when it has a resource that the high-priority job requires. The procedure of assigning a shared resource a priority ceiling higher than the priority of any activity that may use the resource is referred to as setting a priority ceiling. Priority inheritance with ceiling, which combines the two approaches, ensures that priority inheritance only takes place when the low-priority task is in possession of a shared resource that is subject to a priority ceiling.

Priority Scheduling Algorithms

Priority scheduling algorithms are used to determine which tasks or activities should be completed first and in what order. Several priority scheduling strategies are used by various operating systems, including −

First-Come-First-Serve (FCFS)

With this algorithm, tasks are completed in the order they are received. A job's priority is not taken into account, therefore if there are a lot of low-priority tasks waiting in line, the task with the highest priority may have to wait a very long time.

Shortest-Job-First (SJF)

The job that can be completed in the smallest amount of time comes first in this method. A task's priority is not taken into consideration, therefore the job with the greatest priority could have to wait a long time if its execution takes longer than a task with a lower priority.

Priority Scheduling

This algorithm prioritizes activities, with higher priority tasks being carried out before lower priority duties. The same-priority tasks are completed in FCFS sequence.

Round Robin Scheduling

Using this approach, jobs are carried out in a cycle where each job has a set window of time (or quantum) to run. If a job does not complete within its time slice, it is preempted and put at the end of the line. In this method, priority is considered, and tasks with higher priorities are given preference over those with lower priorities.

Multilevel Feedback Queue Scheduling

According to this method, tasks are separated into several queues according to their importance. Tasks are transferred to lower priority queues if they do not finish within their time slice, and the highest priority queue is completed first. Tasks that consume a lot of CPU time are transferred to lower priority queues so that other jobs can have a chance to execute, and the priority of a task is dynamically altered based on its behavior.

Real-Time Operating Systems

Real-Time Operating Systems (RTOS) are types of operating systems created to manage operations with exact timing constraints, such as those present in industrial control systems, medical equipment, and automotive systems. Priority assignment in RTOS is essential to ensuring that the most important activities are completed on schedule. The typical scheduling mechanism used by RTOS assigns jobs static priorities at the time of creation.

Moreover, RTOS employ a number of strategies to guarantee that higher priority operations do not obstruct or delay vital jobs. These methods include proactive scheduling, deadline-based scheduling, and priority inversion prevention.

Applications

Various scheduling algorithms and priority assignment methods are used by various operating systems. These are a few instances −

Windows

The scheduling algorithm used by Windows allocates priorities depending on the priority levels of the threads. From 0 to 31, there are several degrees of thread priority; higher levels have greater priorities. Moreover, Windows has a preemptive scheduling system that makes sure higher priority threads take precedence over lower priority threads.

Linux

A dynamic priority scheduling technique employed by Linux determines a task's priority depending on how much CPU time it has consumed. Moreover, Linux makes use of the Fully Fair Scheduler (CFS), which distributes CPU time across jobs according to priority and CPU consumption.

Android

The scheduling algorithm used by Android allocates priorities depending on the levels of each process's priority. Between -20 and 19, there are several levels of process priority; lower numbers indicate higher priorities. The O(1) scheduler, which is also used by Android, makes sure that higher priority jobs take precedence over lower priority ones.

Conclusion

Each operating system must include priority assignment since it guarantees effective resource usage and enhances system performance. Depending on their needs, several operating systems employ various priority assignment strategies and scheduling algorithms. When a lower priority job prevents or delays a higher priority task, it is known as priority inversion. Priority inversion can be prevented using a number of strategies, including priority inheritance, priority ceilings, and priority inheritance with ceiling. Fixed-priority scheduling algorithms and other methods are used by real-time operating systems to make sure that crucial operations are completed on schedule. Generally, the effective and dependable operation of contemporary computer systems depends on priority assignment.