1. Real-Time Operating System Design
Alvin AuYoung
ee290O
4/19/01
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2. Overview Introduction - Motivation - Goals Design Issues Methodologies
- Priority-based Kernel
- RT extensions of timesharing systems
- Research operating systems
Case study: Java-OS
Conclusions
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3. Motivation
Most current operating systems are not suitable for developing or deploying applications with real-time constraints
i.e. scheduling policies, process synchronization, system architecture
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4. Goals
Create an environment suitable for developing applications with hard real-time constraints on task execution in a reactive environment. Two key ideas:
Predictability: Predict direct consequences of any scheduling decision.
Schedulability Guarantees: Verify the schedulability of a given set of tasks.
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5. Design Issues
How can I guarantee predictability and schedulability with the different components of my system?
Architecture, Memory Management, Task structure, Kernel, etc...
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6. Methodologies Priority-based Kernels VxWorks, OS9, pSOS
RT extensions of timesharing systems
RT-Mach, RT-Unix
Research operating systems
Spring, HARTIK
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7. Priority-based kernels
Objective:
- High performance for average response time to external events.
Characteristics:
- fast context switching
- small footprint
- efficient interrupt handling
- preemptable primitives
- fast communication mechanisms
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8. Priority-based kernels
Driven by a real-time clock
Task Scheduling with fixed priorities
Synchronization tools, limitations
Communication protocols
Problem?
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9. Real-Time extensions of timesharing OS
Objective:
Extend current (commerical) time-sharing systems to satisfy real-time constraints.
Characteristics:
- Reuse standard peripherals and interfaces
- Speedier development
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10. Real-Time extensions of timesharing OS
- Reused Kernel
- Task Scheduling with fixed priorities
Tasks vs Threads? (N. Wirth)
- Re-implemented User-Level Threads
- Nonpreemptable syscalls, interrupts
problems?
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11. Research Operating Systems
Objective: The ability to treat tasks with explicit timing constraints, such as periods and deadlines
Characteristics:
- Scheduling guarantee mechanisms
- Characterize tasks with additional parameters
- Avoidance of nondeterministic blocking time
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12. Research Operating Systems
Case Study: Spring Kernel
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13. A Java Solution: JMD
A Java Kernel for Embedded Systems in Distributed Process Control
Java Microcontrolled Device (JMD)
- JMD applications (high level)
- JMD-OS API
- JMD Kernel
- MicroKernel (low level)
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14. A Java Solution: Task Model
Task Methods and Behaviors:
parameterized by:
- period of execution
- deadline
- WCET
- Priority Class (1 - 10)
Variable Scheduling Algorithm
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15. A Java Solution: Conclusions
Not yet RTSJ compliant
Flexible Kernel provides abstraction and possibility for future development.
Maybe in the future?
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