Chapter 5: Process Scheduling
5.1Basic Concepts
Maximum CPU utilization obtained with multiprogramming
CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait
CPU burst distribution
Histogram of CPU-burst durations(大量的短CPU bursts以及少量的长CPU bursts)
CPU Scheduler
Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them
CPU scheduling decisions may take place when a process:
1.Switches from running to waiting state
2.Switches from running to ready state
3.Switches from waiting to ready
4.Terminates
Scheduling under 1 and 4 is nonpreemptive/cooperative(进程只有终结或者跳到wait状态的时候才会释放CPU)
All other scheduling is preemptive(需要计时器)
抢占式的缺点:共享数据不一致
Dispatcher
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
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switching context
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switching to user mode
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jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running
Scheduling Criteria
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CPU utilization – keep the CPU as busy as possible
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Throughput – # of processes that complete their execution per time unit
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Turnaround time – amount of time to execute a particular process
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Waiting time – amount of time a process has been waiting in the ready queue
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Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)(turnaround time会被输出设备的速度限制)
Scheduling Algorithm Optimization Criteria
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Max CPU utilization
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Max throughput
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Min turnaround time
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Min waiting time
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Min response time
First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3, The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17
Suppose that the processes arrive in the order: P2 , P3 , P1, The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case
Convoy effect: short process behind long process(护送效应)
Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time
Two schemes:
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nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst
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preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)