1. Real-time Linux Evaluation
Kalynnda Berens, GRC

2. Definitions
Embedded System - A combination of hardware and software which together form a component of a larger machine. An embedded system is designed to run on its own without human intervention, and may be required to respond to events in real time.
Real-time OS - an operating system that provides a required level of service in a bounded response time
Hard real-time – absolute deadlines that must be met
Soft real-time – time tolerance within which an event can occur

3. What is Linux? Unix “clone”, first created in 1992 by Linus Torvalds
Open source operating system
Commonly used in server environment, now used in desktops and embedded systems
Not useful for real-time systems because:
Fairness scheduling, with limited priority scheduling
Unpredictable, sometimes high latency
Non-deterministic behavior
Coarse timing resolution

4. Real-Time Linux
Modifications or extensions to standard Linux to allow soft or hard real-time applications to meet timing constraints.
One approach uses a separate real-time “kernel” that runs Linux as the lowest priority task.
Another approach modifies standard Linux for preemptibility, latency, timing, and/or scheduling.
Both open source and commercial versions are available.

5. Real-time Linux Projects
Flight Linux-a customized copy of a standard Linux distribution, adapted to the unique environment of a spacecraft embedded control computer.
STS-83 experiment used Debian GNU/Linux.
RACSI - the remote Automatic Transfer Vehicle (ATV) control at ISS.
GOAS - Ground Operator Assistant System for the rendezvous operations of ATV.
PLATINO numerical control
iArte - Industrial Automation Real Time Environment
FAA Level C certified flight simulator
Cascade – a RTLinux based GPS receiver
Space shuttle experiment, STS-83 experiment, studied the growth of plants in microgravity and used Debian GNU/Linux.
RACSI is a laptop computer running a software package operated by an astronaut onboard the station. RACSI double monitors and checks the ATV mission and provides two simple command capabilities: temporarily interrupt the mission or command a collision-avoidance maneuver.
GOAS, the Ground Operator Assistant System for the rendezvous operations of ATV. Used on ground, GOAS is a software tool to monitor the ATV mission and intervene in case of a problem. GOAS provides complex command and control capabilities to replan the entire mission if necessary.
PLATINO numerical control based on standard PC with LINUX O.S. and RTAI real time extension
iArte
(Industrial Automation Real Time Environment)
FAA Level C certified flight simulator
Cascade is a RTLinux (a realtime augmentation to Linux) based GPS receiver

6. Independent Kernel Variants
Linux Process
Linux Process
User Space
Kernel Space
Real-Time Task
Real-Time Task
Standard Linux Kernel
Real-Time Micro Kernel
Hardware

7. Kernel Modification Variants
Linux Process
Linux Process
Real-time
Process
User Space
Kernel Space
Modified Linux Kernel
Preemptible, Micro Timing, and/or Low Latency
Loadable Module
Loadable Module
Hardware

8. Real-time Linux Variants
RTAI (Real-time Application Interface)
Kernel, User space hard real-time (LXRT)
Open source
RT-Linux
Kernel
Commercial, open source
OnCore OS
Separate RTOS
Commercial
KURT (Kansas University Real-time)
Modified
HardHat

9. Testing Real-Time Linux
Requirements derived from
VxWorks 5.4 Test plan, MSFC
VxWorks AE Test plan, GRC
RTOS properties
Specific test scenarios will be derived from the requirements, plus…
Variant-specific attributes
Known errors, defects, etc.
Results of static analysis (open source)

10. Test Plan VxWorks 5.4 Linux RTOS Test Plan Variant Specific VxWorks
Test Plans
Variant
Specific
VxWorks AE
Errors
RTOS
Testing
Static
Analysis

11. Tools Linux Trace Toolkit Dynamic Probes kGDB (kernel debugger)
Linux Kernel Crash Dump
Linux Test Project
LMBench
Ballista
strace
System Call Tracker

12. Requirements Scheduling Hardware Interface
Communication and Synchronization
Memory Management
Task Management
Timing
Configuration
External Communications
Error Reporting
Embedded Features
File system
Reliability

13. Error Catalog

14. Test Setup Development System (Mandrake 8.1 Linux) Ethernet Connection
Embedded
PowerPC
Embedded
Pentium PC/104

15. Future Work Static analysis of open source variants
Completion of variant-specific test plans
Test scenario creation
Test procedures for each scenario
Testing on embedded hardware
Test results and report