聊城大学计算机学院计算机操作系统（1）英文课件 Chapter9.ppt

Uniprocessor Scheduling Chapter 9 Aim of Scheduling Assign processes to be executed by the processor(s) Response time Throughput Processor efficiency Long-Term Scheduling Determines which programs are admitted to the system for processing Controls the degree of multiprogramming More processes, smaller percentage of time each process is executed Medium-Term Scheduling Part of the swapping function Based on the need to manage the degree of multiprogramming Short-Term Scheduling Known as the dispatcher Executes most frequently Invoked when an event occurs Clock interrupts I/O interrupts Operating system calls Signals Short-Tem Scheduling Criteria User-oriented Response Time Elapsed time between the submission of a request until there is output. System-oriented Effective and efficient utilization of the processor Short-Term Scheduling Criteria Performance-related Quantitative Measurable such as response time and throughput Priorities Scheduler will always choose a process of higher priority over one of lower priority Have multiple ready queues to represent each level of priority Lower-priority may suffer starvation Allow a process to change its priority based on its age or execution history Decision Mode Nonpreemptive Once a process is in the running state, it will continue until it terminates or blocks itself for I/O Preemptive Currently running process may be interrupted and moved to the Ready state by the operating system Allows for better service since any one process cannot monopolize the processor for very long Process Scheduling Example First-Come-First-Served(FCFS) Each process joins the Ready queue When the current process ceases to execute, the oldest process in the Ready queue is selected First-Come-First-Served(FCFS) A short process may have to wait a very long time before it can execute Favors CPU-bound processes I/O processes have to wait until CPU-bound process completes Round-Robin Uses preemption based on a clock An amount of time is de