Multilevel queue scheduling
- 1. Multilevel Queue Scheduling Principles of Operating System
- 2. ➢ CPU scheduling ➢ Multilevel queue scheduling ➢ First priority preemptive scheduling method. ➢ Time slicing method ➢ Advantage and Disadvantage ➢ Conclusion Overview
- 3. What is CPU Scheduling? CPU Scheduling is a process of determining which process will own CPU for execution while another process is on hold. The Processes that needs to be executed are kept into the ready queue and then they are executed one by one in the CPU.
- 4. But… In real life, there are more than one type of processes like.. System Process Interactive Process Background Process If we have different types of processes then why there’s just one ready queue for all of them!!
- 5. MULTILEVEL QUEUE SCHEDULING ● Multilevel queue scheduling is a type of CPU scheduling in which the processes in the ready state are divided into different groups, each group having its own scheduling needs. ● The ready queue is divided into different queues according to different properties of the process like memory size, process priority, or process type.
- 6. Suppose there are 3 processes ready to execute: 1. System process The CPU stops executing other processes when system interrupts. That’s why it has the highest priority. 2. Interactive process Process where we can directly interact with the application. That’s why it’s priority is less than system process. 3. Background process Where we give multiple processes to the system and system will keep executing the processes in the background. It has the lowest priority.
- 7. ● There must be different queue for every process, that means each process will have its own queue. ● Each queue is assigned an absolute priority over other queues System process queue > Interactive process queue > Background process queue ● This method is called first priority preemptive scheduling method. ● Overall gist of this method is that run the processes with higher priority first and then the processes with subsequent priority.
- 8. Time Slicing method: In this method, processes from each queue are run for a fixed amount of time, and then we move on to the next queue. When we arrive at the last queue, we run it for a specific amount of time, and then we move back to the first queue. For example, queue one runs for 50% of CPU time, queue two runs for 30% of CPU time, and queue three runs for 20% of CPU time. In this way, the total CPU time is divided among each queue to run their processes.
- 9. We need to run the processes in all the queues in the same manner, or we can have different implementations for each queue. There are many algorithms using which we can decide the process to be run first and for how long. For example: ● The system process queue can use FCFS(First Come, First Serve) scheduling. ● The interactive process queue can use SJF(Shortest Job First) scheduling. ● The batch process queue can use RR(Round Robin) scheduling.
- 11. Example:
- 12. Advantages: It allows us to apply different scheduling algorithms for different processes. It offers a low scheduling overhead, i.e., the time taken by the dispatcher to move the process from the ready state to the running state is low. Dispatcher performs the task of allocating the selected process to the CPU. Disadvantages: There are chances of starvation for lower priority processes. If higher priority processes keep coming, then the lower priority processes won't get an opportunity to go into the running state. Multilevel queue scheduling is inflexible.
- 13. Conclusion: ● Multilevel Queue Scheduling is a part of CPU scheduling. ● For each different types of processes has its own queue which executes from highest priority to the lowest priority. ● There are many algorithms that can be used to run the process. ● Multilevel Queue Scheduling has its advantages as well as disadvantages like Starvation, that’s why we use Multilevel feedback Queue Scheduling
- 14. THANK YOU Presented by: 428_Kuldeep Pandey 429_Aditi Pawaskar 430_Aakanksha Rajbhar