*Real-Time Scheduling

CIS 700

*OutlineReal-time systems

Real-time scheduling algorithmsFixed-priority algorithm (RM)Dynamic-priority algorithm (EDF)

*Real-Time WorkloadJob (unit of work)a computation, a file read, a message transmission, etcAttributesResources required to make progressTiming parametersReleased Absolute deadlineExecution time

*Real-Time TaskTask : a sequence of similar jobsPeriodic task (p,e)Its jobs repeat regularlyPeriod p = inter-release time (0 < p)Execution time e = maximum execution time (0 < e < p)Utilization U = e/p510150

*SchedulabilityProperty indicating whether a real-time system (a set of real-time tasks) can meet their deadlines(4,1)(5,2)(7,2)

*Real-Time SchedulingDetermines the order of real-time task executions Static-priority schedulingDynamic-priority scheduling(4,1)(5,2)(7,2)

*RM (Rate Monotonic)Optimal static-priority schedulingIt assigns priority according to periodA task with a shorter period has a higher priorityExecutes a job with the shortest period

(4,1)(5,2)(7,2)T1T2T3

*RM (Rate Monotonic)Executes a job with the shortest period(4,1)(5,2)(7,2)T1T2T3

*RM (Rate Monotonic)Executes a job with the shortest period(4,1)(5,2)(7,2)Deadline Miss !T1T2T3

*Response TimeResponse time Duration from released time to finish time(4,1)(5,2)(10,2)T1T2T3

*Response TimeResponse time Duration from released time to finish time

(4,1)(5,2)(10,2)Response TimeT1T2T3

*Response TimeResponse Time (ri) [Audsley et al., 1993]

HP(Ti) : a set of higher-priority tasks than Ti

(4,1)(5,2)(10,2)551010T1T2T3

*RM - Schedulability AnalysisReal-time system is schedulable under RM if and only if ri pi for all task Ti(pi,ei)

*RM Utilization BoundReal-time system is schedulable under RM if Ui n (21/n-1)

Liu & Layland, Scheduling algorithms for multi-programming in a hard-real-time environment, Journal of ACM, 1973.

*RM Utilization BoundReal-time system is schedulable under RM if Ui n (21/n-1)

Example: T1(4,1), T2(5,1), T3(10,1),

Ui = 1/4 + 1/5 + 1/10 = 0.55 3 (21/3-1) 0.78

Thus, {T1, T2, T3} is schedulable under RM.

*RM Utilization BoundReal-time system is schedulable under RM if Ui n (21/n-1)

Chart1

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The Number of Tasks

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RM Utilization Bounds

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The Number of Tasks

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*EDF (Earliest Deadline First)Optimal dynamic priority schedulingA task with a shorter deadline has a higher priorityExecutes a job with the earliest deadline(4,1)(5,2)(7,2)T1T2T3

*EDF (Earliest Deadline First)Executes a job with the earliest deadline(4,1)(5,2)(7,2)T1T2T3

*EDF (Earliest Deadline First)Executes a job with the earliest deadline(4,1)(5,2)(7,2)T1T2T3

*EDF (Earliest Deadline First)Executes a job with the earliest deadline(4,1)(5,2)(7,2)T1T2T3

*EDF (Earliest Deadline First)Optimal scheduling algorithm if there is a schedule for a set of real-time tasks, EDF can schedule it.(4,1)(5,2)(7,2)T1T2T3

*Processor Demand BoundDemand Bound Function : dbf(t)the maximum processor demand by workload over any interval of length t

(4,1)(5,2)(7,2)tT1T2T3

*EDF Utilization BoundReal-time system is schedulable under EDF if and only if Ui 1

Liu & Layland, Scheduling algorithms for multi-programming in a hard-real-time environment, Journal of ACM, 1973.

*RM vs. EDFRate MonotonicSimpler implementation, even in systems without explicit support for timing constraints (periods, deadlines)Predictability for the highest priority tasks

EDFFull processor utilizationMisbehavior during overload conditions

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