10 Questions
What is the main purpose of a scheduler in an operating system?
Allocating resources among competing processes
Which type of scheduler determines the degree of multiprogramming and balances I/O-bound and CPU-bound processes?
Long-term Scheduler
What happens to a process after it has either completed or aborted in the ready queue?
It is discarded
Which scheduler is responsible for swapping processes between main memory and disk?
Medium-term Scheduler
What role does the dispatcher play in the execution of processes in a queue?
Selects a process from the queue to execute
Which component of the operating system makes decisions on which process to execute next?
Scheduler
What is the primary goal of the types of schedulers in an operating system?
Minimizing the turnaround time, waiting time, and response time of processes
Which scheduler optimizes system performance by balancing I/O-bound and CPU-bound processes?
Dispatcher
In what state does a process exist after it has been interrupted and transferred to the waiting queue?
Blocked state
What does a Long-term Scheduler focus on when selecting processes to bring into the ready queue?
Balancing the number of I/O-bound and CPU-bound processes for system optimization
Study Notes
Process Scheduling
- Process scheduling is a function of the operating system that handles the removal of a running process from the CPU and selects another process to execute based on a particular strategy.
Categories of Scheduling
- Non-preemptive: The resource cannot be taken from a process until the process completes execution.
- Preemptive: The OS allocates resources to a process for a fixed amount of time, and the process switches from running state to ready state or from waiting state to ready state.
Process Scheduling Queues
- Job Queue: Keeps all the processes in the system.
- Ready Queue: Keeps a set of all processes residing in main memory, ready and waiting to execute.
- Device Queues: Keeps processes blocked due to unavailability of an I/O device.
Two-State Process Model
- Running: A new process enters the system in the running state.
- Not Running: Processes not running are kept in a queue, waiting for their turn to execute.
Schedulers
- Long-term Scheduler (Job Scheduler): Selects which processes should be brought into the ready queue, determines the degree of multiprogramming, and optimizes system performance.
- Short-term Scheduler (CPU Scheduler): Decides which process to execute next, invoked frequently (e.g., after every clock interrupt).
- Medium-term Scheduler: Acts as a bridge between long-term and short-term schedulers, controlling the degree of multiprogramming by swapping processes between main memory and disk.
Scheduling Algorithms
- Shortest Job Next (SJN) or Shortest Job First (SJF): A non-preemptive algorithm that assigns priority to processes based on their duration or resource requirements.
- Shortest Remaining Time (SRT): The preemptive version of SJN, allocating the processor to the job closest to completion.
- Round Robin Scheduling: A preemptive algorithm that provides each process a fixed time to execute (called a quantum).
- Multiple-Level Queues Scheduling: Uses other existing algorithms to group and schedule jobs with common characteristics.
Test your knowledge on Shortest Job Next (SJN) or Shortest Job First (SJF) priority scheduling, a non-preemptive algorithm commonly used in batch systems. Understand how processes are executed based on priority levels and resource requirements.
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