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os6 - Modified from Lecture Slides for Operating System...

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Operating Systems : CPU Scheduling (Chapter 6) Sung-Yong Park, Ph.D. Distributed Computing & Communication Laboratory Department of Computer Science Sogang University Modified from Lecture Slides for Operating System Concepts (6 th Edition) by Silberschatz et. al. Sogang University Distributed Computing & Communication Lab. Page 2 Basic Concepts Multiprogramming – maximize CPU utilization -> need scheduling. CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. CPU burst distribution – generally characterized as exponential, with many short CPU bursts, and a few long CPU bursts.
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Sogang University Distributed Computing & Communication Lab. Page 3 Histogram of CPU-burst Times I/O bound program CPU bound program Sogang University Distributed Computing & Communication Lab. Page 4 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 non-preemptive . All other scheduling is preemptive .
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Sogang University Distributed Computing & Communication Lab. Page 5 Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode 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. Sogang University Distributed Computing & Communication Lab. Page 6 Scheduling Criteria CPU utilization – keep the CPU as busy as possible. Throughput – # of processes that complete their execution per time unit. Turnaround time – amount of time to execute a particular process (from submission to completion). Waiting time – amount of time a process has been waiting in the ready queue. 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).
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Sogang University Distributed Computing & Communication Lab.
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