Real Time Systems

1
Real Time Systems

• Investigate and present the functionality of Real
  Time Java as provided by the more popular
  compilers and its applicability to Real Time
  System Programming.

2
Requirements

• The real-time Java expert group created
  principles to limit the changes needed to the
  Java VM.
• These are
• The specifications should not include any
  requirements that restrict its use to particular
  java environments.
• The RTSJ must not prevent existing, properly
  written non real-time applications executing on
  the real-time machine.

3
Requirements 2

• The Write once, run anywhere philosophy should
  be pursued as much as possible
• The specification should include the current
  features but allow for the addition of new
  features in the future.
• The most important feature should be the
  predictability of the execution.
• The RTSJ must not include new keywords or modify
  the compiler.
• The RTSJ should not mandate algorithms but,
  rather require that implementers meet the
  specifications.

4
Standard Java issues

• The Java Garbage collector causes a few problems,
  these are
• Dynamic allocation of memory
• Fragmentation of the memory
• Scheduling

5
Standard Java issues

• Problem with inverse priority low level threads
  prevent higher level ones running.
• The java virtual machine does not implement a
  mechanism for determining the execution of tasks

6
Available Technology

• KVM and J2ME
• PERC
• Chocolate
• The Jamaica Virtual Machine

7
KVM and J2ME

• Kilio Virtual machine (KVM)
• Possible to run java programs on any 16-bit
  processor with 128k memory.
• Does not take real-time specifications into
  account but does improve performance.
• Java 2 Micro Edition(J2ME)
• Designed for small embedded devices
• Does not fully comply with the RTSJ
• With KVM offers a strong embedded development
  platform .

8
PERC

• Real-time java virtual machine
• Lacks some libraries
• Improved features
• Garbage collector
• Dispatcher management
• Memory Management

9
Chocolate A Reservation-Based Real-Time Java
Environment

• Limited to Windows NT
• Many new features such as
• Memory Management
• Synchronization and Resource-Sharing
• Thread Termination
• Physical Memory Access

10
The Jamaica Virtual Machine

• The Jamaica VM is the only Java implementation
  with an efficient hard real-time garbage
  collector.
• Provides the user with hard real-time
• Compatible with standard Java syntax
• Fast execution
• High priority threads can override lower ones

11
Features of the RTSJ

• Improved scheduling by introduction of two new
  thread types.

• Improved synchronisation introduction of the
  priority inheritance protocol prevents priority
  inversion.

12
Features of the RTSJ

• Resource Management
• Physical Memory Access
• RawMemoryAccess class
• PhysicalMemoryAccess class
• Dynamic Memory Management
• Physical memory, used for non-volatile RAM
• Immortal memory, as the name suggests!
• Scoped memory, destroyed when out of scope
  without GC involvement.

13
JTime

• A JVM from TimeSys.
• Complies to the RTSJ
• provides everything that you expect from a Java
  runtime environment, plus predictability
• Underwent testing by Boeing with excellent
  results.
• RT Java for Distributed Real-Time Embedded (DRE)
  Systems?
• An avionics system for example.

14
JTime Threads

• An experiment of thread throughput for each
  thread type
• RealTimeThread
• NoHeapRealTimeThread
• Standard Java thread

Aim was to record the time taken to perform a
computationally intensive algorithm, similar to
what is found within an avionics system.
15
Priority Inheritance

• JTime adds a strict, real-time priority scheduler
  with greater precision and hundreds of
  precisely-specified pre-emptive priorities.
• An experiment to record the time taken to boost
  and un-boost priorities within JTime.

Uses a three thread case of priority
inheritance LPT, MPT HPT.
16
Context Switching

• An experiment to record the time taken to
  context switch using threads within JTime.
• A two thread case was used by executing a HPT
  with a contending LPT their run() methods
  simply called yield().

17
Summary

• RTSJ implementations are capable of providing
  sufficient real-time characteristics.
• Massive support from the real-time community
  shows promise.
• More testing needed for its use within DRE
  systems.