lsli14 - 14-1 14-1 Time and Scheduling Outline of material...

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14-1 14-1 Time and Scheduling Outline of material in this set: Time measures. Accounting for CPU time, e.g. 50% idle. Performance measures. Measures of CPU performance. Task states. Label indicating a task’s needs. Scheduling data. Information OS uses to schedule tasks. Scheduling events. Actions which cause the OS to stop one task and start another. Scheduling algorithms. How the OS chooses which task to run. 14-1 EE 4770 Lecture Transparency. Formatted 13:27, 23 December 1997 from lsli14. 14-1
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14-2 14-2 Time Measures At any time a CPU will be doing one of three things: Running a task in user mode, running in privileged mode, idle (no tasks to run). For an understood interval, T , let t u ( T ) denote time CPU in user mode, t p ( T ) denote time CPU in privileged mode, t i ( T ) denote time CPU is idle . The duration of the interval, t ( T ), is the sum of these . . . t ( T ) = t u ( T ) + t s ( T ) + t i ( T ). T sometimes omitted for brevity. 14-2 EE 4770 Lecture Transparency. Formatted 13:27, 23 December 1997 from lsli14. 14-2
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14-3 14-3 Performance Measures Several different measures of performance are used. Each measures a different aspect of performance. Utilization [of the CPU]. How efficiently CPU time is being used. Throughput [of the system]. What rate ( e.g. tasks/hour) work is getting done. Turnaround time [of a particular or average task]. The time to complete an individual task . . . . . . or the average time to complete a task. Response time [of a particular or average task]. The time between a particular event and response. (Usually the task responding to user input.) 14-3 EE 4770 Lecture Transparency. Formatted 13:27, 23 December 1997 from lsli14. 14-3
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14-4 14-4 Utilization The utilization of a CPU over T , denoted U ( T ) is given by: U ( T ) = t u ( T ) + t p ( T ) t ( T ) , where t u ( T ) is the user time over interval T , t p ( T ) is the privileged time over interval T , and t ( T ) is the total duration of interval T . Utilization is in the range [0 , 1]. Accountants want utilization to be high . . . . . . users want it to be low (when they run their tasks). Throughput Let n ( T ) be the number of tasks which complete in time period T . Then throughput is given by θ ( T ) = n ( T ) t ( T ) . The popular SPECrate benchmarks measure throughput. 14-4 EE 4770 Lecture Transparency. Formatted 13:27, 23 December 1997 from lsli14. 14-4
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14-5 14-5 Turnaround Time Let a task be submitted at t 1 . Let the task be completed at t 2 . Then the turnaround time for the task is t 2 - t 1 . Users want turnaround time to be short. When utilization is low, turnaround time is usually short. The SPECint and SPECfp benchmarks measure turnaround time on an unloaded system (in contrast to the SPECrate benchmarks). 14-5 EE 4770 Lecture Transparency. Formatted 13:27, 23 December 1997 from lsli14. 14-5
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14-6 14-6 Response Time Response time defined for an event and response . Event is something external that task senses.
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