Discussion: Scheduling

1
Discussion Scheduling

• Haibo Zeng
• Amit Mahajan

2
Outline

• Problem multi-program scheduling on a single
  processor
• Optimum fixed priority scheduler rate
  monotonic scheduling
• Optimum dynamic scheduling algorithm deadline
  driven scheduling
• Mixed scheduling algorithm

3
Introduction

• Environment
• hard-real-time vs. soft-real-time
• Scheduling
• Preemptive and priority driven
• Fixed priority vs. dynamic

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Assumption of environment

• A1 The requests for all tasks with hard
  deadlines are periodic
• A2 Task deadline is its next request
• A3 The tasks are independent
• A4 Run-time for each task is constant
• A5 Nonperiodic tasks have no deadlines

5
Outline

• Problem multi-program scheduling on a single
  processor
• Optimum fixed priority scheduler rate
  monotonic scheduling
• Optimum dynamic scheduling algorithm deadline
  driven scheduling
• Mixed scheduling algorithm

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Rate Monotonic Scheduling

• According to their request rates only
• Higher request rates, higher priorities
• Optimum fixed priority scheduling feasible if
  any feasible fixed priority assignment exists
• Proof
• Use the concept critical instant to analyze the
  case of scheduling two tasks

7
RMS (contd)

• Proof
• Use the concept critical instant to analyze the
  case of scheduling two tasks
• Result assign higher priorities to task
  with shorter request period independent of
  their run-times.
• Generalize this result to m tasks

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RMS (contd)

• Available Processor Utilization can be as low as
• Analysis
• Right hand side of the inequality is monotonic
  decreasing with m

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RMS (contd)

• Question
• Why utilization factor cant reach 100?
• Answer
• Processor idle time example
• Question
• How to relax the utilization bound?
• Answer
• For i1,2,,m-1,
• Better choice dynamic priority assignment

10
Outline

• Problem multi-program scheduling on a single
  processor
• Optimum fixed priority scheduler rate
  monotonic scheduling
• Optimum dynamic scheduling algorithm deadline
  driven scheduling
• Mixed scheduling algorithm

11
Deadline Driven Scheduling

• According to their request rates earliest
  deadline first (EDF)
• No processor idle time before overflow
• Schedulable iff processor use is less than 1
• Optimum scheduling algorithm feasible if any
  feasible assignment exists

12
Outline

• Problem multi-program scheduling on a single
  processor
• Optimum fixed priority scheduler rate
  monotonic scheduling
• Optimum dynamic scheduling algorithm deadline
  driven scheduling
• Mixed scheduling algorithm

13
Mixed scheduling algorithm

• Nice for many applications
• Interrupt hardware fixed priority scheduler
• Other software tasks dynamic priority scheduler
• Scheduling algorithm
• K tasks of shortest periods RMS
• Remaining slower paced tasks EDF

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Mixed scheduling algorithm (contd)

• Comparison with RMS and EDF
• Still cant reach 100 utilization
• But much better than RMS
• Example with 3 tasks
• T13, T24, T35
• C11, C21, C31(rate-monotonic), 2(mixed)
• RMS U 1/3 1/4 1/5 78.3
• Mixed scheduling algorithm
  U 1/3 1/4 2/5 98.3

15
Questions

• Overhead we ignored?
• Dynamic scheduling
• Preemption
• If programs are nonterminating, how about the
  resources?
• Are the assumptions about environment always
  fine?
• If A1 or A4 dont hold, what do we do?
• Is A3 suitable for embedded systems?