CPU Scheduling Dishes take varying amounts of time to prepare - PowerPoint PPT Presentation

The Problem You are the cook at the State Street Diner Customers enter and place orders 24 hours a day CPU Scheduling Dishes take varying amounts of time to prepare (Chapters 7-11) What are your goals? Minimize average latency Minimize maximum latency Maximize throughput Which strategy achieves your goal? Kernel Operation Context matters! (conceptual, simplified) What if instead you are: Initialize devices Initialize “first process” the owner of an expensive container ship, and while (TRUE) { have cargo across the world while device interrupts pending the head nurse managing the waiting room of an - handle device interrupts emergency room U while system calls pending P C g n i l u a student who has to do homework in various d - handle system calls e h c S classes, hang out with other students and if run queue is non-empty (occasionally) sleep - select a runnable process and switch to it otherwise - wait for device interrupt }

Schedulers in the OS Scheduling processes OS keeps PCBs on different queues CPU scheduler selects next process to run from the ready queue Ready processes are on ready queue - OS chooses one to dispatch Disk scheduler selects next read/write Processes waiting for I/O are on appropriate operation device queue Network scheduler selects next packet to Processes waiting on a condition are on an appropriate condition variable queue send or process OS regulates PCB migration during life cycle Page Replacement scheduler selects page to of corresponding process evict Why scheduling is Terminology and Metrics challenging Processes are not created equal! Job/Task CPU-bound process: long CPU bursts A user request: e.g., mouse click, web request, mp3 encoding, compilation, scientific applications shell command… Turnaround time CPU bursts I/O-bound process: short CPU bursts Time elapsed between a job’ s arrival and its index a file system, browse small web pages completion Throughput Problem Number of tasks completed per unit of time don’ t know jobs type before running it jobs behavior can change over time

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