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CS 101 – Oct. 28

CS 101 – Oct. 28. Operating Systems (continued) Process states √ Scheduling processes on the CPU System load Types of OS. Process scheduling. Extensively studied in CS ! Look carefully at examples pp. 340-342. 3 common techniques Round robin (we saw this one already)

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CS 101 – Oct. 28

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  1. CS 101 – Oct. 28 • Operating Systems (continued) • Process states √ • Scheduling processes on the CPU • System load • Types of OS

  2. Process scheduling • Extensively studied in CS ! • Look carefully at examples pp. 340-342. • 3 common techniques • Round robin (we saw this one already) • First-come, first-served • Shortest Job Next

  3. Scheduling • Round-robin • Fair: give each process an equal time quantum. • Usually not long enough to finish, so pre-empt. Pre-emption incurs some overhead, so there are alternative strategies: • First-come first served • Do the tasks in the order in which they are requested • Shortest job next • Try to minimize the average completion time. Do the easy/short tasks first.

  4. Measuring a schedule • To tell how good a schedule is, we can compute the average turnaround time. • People are usually interested in how long it takes for their jobs to get done. • Turnaround time = (time @ finish – time @ request)

  5. Example First-come, first-served: Process 1 can execute from t=0 to t=20 Process 2, t=20 to t=50 Process 3, t=50 to t=90 Process 4, t=90 to t=100 What is the average turnaround time?

  6. Examples

  7. System load • A measure to show how “busy” the CPU is • At an instant: how many tasks are currently running or ready • If load > 1, we have “overloaded” system, and work is starting to back up. Should stop requesting more jobs.  • Typically reported as an average over the last 1, 5 or 15 minutes.

  8. Cutting Edge • Real-time OS • Assumes all jobs have deadlines • High Performance Computing • Using multiple CPUs at once • Technique is called “parallel processing” • Distributed OS • Your running program and/or files may be on a different machine

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