CS 452/652 Fall 2022 - Lecture 20

Real-Time Scheduling / Theory and Practice

CPU Scheduling

• embedded (special-purpose) system
  • hardware / operating system / application co-design
  • guaranteed response latency - predictability! (too early?)
  • hard vs. firm vs. near vs. soft real-time
  • hard: "no delay"
  • firm: rate delay miss ok
  • near: some delay ok
  • soft: statistical distribution of response times ok, degrading utility
• general-purpose computer system
  • hardware - operating system - application: independent designs
  • throughput and fairness objectives
  • hard real-time almost impossible

Cyclic Executive

• polling loop w/ strict timing
• components: subroutine or coroutine
  • subroutine runs to completion
  • coroutine suspends/resumes (need stack)
• known upper bound for each component
• sleep to fill gaps → predictable timing

  t = current_time();
  if (C1) then A1;
  ...
  sleep(t + target - current_time());
  

• monolith, limited flexibility

Real-Time Task Scheduling

• separate OS kernel and application tasks for flexibility
  • dynamic creation of tasks
  • tasks annotated with real-time requirements
• periodic (arrival, run-time) vs. aperiodic tasks (sporadic)
  • aperiodic much more difficult to analyze
• admission control: set of feasible real-time tasks
• selection mechanism
  • static priority vs. dynamic/deadline-based
  • hybrid: can have background tasks
• mathematical model

Fundamentals

• classic paper: Scheduling Algorithms for Multiprogramming in a Hard-Real-Time Environment (Liu//Layland)
  • periodic tasks; deadline = arrival of next request
  • immediate preemption
  • rate-monotonic (RM) scheduling
    • assign priorities according to arrival rate (not runtime)
    • admission analysis based on total CPU utilization, approximated as < 70%
    • starvation under overload
  • earliest deadline first (EDF) scheduling
    • dynamic priorities → sorting / computational overhead
    • optimal for CPU utilization < 100%
    • no guarantees under overload
• refinement for networking: Generalized Processor Sharing
  • other populer name: Weighted Fair Queueing
  • variation and extension of Liu/Layland's EDF
  • adding work conservation as objective
  • network traffic: transmission rate, burst, non-preemptive packet transmission
  • later work uses traffic characteristics to build O(1) scheduling algorithms
• all of the above are single resource!
• optimal multi-processor scheduling is NP-hard
• also keep in mind effects from resource sharing

Train Control

• rate-monotonic scheduling seems sufficient
  • scheduling analysis vs. latency analysis
• scheduling:
  • service tasks, identical engineers - arrival, runtime?
  • interrupt handling latency minimal
  • assess how many trains can be controlled
  • CPU not bottleneck - track limits number of trains running!
• train control latency:
  • central event query and sensor attribution
  • central train/track operations - buffering?