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Real-Time Operating System Design. Alvin AuYoung ee290O 4/19/01. Real-Time OS Design - 1. Overview. Introduction - Motivation - Goals Design Issues Methodologies - Priority-based Kernel - RT extensions of timesharing systems - Research operating systems Case study: Java-OS

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Real time operating system design

Real-Time Operating System Design

Alvin AuYoung

ee290O

4/19/01

Real-Time OS Design - 1


Overview
Overview

Introduction

- Motivation

- Goals

Design Issues

Methodologies

- Priority-based Kernel

- RT extensions of timesharing systems

- Research operating systems

Case study: Java-OS

Conclusions

Real-Time OS Design - 2


Motivation
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

Real-Time OS Design - 3


Goals
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.

Real-Time OS Design - 4


Design issues
Design Issues

How can I guarantee predictability and schedulability with the different components of my system?

Architecture, Memory Management, Task structure, Kernel, etc...

Real-Time OS Design - 5


Methodologies
Methodologies

Priority-based Kernels

VxWorks, OS9, pSOS

RT extensions of timesharing systems

RT-Mach, RT-Unix

Research operating systems

Spring, HARTIK

Real-Time OS Design - 6


Priority based kernels
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

Real-Time OS Design - 7


Priority based kernels1
Priority-based kernels

Driven by a real-time clock

Task Scheduling with fixed priorities

Synchronization tools, limitations

Communication protocols

Problem?

Real-Time OS Design - 8


Real time extensions of timesharing os
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

Real-Time OS Design - 9


Real time extensions of timesharing os1
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?

Real-Time OS Design - 10


Research operating systems
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

Real-Time OS Design - 11


Research operating systems1
Research Operating Systems

Case Study: Spring Kernel

Real-Time OS Design - 12


A java solution jmd
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)

Real-Time OS Design - 13


A java solution task model
A Java Solution: Task Model

Task Methods and Behaviors:

parameterized by:

- period of execution

- deadline

- WCET

- Priority Class (1 - 10)

Variable Scheduling Algorithm

Real-Time OS Design - 14


A java solution conclusions
A Java Solution: Conclusions

Not yet RTSJ compliant

Flexible Kernel provides abstraction and possibility for future development.

Maybe in the future?

Real-Time OS Design - 15