Real-time Java Threads Insik Shin Real-time Systems Group University of Pennsylvania

KVM Threading System  What kind of information for a thread? data structures  How to maintain such information?  How to support multi-threading? scheduling policy context switching mechanism  Additional features for supporting threads? timer, monitor

Thread Information  pointer to ‘run’ method  priority  virtual machine execution stack  virtual machine registers (ip, fp, sp)  timer & monitor  state (active | wait)  thread queues: active, wait, timer, monitor

Thread Information in KVM  pointer to ‘run’ method  priority  virtual machine execution stack  virtual machine registers (ip, fp, sp)  timer & monitor  state (active | wait)  thread queues: active, wait, timer, monitor Java-level thread information implementation-level thread information

Data Structures in KVM  Two separate structures THREAD  internal (VM-level) structure  information for implementing threading system JAVATHREAD  information defined in Java class ‘Thread.java’ Advantages  JAVATHREAD needs to be subclassed  Implementation independent of the Java language for portability & extension reasons

JAVATHREAD in KVM  JAVATHREAD COMMON_OBJECT_INFO long priority current priority THREAD VMthread pointer to internal THREAD INSTANCE target pointer to the object whose ‘run’

THREAD in KVM  THREAD THREAD nextAliveThread active thread queue THREAD nextThread all thread queue JAVATHREAD javathread java-level thread informatin STACK stack execution stack BYTE* ipStore program counter FRAME fpStore frame pointer Cell* spStore execution stack pointer MONITOR monitor thread monitor queue THREAD nextAlarmThread thread timer queue Long wakeupTime[2] wakeup time Void (*wakeupCall)(THREAD) callback when timer expires state thread state

THREAD & JAVATHREAD in KVM  THREAD & JAVATHREAD JAVATHREAD javathreadTHREAD VMthread

Thread Queues in KVM nextThread nextAliveThread monitor nextAlarmThread

Thread Queues in KVM nextThread nextAliveThread monitor nextAlarmThread CurrentThread RunnableThreads

Thread Context Switching in KVM IP, FP, SP VM Registers CurrentThread 1.store execution environment

Thread Context Switching in KVM IP, FP, SP VM Registers 1.store execution environment CurrentThread 2. Pick a thread from ‘RunnableThreads’ queue

Thread Context Switching in KVM IP, FP, SP VM Registers 1.store execution environment CurrentThread 2. Pick a thread from ‘RunnableThreads’ queue 3. Put the old thread into ‘RunnableThreads’ queue

Thread Context Switching in KVM IP, FP, SP VM Registers 1.store execution environment CurrentThread 2. Pick a thread from ‘RunnableThreads’ queue 4. load execution environment 3. Put the old thread into ‘RunnableThreads’ queue

Thread Scheduling in KVM  Scheduling policy Round-robin with the following quantum: quantum = 500 * priority  Scheduler data structure THREAD CurrentThread THREAD RunnableThreads

Thread operations in KVM  Constructors & deconstructors BuildThread, DismantleThread  Thread activation & deactivation initThreadBehavior, startThread, stopThread, suspendThread, resumeThread, removeFirstRunnableThread  Multitasking operations switchThread storeExecutionEnvironment, loadExecutionEnvironment  Timer, Queue, Monitor operations

Java API & KVM Thread.java public native void start(); KVM : nativeCore.c Java_java_lang_Thread_start() { … startThread(); … } KVM : thread.c startThread() { … }

Real-time Thread Scheduling  What kind of features to be added? Real-time thread information Real-time (priority) scheduler Synchronization (monitor) Timer, Asynchrony  How to extend KVM for real-time threads?

Real-time Thread Information  Scheduling parameters priority, importance  Release parameters cost, deadline, overrunHandler, missHandler start, period, minInterarrival  Memory parameters maxMemoryArea, maxImmortal  Processing group parameters  Scheduler

Data Structures in KVM  Two separate structures THREAD  internal (VM-level) structure  information for implementing threading system JAVATHREAD  information defined in Java class ‘Thread.java’ Advantages  JAVATHREAD needs to be subclassed  Implementation independent of the Java language for portability & extension reasons

Real-time Thread Structure  Two separate structures THREAD  internal (VM-level) structure  information for implementing threading system JAVATHREAD  information defined in Java class ‘Thread.java’ RT_JAVATHREAD  information defined in Java class ‘RealtimeThread.java’

Real-time Thread Structure  RT_JAVATHREAD extends JAVATHREAD JAVATHREAD SchedulingParameter sp ReleaseParameter rp MemoryParameter mp ProcessingGroupParameter pgp Scheduler sch COMMON_OBJECT_INFO long priority THREAD VMthread INSTANCE target

THREAD in KVM THREAD nextAliveThread active thread queue THREAD nextThread all thread queue JAVATHREAD javathread java-level thread informatin STACK stack execution stack BYTE* ipStore program counter FRAME fpStore frame pointer Cell* spStore execution stack pointer MONITOR monitor thread monitor queue THREAD nextAlarmThread thread timer queue Long wakeupTime[2] wakeup time Void (*wakeupCall)(THREAD) callback when timer expires state thread state

THREAD in KVM THREAD nextAliveThread active thread queue THREAD nextThread all thread queue JAVATHREAD javathread java-level thread informatin STACK stack execution stack BYTE* ipStore program counter FRAME fpStore frame pointer Cell* spStore execution stack pointer MONITOR monitor thread monitor queue THREAD nextAlarmThread thread timer queue Long wakeupTime[2] wakeup time Void (*wakeupCall)(THREAD) callback when timer expires state thread state bool_t realtime

Thread Structures for Real-time Java realtime = falserealtime = true non-realtime thread realtime thread

Real-time Thread Supports  We can use the same mechanisms as in KVM for the followings: thread queue managements context switching

Real-time Thread Scheduler  RTSJ suggests PriorityScheduler for RTJ  Each RT thread can be associated with any scheduler multiple schedulers at the same time Two-level scheduling

Two-level Scheduling Meta-level scheduler Priority scheduler Round-robin scheduler EDF scheduler

Thread Scheduling in KVM  Scheduling policy Round-robin with the following quantum: quantum = 500 * priority  Scheduler data structure THREAD CurrentThread THREAD RunnableThreads

Real-time Thread Scheduling  Scheduling policy Meta-level scheduling policy (priority)  Scheduler data structure SCHEDULER THREAD CurrentThread THREAD RunnableThreads

Java API & KVM Thread.java public native void start(); KVM : nativeCore.c Java_java_lang_Thread_start() { … startThread(); … } KVM : thread.c startThread() { … }

Real-time Java API & KVM RealtimeThread.java public native void start(); KVM : nativeCore.c Java_javax_rtgedition_RealtimeThread_start() { … startThread(); … } KVM : thread.c startThread() { … }