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Optimizing CPU Scheduling: Concepts, Algorithms, and Objectives

Learn about CPU scheduling, CPU-I/O burst cycles, optimization criteria, and scheduling algorithms like FCFS, SJF, and RR.

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Optimizing CPU Scheduling: Concepts, Algorithms, and Objectives

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  1. CPU Scheduling ( Basic Concepts) CPU scheduling • Which process will get the CPU when it is available? This is decided by the O.S. and is called CPU scheduling. • The part of the O.S. which performs scheduling is called Scheduler. • The scheduler selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. • The objective is to maximize CPU utilization in multiprogramming. CPU–I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait.

  2. Alternating Sequence of CPU And I/O Bursts execute CPU Burst I/O Wait I/O Burst execute CPU Burst I/O Wait I/O Burst

  3. Important Definitions

  4. Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time

  5. Scheduling Algorithms • First-Come, First-Served (FCFS) Scheduling ( non preemptive) • This algorithm favors: • Longer jobs against the shorter jobs ( W/B is less for longer jobs) • CPU bound processes against I/O bound processes

  6. Shortest Job first Scheduling Now the jobs appear as: • Now: • Longer jobs are not favored against the shorter jobs ( W/B is less for longer jobs) • A longer job can face starvation • CPU bound processes are still favored against I/O bound processes

  7. Round-Robin (RR) Scheduling (Preemptive) • A queue of ready processes is maintained • A time quantum q (10-100 msec) is allocated to each process. • I f a process does not finish in the allocated time it goes back to the end of the queue (circular queue) • If there are n processes, each process gets turn at most after (n-1) q units. Gannt Chart for q = 20 msec CPU CPU • Performance • If q is large, this scheme degenerates to FCFS • If q is small, more context switches (each process finishes in more turns)

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