Sriram Ananthanarayanan

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Issues in Real-Time Java DesignCOSC
6384Instructor Dr. Albert Cheng

• Sriram Ananthanarayanan
• April 14, 2005

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Outline

• Where Java falls short
• RTSJ
• Implementations
• RTJPerf
• Problems with RTSJ
• Garbage Collection
• Goals
• References

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Where Java falls short

• Traditional Java not appropriate for development
  of real-time software
• Garbage Collection
• In real-time systems it is important that memory
  is available always
• Garbage Collection runs as a low priority
  background process
• Can impose long delays
• JRE does not allow applications to determine how
  much memory they require and total memory
  available

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Where Java falls short

• Task Scheduling
• No mechanism for scheduling tasks
• No way to determine other tasks running on the
  system and their priorities
• Lack of Support for Predictability
• WCET Analysis
• Scheduling Analysis
• Periodic processes
• Priority Inversion

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RTSJ

• Requirements
• Backward Compatibility with non real-time Java
  programs
• Support the principle Write Once Run Anywhere
  but not at the expense of predictability
• Requires no syntactic extensions to Java Language
• Scheduling
• No guarantee that highest priority runnable
  thread will always execute
• Defines only 10 priority levels
• Weak definition of scheduling
• Lacks predictability

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RTSJ

• Scheduling
• Allow new schedulers other than Timer class
• Provide a Priority Scheduler
• Provision for adding more schedulers
• Memory Management
• Garbage Collection
• Elminate GCs effect on real-time threads
• Different types of memories
• Scoped Memory
• Physical Memory
• Immortal Memory
• Heap Memory

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RTSJ

• Synchronization
• To maintian proper behavior based on execution
  rules
• Thread executes in the order of execution
  eligibilty
• FIFO when the eligibilities are equal
• Real-time features not in Java
• Asynchronous Event Handling
• Asynchronous Transfer of Control
• Asynchronous Thread Termination
• Physical Memory Access

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Implementations

• Several Implementations
• Timesys RTSJ RI
• Official Implementation of RTSJ Reference
  Implementation
• CVM
• J2ME platform targetted for embedded
  systems
• jRate
• Open-Source RTSJ based extension of the
  GNU Compiler for Java

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Evaluating RTSJ Implementations

• RTJPerf
• An open-source benchmarking suite developed at
• UC Irvine
• Features evaluated
• Memory
• Asynchrony
• Threads
• Timer

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Evaluating RTSJ Implementations

• Tests that can be performed
• Allocation Time Test
• Asynchronous Event Handler Dispatch Delay Test
• Asynchronous Event Handler Priority Inervsion
  Test
• Context Switch Test
• Periodic Thread Test
• One Short Timer Test

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Problems with RTSJ

• Semantics not well defined
• Adds complexity to the VM
• Adds complexity to the programming model
• Alternative Approaches
• Ravenscar Java
• JNI
• Real-Time Core
• JOP (Java Optimized Processor)

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Garbage Collection

• Conservative Garbage Collection Algorithms
• Reference Counting
• Mark-Sweep
• Mark-Compact
• Copying Algorithms
• Different requirements for Real-Time Systems
• GC needs to keep deadlines due to the
  predictability requirement
• Guarantee that it never exceeds its free memory
• Handle Memory Fragmentation
• Not all GC algorithms are schedulable

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Approach I

• Revise the Memory Model
• Define new Memory Areas
• Scoped Memory
• Physical Memory
• Raw Memory
• Immortal Memory
• Heap Memory
• Assignment Rules between different memory areas
  have to be checked by the implementation
• This approach is followed by RTSJ

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Approach II

• Real-Time Garbage Collection
• Incremental
• Exact
• Non-fragmenting
• Operate as a mid-priority process separating high
  priority hard real time processes from low
  priority soft real time processes
• Hard upper bounds for execution time of any
  operation
• Hard upper bound for pre-emption delay

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Approach II

• Implementations
• IVM (Infinitesimal Virtual Machine)
• Implemented from scratch instead of rewriting an
  existing JVM implementation
• JVM (Jamaica Virtual Machine)
• Metronome

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Goals

• Detailed Analysis of RTSJ including design
  issues, advantages, disadvantages
• Evaluate and compare Timesys RI with latest
  version of JDK using RTJPerf
• Analyze the Real-time Garbage Collection
  algorithms
• Provide some further extension and scope of
  improvements in design of RTSJ

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References

• Evaluating Real-Time Java Feature and Performance
  for Real-time Embedded Systems - Angelo Corsaro,
  Douglas C. Schmidt
• Implications of Real-Time Garbage Collection in
  Java Anders Ive
• Adding Real-Time Capabilities to Java - Kevin
  Nelsen
• Restriction of Java for Embedded Real-Time
  Systems Martin Schoeberl