lec05_06_common - Overview of Commonly used Approaches to...

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1 Overview of Commonly used Approaches to Real-Time Scheduling - Chapter 4 - Overview of Chapter 4 topics • The nature of the “game” • Overview of common approaches – Clock-Driven Approach – Weighted Round-Robin Approach – Priority-Driven Approach • EDF, LST, RM • How to prove a scheduling is optimal • Optimality in limited sense
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2 How to schedule? • Four control-law tasks – Task 1 (left arm control): p1=30ms – Task 2 (right arm control): p2=30ms – Task 3 (left leg control): p3=50ms – Task 4 (right leg control): p4=50ms time Task1 time Task2 time Task3 time Task4 The Clock Driven Approach • It is also known as cyclical executive or timer driven approach. • Scheduling decisions are made at specific time instants. These instants are chosen a priori before the system begins its execution. • The timer kicks off the execution of a segment of code from a table It is still in use and useful for – small and simple embedded systems – safety critical flight control systems (slowly phased out by static priority scheduling) T 1,1 T 1,2 T 2 T 3 T 4 T 5 T 6
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3 Weighted Round Robin (WRR) Approach It is the old time token passing approach with a twist. When the token pass to you, you get a slice of CPU. If you have nothing to do or you finish early, the token is given to the next guy right away – scheduling message transmissions in ultrahigh-speed networks • A time slice is typically in the order of tens of milliseconds. • WRR let some tasks to hold the token longer (larger slice) than others • Quiz: How does WRR or RR differ with Clock driven approach? Ready queue (FIFO) CPU Weighted Round Robin (WRR) Approach Ready queue (FIFO) Link J1, 6 J2 ,2 J3 ,4 2 1 1 J1 time 0 5 10 15 20 J2 J3 J1 J2 J3 J1 J3 J3 round1 round2 round3 round4 CPU 2 1 1
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4 Priority Driven Approach • This approach assigns priority to tasks. The highest
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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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lec05_06_common - Overview of Commonly used Approaches to...

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