Operating Systems CPU Scheduling Quiz

CPU Scheduling and Scheduling

• CPU scheduling is the process of determining which process will own the CPU for execution while another process is held.
• The main task of CPU scheduling is to ensure that whenever the CPU is idle, the operating system (OS) selects at least one of the processes available in the ready queue for execution.
• This selection is carried out by the CPU scheduler.
• It chooses one of the processes in memory that are ready for execution.

Types of CPU Scheduling

• Preemptive Scheduling: Tasks are assigned priorities. A higher priority task might interrupt a lower priority task, even if the lower priority task was already running. The lower priority task resumes when the higher priority task completes.
• Non-preemptive Scheduling: The CPU is allocated to a specific process. The process remains in control of the CPU until it releases it, either by switching context or terminating. This method doesn't require special hardware like a timer.

When Scheduling is Preemptive or Non-Preemptive

• If only conditions 1 (process switches from running to waiting) and 4 (process finishes execution) apply, the scheduling is non-preemptive.
• All other conditions indicate preemptive scheduling.

Scheduler

• The scheduler chooses which process runs next when multiple processes are in the ready queue.
• A process can move between various scheduling queues throughout its lifetime.
• The scheduler selects processes from these queues.
• There are three types of schedulers based on the scheduling queue and operating system.
  • Long-term scheduler (Job scheduler)
  • Medium-term scheduler
  • Short-term scheduler (Process scheduler)

Swapping

• Swapping is the process of swapping processes in and out of main memory.
• Swap-out: When main memory is limited, processes are moved to secondary storage (e.g., hard disk).
• Swap-in: A process is loaded back into main memory.

Operations on Processes

• Process Creation: New processes can be created by a running process via a system call. The creating process is the parent, and the new processes are children.
• Process Resources: Subprocesses can either receive resources directly from the parent or share resources with the parent process.
• Process Execution: Parent and child processes can execute concurrently or the parent can wait for its children to terminate. Parent and child processes can share or have separate address spaces.
• Process Identification: Each process is identified by a unique process identifier (PID).
• Process Creation: A new process is created using the fork system call.
• Process Communication: Processes sharing an address space can communicate directly.

Process Termination

• Process termination occurs when a process finishes its execution. This can be normal or abnormal termination, with the OS potentially initiating the termination.
• Resources held by the terminated process are deallocated.
• Child processes might return data to the parent process.
• A parent process might terminate its children due to tasks no longer required or if an error occurs.

CPU Scheduling Terminologies

• Burst Time/Execution Time: Time required for process execution. Often referred to as CPU bursts.
• Arrival Time: When a process enters the ready state.
• Finish Time: When a process completes and leaves the system.
• Multiprogramming: Multiple programs residing in memory at the same time.
• Jobs: Programs without user interaction.
• Users: Programs with user interaction.
• Process: The basis of all job and user programs.
• CPU/IO Burst Cycle: The process alternates between CPU execution and I/O operations. CPU bursts are typically shorter than I/O.

CPU Scheduling Criteria

• Maximize:
  • CPU Utilization: Keeping the CPU busy as much as possible in the system.
  • Throughput: The number of processes completed per unit of time.
• Minimize:
  • Waiting Time: The time a process waits in the ready queue.
  • Response Time: The time it takes for a process to respond to a request.
  • Turnaround Time: The total time a process spends in the system from submission to completion.

Interval Timer

• An interval timer is closely related to preemption.
• A process receives allocated CPU time, and a timer is set for a specific interval.
• This timer interrupt and process preemption force the processes to return the CPU before the burst completes.
• This prevents processes from monopolizing the system resources.

Types of CPU Scheduling Algorithms

• First Come First Serve (FCFS): Processes are executed in the order they arrive. Simple to implement but can lead to longer wait times for shorter processes.
• Shortest Job First (SJF): Processes with the shortest execution time are given priority. Improves efficiency but requires knowing the process's execution time in advance.
• Shortest Remaining Time (SRT): A preemptive version of SJF. If a shorter process arrives while a longer process is running, the shorter process preempts the longer process.
• Priority Scheduling: Processes are assigned priorities, with higher priority processes being executed first. Requires a method assigning priorities.
• Round Robin: Processes are executed in a circular fashion, with each process getting a time slice. Each process is given a slot, thus preventing starvation.
• Multilevel Queue Scheduling: Processes are sorted into various queues based on characteristics (priority, memory size, etc.). Separate scheduling algorithms can operate on each queue.

Deadlocks

• A set of processes are blocked, each holding a resource and waiting for another resource held by a blocked process.

Methods of Handling Deadlocks

• Ignoring the Deadlock: The OS assumes deadlocks won't occur, which is practical in many OS functions involving many users.
• Deadlock Prevention: Prevents the occurrence of one or more deadlock conditions (Mutual Exclusion, Hold and Wait, No Preemption, Circular Wait).
• Deadlock Avoidance: Uses algorithms to ensure the system remains in a safe state at all times, avoiding deadlocks.
• Deadlock Detection and Recovery: Detects deadlocks when they arise and recovers the system by preempting resources or terminating processes.

Test your knowledge of CPU scheduling in operating systems with this quiz. It covers key characteristics of preemptive and non-preemptive scheduling, scheduling algorithms, and roles of different types of schedulers. Perfect for students studying operating system concepts.