CPU Scheduling Algorithms

CPU Scheduling Algorithms

• 2.
  CPU SCHEDULING ALGORITHMS
• 3.
  Introduction Scheduling management Scheduling decisions SchedulingCriteria CPU scheduling algorithms • First-Come, First-Served (FCFS) Scheduling • Shortest Job First (Sjf) Scheduling • Priority Scheduling OVERVIEW
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   Processes aremanaged through the use of multiple queues of PCB's.  The job queue contains all jobs submitted to the system, but not yet in main memory.  The ready queue contains all jobs in main memory ready to execute.  Each I/O device has a queue of jobs waiting for various I/O operations.  All of these events are signaled by interrupts SCHEDULING MANAGEMENT
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   CPU schedulingdecisions may take place when a process: • Switches from running to waiting state. • Switches from running to ready state. • Switches from waiting to ready state. • Terminates. SCHEDULING DECISIONS
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  Each scheduling algorithmfavors particular criteria:  CPU utilization  Throughput  Turnaround time  Waiting time  Response time SCHEDULING CRITERIA
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  SCHEDULING ALGORITHMS First-Come, First-Served(FCFS) Scheduling
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  • Jobs areexecuted on first come, first serve basis. • Easy to understand and implement. • Poor in performance as average wait time is high. INTRODUCTION
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  • Suppose thatthe processes arrive in the order: P1 , P2 , P3 EXAMPLE Process Burst Time P1 24 P2 3 P3 3
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  SCHEDULING ALGORITHMS Shortest JobFirst (Sjf) Scheduling
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   Best approachto minimize waiting time.  Impossible to implement.  Processer should know in advance how much time process will take. INTRODUCTION
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   Non preemptive: •Once CPU given to the process it cannot be preempted until completes its CPU burst.  Preemptive • If a new process arrives with CPU burst length less than remaining time of current executing TWO SCHEMES:
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   Non preemptive: •Once CPU given to the process it cannot be preempted until completes its CPU burst.  Preemptive • If a new process arrives with CPU burst length less than remaining time of current executing EXAMPLE
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  EXAMPLE Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4
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  SCHEDULING ALGORITHMS Priority Scheduling
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   Each processis assigned a priority.  Process with highest priority is to be executed first and so on.  Processes with same priority are executed on first come first serve basis.  Priority can be decided based on following: • memory requirement • time requirements • any other resource requirement. INTRODUCTION
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  A priority number(integer) is associated with each process. The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority). • Preemptive • Nonpreemptive INTRODUCTION
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  EXAMPLE