Scheduling for Embedded Real-Time Systems

1. Scheduling for Embedded Real-Time Systems Amit Mahajan and Haibo

2. Overview • Real-Time Systems • Scheduling problem in Real-Time Systems • Control Dominated Systems • Dataflow Systems • Conclusions

3. Real-Time Systems • Are composed of several distinct cooperating tasks • Are usually non-terminating • React faster than the rate of the environment i.e. the composing tasks have tight timing constraints

4. Scheduling problem in Real-Time Systems Given • A set processing units with “known” processing capabilities • A set of tasks with “known” characteristics and constraints on the completion time Derive an order of execution of these tasks

5. Overview • Real-Time Systems • Scheduling problem in Real-Time Systems • Control Dominated Systems • Dataflow Systems • Conclusions

6. Static Scheduling Policies • Tasks execute in a fixed order determined offline • Suited best to the cases where the time when the tasks become enabled are known well in advance • Easy to debug but usually give low processor usage

7. Static Scheduling Policies (contd.) • Round Robin • Tasks checked for readiness in a predetermined order and executed immediately if they are ready • Easy to give bounds on the time between a request for an execution and the corresponding execution • Cyclic Scheduling • Like round robin except that tasks may appear more than once in one cycle • Used in dataflow systems

8. Dynamic Scheduling Policies • Order of task execution decided at runtime • Usually based on some sort of priority of tasks • May or may not be pre-emptive • Suited for systems with irregular task activations • High processor usage but difficult to debug

9. Dynamic Scheduling Policies (contd.) • Work by Liu and Layland. Under strict assumptions • RMS • Static priorities • Priority of a task is proportional to its rate • EDF • Dynamic priorities • Priority of a task is inversely proportional to the time remaining to its deadline • Under a set of relaxed conditions, Audsley et al. proposed an optimal priority assignment algorithm for static priority scheduling

10. Schedulability Analysis • To determine if a set of tasks meets its timing constraints • Common approach – worst case response time (WCRT) analysis • Once WCRT is computed, checking whether a task meets its deadline is trivial in many cases • With Liu and Layland assumptions of preemptive static priority scheduling and independent periodic tasks with fixed runtimes, WCRT analysis is easy

11. WCRT analysis for realistic real-time embedded systems • Real-time embedded systems frequently flout the Liu layland assumptions • state- dependent run-times, dependency between tasks and non-periodic events in the environment • Balarin and Sangiovanni proposed a conservative extension to WCRT analysis for systems in which • Scheduling is either preemptive or nonpreemptive static priority • The environment only needs to respect the timing between successive occurrences of events FOR MOST SYSTEMS WITH STATE AND INPUT DEPENDENT RUNTIMES, TRACTABLE WCRT ANALYSIS IS A MYSTERY!

12. Synchronous Scheduling • E.g Esterel • Synchronous assumption • Given a set of communicating tasks, each represented by a FSM, the computation of the output as well as the communication among the FSMs takes place in zero time • The synchronicity hypothesis reduces the set of FSMs to a single product machine • For synchronous systems, the structure of the product machine is equivalent to the static scheduling of the component FSMs

13. Synchronous Scheduling (contd.) • Advantages • Deterministic response • Ease of debugging and verification • Problems • May lead to inefficient execution time as compared to task-based scheduling • Some tasks in a system may not be representable as FSM • Undelayed feedback loops are a cause of great trouble

14. Overview • Real-Time Systems • Scheduling problem in Real-Time Systems • Control Dominated Systems • Dataflow Systems • Conclusions

15. Dataflow Systems • Dataflow graphs used for scheduling purposes in DSP systems • Explicit representation of interaction between tasks • More emphasis on the throughput of the system • Two types of dataflow models • SDF • static compile time scheduling possible • DDF or control DF • more powerful than SDF but require some dynamic scheduling Most DSP algorithms don’t require much decision making – SDF is a good enough model

16. SDF • Used in many block-diagram based rapid prototyping and code generation for programmable DSPs • For uniprocessor scheduling • Code generator steps through the schedule and generates a sequence of actor invocations • In-line code used instead of sub-routine calls • Loops used to prevent explosion in code size due to multiple appearances of an actor in a schedule

17. Research Issues in SDF scheduling • Minimize code and buffer size used for actors. Approaches are • Minimizing for one of the criterions. Among the existing solutions, choosing the solution that minimizes the second criterion • Exploring solutions that optimize based on a tradeoff between the two constraints i.e. avoiding the extremes • Minimizing activation schedules

18. Conclusions • Dataflow systems • Regularity of inputs and control lends itself well to static scheduling • Good techniques exist scheduling on one or several processors with tradeoff between code-size, buffer size, execution time etc • Control-dominated systems • Efficient scheduling schemes known for a very small class of systems • Lack of a good model to capture inter-task dependencies • Lack of good techniques to determine the WCRT on complex processors without making unrealistic assumptions