best effort scheduling algorithms

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Scheduling Basic Concepts Ref: Hard Real-Time Computing Systems Giorgio Buttazzo Ref: Real-Time Systems & Software Alan Shaw Processes - Tasks - Threads. - PowerPoint PPT Presentation

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  • Scheduling

    Basic Concepts

    Ref: Hard Real-Time Computing SystemsGiorgio Buttazzo

    Ref: Real-Time Systems & Software Alan Shaw

    Processes - Tasks - Threads

  • Real-Time Systems

    Periodic and Aperiodic

    Hard & Soft Deadlines

    Scheduling of Tasks Static & Dynamic

    Sharing of Resources and Critical Sections

    Domino Effect

  • Best Effort Scheduling Algorithms

  • Example Cost FunctionsWhat is a good system?

  • Metrics for Performance EvaluationWhat is a good system?

  • Attempting to Minimize Lateness

  • Scheduling Anomalies

  • Precedence Example

  • Adding a processor

  • Reducing Computation Times ?

  • Weaken Constraints

  • Resource Constraints

  • Successive events are separated by at least pThe Sporadic processShaws notation:

  • Cyclic Executives for Scheduling Periodic Tasks: Broken into blocks: Major Cycles (MCT) shortest period where all task periods align all tasks execute in a major cycle Minor Cycles (mct) frames used to allocate tasks or portions of tasks

    Example: Say you have two tasks: T1 = (c1, p1, d1) = (1, 2, 2) T2 = (c2, p2, d2) = (1, 3, 3) |xxxxxx______|xxxxxx______|xxxxxx______|.. |______xxxxxx______|xxxxxx____________| 0 1 2 3 4 5 6 MCT = 6 mct = 2 (or 1) Allocations are: {T1,T2} {T1,T2} {T1}, repeat

  • Can this be scheduled?What is MCT?What is mct?What is a schedule?

  • Ada-like implementation of this scheduling.Clock returns the current timeDelay_until x causes the task to block until at least time x

  • Some principles about periodic scheduling: We assume all processes are released at time t0. Why? The MCT is the least common multiple (lcm) of the periods. Why? The mct must divide the MCT. Why? The mct must be >= the longest computation block. Why? mct
  • Home work #1: (Lecture 4/03)

    From Buttazzo:

    From Shaw: Exercises 2.2: 1, 2, 3, 5, 6

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