Real-time Java Threads

Insik Shin

Real-time Systems GroupUniversity 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_INFOlong priority current priority THREAD VMthread pointer to internal THREADINSTANCE target pointer to the object whose ‘run’

THREAD in KVM THREAD

THREAD nextAliveThread active thread queueTHREAD nextThread all thread queueJAVATHREAD javathread java-level thread informatinSTACK stack execution stackBYTE* ipStore program counterFRAME fpStore frame pointerCell* spStore execution stack pointer

MONITOR monitor thread monitor queueTHREAD nextAlarmThread thread timer queueLong wakeupTime[2] wakeup timeVoid (*wakeupCall)(THREAD) callback when timer expires state thread state

THREAD & JAVATHREAD in KVM THREAD & JAVATHREAD

JAVATHREAD javathread

THREAD 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 RegistersCurrentThread

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 & KVMThread.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

JAVATHREADSchedulingParameter spReleaseParameter rpMemoryParameter mpProcessingGroupParameter pgpScheduler sch

COMMON_OBJECT_INFOlong priorityTHREAD VMthread INSTANCE target

THREAD in KVM

THREAD nextAliveThread active thread queueTHREAD nextThread all thread queueJAVATHREAD javathread java-level thread informatinSTACK stack execution stackBYTE* ipStore program counterFRAME fpStore frame pointerCell* spStore execution stack pointer

MONITOR monitor thread monitor queueTHREAD nextAlarmThread thread timer queueLong wakeupTime[2] wakeup timeVoid (*wakeupCall)(THREAD) callback when timer expires state thread state

THREAD in KVM

THREAD nextAliveThread active thread queueTHREAD nextThread all thread queueJAVATHREAD javathread java-level thread informatinSTACK stack execution stackBYTE* ipStore program counterFRAME fpStore frame pointerCell* spStore execution stack pointer

MONITOR monitor thread monitor queueTHREAD nextAlarmThread thread timer queueLong wakeupTime[2] wakeup timeVoid (*wakeupCall)(THREAD) callback when timer expires state thread state

bool_t realtime

Thread Structures for Real-time Java

realtime = false realtime = 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-robinscheduler

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 CurrentThreadTHREAD RunnableThreads

Java API & KVMThread.java public native void start();

KVM : nativeCore.c

Java_java_lang_Thread_start() {… startThread();…

}

KVM : thread.c

startThread() { …}

Real-time Java API & KVMRealtimeThread.java public native void start();

KVM : nativeCore.c

Java_javax_rtgedition_RealtimeThread_start() {… startThread();…

}

KVM : thread.c

startThread() { …}