4.ppt - ClicktoeditMastersubtitlestyle RealTime Scheduling

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Click to edit Master subtitle style Real-Time  Scheduling
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Prelude: Maximal Parallelism n Question: Given a precedence graph, how many  processors should be used to execute it? n Example – Sequential program: a := x + y;  /* Job J1 */  b := z + 1;  /* Job J2 */  c := a - b;  /* Job J3 */  w := c + 1;  /* Job J4 */  d := a + e;  /* Job J5 */  J 1 J 2 J 3 J 4 J 5 J 6
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Maximal Parallelism n Assign processors 1 and 2: P1:    P2: n Synchronize J1 with J3, and J4 with J6. n Answer: “ maximally parallel ”  – The maximum number of processors that can  be used efficiently is equal to the cardinality of the  J J J J 2 J 3 J 4
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Scheduler and Schedule n Jobs are scheduled and allocated resources according  to the  scheduling and resource access-control  algorithms n scheduler   is a module that allocates processor and  resources to jobs and tasks.  It is defined by the  scheduling and resource management algorithms it  implements.
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Scheduler and Schedule n Specifically, a job is scheduled in a time interval on a  processor. n schedule  is an assignment of all the jobs in the  system on the available processors produced by the  scheduler.
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Valid Schedules Correctness of the scheduler:  A  valid schedule   satisfies: 1. Every processor is assigned to at most one job at any  time 2. Every job is assigned at most one processor at any  time (Jobs do not run in parallel on more than one  processor.)
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Valid Schedules 4. Depending on the scheduling algorithms used, the  total amount of processor time assigned to every  job is equal to its maximum or actual execution  time 5. All the precedence and resource usage constraints  are satisfied
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Scheduling Measures n A valid schedule is a  feasible schedule  if every  job meets its timing constraints. n A hard real-time scheduling algorithm   is  optimal  if using the algorithm the scheduler  always produces a feasible schedule, if the  given set of jobs have feasible schedules.
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Scheduling Measures n Minimizing  maximum or average tardiness   versus minimizing  average absolute lateness [ Jitters in the completion time ]  
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Scheduling Measures n All the jobs have the same release time and  deadline:  The longest response time is the  makespan  of the schedule n Average response time  – performance measure  of soft-deadline jobs
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Scheduling Measures n In a system that has a mixture of jobs with  hard and soft deadlines, the objective of  scheduling is typically to minimize the average  response time of jobs with soft deadlines while  ensuring that all jobs with hard deadlines  complete in time.
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Scheduling Measures n Miss rate and loss rate in soft real-time 
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4.ppt - ClicktoeditMastersubtitlestyle RealTime Scheduling

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