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Predictable Java. Overview Implementations Next steps. Java Objekt 2, 19 August 2009. Anders P. Ravn and Hans Søndergaard . A Real-Time Application. Periodic Event Handlers Aperiodic Event Handlers collected in a mission Mission Handler Each handler has a Memory is Scheduled.
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Predictable Java • Overview • Implementations • Next steps Java Objekt 2, 19 August 2009 Anders P. Ravn and Hans Søndergaard
A Real-Time Application • Periodic Event Handlers • Aperiodic Event Handlers collected in a mission • Mission Handler Each handler • has a Memory • is Scheduled
Periodic handler class Periodic extends PeriodicEventHandler { protected Periodic(.., PeriodicParameters pp, Scheduler scheduler, MemoryArea memory); public void handleEvent() { // the logic to be executed every period } }
Aperiodic handler class Aperiodic extends AperiodicEventHandler { protected Aperiodic(.., AperiodicParameters ap, Scheduler scheduler, MemoryArea memory); public void handleEvent() { // the logic to be executed when an event occurs } }
A simple mission public class Basic extends Mission { protected Basic(.., AperiodicParameters ap, Scheduler scheduler, MemoryArea memoryArea) { ... // initialization } public static void main (String[] args) { new Basic( null, null, new CyclicScheduler(), new LTMemory(10*1024)); } }
…The mission addToMission( new Periodic( null, pp, getScheduler(), new LTMemory(1024))); addToMission( new Periodic( null, pp, getScheduler(), new LTMemory(1024))); ... add(); // mission to its scheduler
Complex mission private Mission[] mission; private int active = 0; static AperiodicEvent event; public ThreeSequentialMissions(...) { mission = new Mission[3]; // set up the three missions mission[0] = new Mission(...); // add handlers for mission 0 // including the mission termination ... mission[1] = new Mission(); ... // start the first mission mission[active].add(); event = new AperiodicEvent(this); }
Changing mission private Mission[] mission; private int active = 0; static AperiodicEvent event; public ThreeSequentialMissions(...) { ... } public void handleEvent() { mission[active].remove(); active = (active + 1) % mission.length; mission[active].add(); }
Level 0 • only Periodic Event Handlers • cyclic scheduler
Level 1 SO 3 has the highest priority • periodic and aperiodic event handlers • fixed-priority preemptive scheduler
Implementations • On top of RTSJ (adapter) • Native implementation using • JamVM • Xenomai/Linux • Mechatronic Brick
Schedulers • Cyclic scheduler • Only one mission, with periodic handlers • Using cyclic executive model • The Scheduler: a periodic RT_TASK • period = gcd of all the handler periods • Fixed priority pre-emptive scheduler • Missions with periodic and aperiodic handlers • Implemented as RT_TASKs
Memory • No GC • Two types of memory • Immortal memory = Heap (without GC) • Lifetime = lifetime of the application • Scoped memory • Each handler has its own private memory • Lifetime = lifetime of the handler • Object allocation rewritten in JamVM
Synchronization • Java’s locking model (synchronization) • does not fit to avoid priority inversion • Xenomai's MUTEX enforce priority inheritance • Rewritten JamVM • synchronized methods in classes for shared objects: • uses MUTEX lock/unlock
Next steps • Eclipse plugins • Homepage for Predictable Java • Different tools for static analysis • Examples • ...
References • A predictable Java profile - rationale and implementations Thomas Bøgholm, René R. Hansen, Anders P. Ravn, Bent Thomsen, and Hans Søndergaard JTRES’09. 23-25 September 2009. • Safety-Critical Java JSR 302: Safety Critical Java Technology http://jcp.org/en/jsr/detail?id=302 • Java for Safety-CriticalApplications Thomas Henties, James J. Hunt, Doug Locke, Kelvin Nilsen, Martin Schoeberl, Jan Vitek. SafeSert 2009. • Safety Critical Specification for Java Version 0.5. August 2008. Draft.
Periodic event handler execution Periodic Rt_task: for (;;) { JNI-callback-to-scheduler-dispatch(handlerNo) rt_task_wait_period(NULL); } class PriorityScheduler extends Scheduler { create Native handlers, start them, etc. .. void dispatch(int handlerNo) { EventHandler evh = getEvh(handlerNo); evh.getMemoryArea().enter(evh); } } public abstract class MemoryArea { long memSize; int memID; // a reference to the MEM_AREA public void enter (Runnable logic) { Native.enterNativeMemArea (memID); logic.run(); Native.leaveNativeMemArea (memID); } ... } abstract class PeriodicEventHandler { .. public final void run() { handleEvent(); // abstract method } } class MyPeriodicEvH extends PeriodicEventHandler { .. public void handleEvent() { // the logic to be executed every period } }
Xenomai Architecture • Xenomai runs on top of the Linux kernel and handles all incoming interrupts first, before the Linux kernel • Adeos (Adaptive Domain Environment for Operating Systems) • a nanokernel hardware abstraction layer (HAL) • operates between computer hardware and the operating system • Xenomai has some skins • Native, POSIX, etc.
Native Xenomai API • RT services for • RT_TASKs • rt_task_create, rt_task_start, rt_task_set_priority • rt_task_set_periodic, rt_task_wait_period • rt_task_suspend, rt_task_resume • .. • RT_EVENTs • RT_INTERRUPTs • RT_MUTEXs