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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