lec13_aper1_fix - Scheduling of Aperiodic and Sporadic Jobs...

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1 Scheduling of Aperiodic and Sporadic Jobs in Priority-Driven Systems - Chapter 7 – (Fixed-Priority Framework) Types of Aperiodic Requests • The jobs of an aperiodic task have random release times – Soft aperiodic tasks: • random arrivals such as a Poisson distribution: • the execution time can also be random such as exponential distribution • typically it models users’ requests. – Firm aperiodic tasks (Sporadic tasks): there is a minimal separation between 2 consecutive arrivals there is a worst-case execution time bound models emergency requests such as the warning of engine overheat Processor Acceptance Test Sporadic Task Periodic Task Aperiodic Task rejection
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2 Interrupt Handling, Background, Polling 0369 10 0 T1 = (3,1) T1 = (5,2) Deadline miss 12 0.2 5 1.2 10 0 T1 = (3,1) T1 = (5,2) 12 5 10 0 T1 = (3,1) T1 = (5,2) 12 5 S = (2.5,0.5) 2.5 5 7.5 10 12.5 Interrupt Handling Background Polling Interrupt Handling or Background Service • One way to serve aperiodic requests is handle them right at the interrupt handler. – This gives the best response time but can greatly interrupt the hard real-time periodic tasks. – Use it as last resort only such as pending power failure exception handling • Another simple method is to give background class priority to aperiodic requests. This works as well but the response time is not too good. For example: – Priority levels 1 to 246 for periodic tasks – Priority levels 247 to 256 for aperiodic tasks
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3 Polling - 1 The simplest form of integrated aperiodic and periodic service is polling. – For each aperiodic task, we assign a periodic service with budget e s and period p s . This creates a server ( e s , p s ) – The aperiodic requests are buffered into a queue – When polling server starts, • Resumes the existing job if it was suspended in last cycle. • it checks the queue. – The polling server runs until • All the requests are served • Or suspends itself when the budget is exhausted. – Remark: a small improvement is to run the tasks in background priority instead of suspend. This background mode can be applied to all the servers discussed later. Polling - 2 • A polling server is just a periodic task and thus the schedulability of periodic tasks is easy to analyze. For example, if we use L&L bound, • Quiz: How can we analyze the aperiodic performance for each polling server? • Answer: Using M/M/1 for 1 st cut analysis. Server size is e s /p s . Service time is (0.5 p s + average request service time)… () 1 2 ) 1 ( ) 1 /( 1 1 + + + = n s s n i i i n p e p e [] 1 where E[S] is the average service time and is the average utilization of the bandwidth assigned to aperiodic jobs ES w ρ = Average response time In M/M/1 queue
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4 Deferrable Server - 1 • Comparing polling with interrupt handling, interrupt handling serves aperiodic requests right away whereas the
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This note was uploaded on 07/02/2008 for the course COMPUTER S 664 taught by Professor Leechangkyu during the Spring '08 term at Korea University.

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lec13_aper1_fix - Scheduling of Aperiodic and Sporadic Jobs...

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