Basanta jtr2009

Este trabajo ha sido publicado en IEEE-ISORC’08 (Orlando-USA)

Simplifying the dualized threading model of RTSJ

Pablo Basanta-ValMarisol García-Valls

Iria Estévez-Ayres

mailto: [email protected]://www.it.uc3m.es/drequiem/

XII Jornadas de Tiempo Real 5-6 de febrero de 2009 (Leganés)

Roadmap

1. Introduction– Motivation

2. RealtimeThread++ extension– Interface changes– Example of use– Low level (VM) changes

3. Taking advantage of the extension– GC interference avoidance– Extended event handlers– Enhanced real-time distributed architectures

4. Empirical results– Absolute and relative penalties

5. Concluding remarks


Introduction

• Java is a successful high-level language– Bytecode portability, garbage collection, clear syntax, a number of

libraries, ….– Productivity may be higher than with other languages (C/C++)

• Java has extended its radius of action to real-time systems– Ideally: real-time portability, real-time automatic memory

management, and real-time libraries– And also: lower development times, and improved maintainability


Current Status of Real-Time Java

• Specifications– Real-Time Specification for Java (RTSJ)

– Distributed Realtime Specification for Java (DRTSJ)

• Compliant virtual machines– Sun’s JRTS, Aicas’s Jamaica, TimeSys’s JTime, IBM Real-Time

Websphere, Purdue’s OVM, …

• Plug-ins for development environments– Netbeans (Sun) and Eclipse (IBM, Aicas)


RealtimeThread++• An extension not identified in JSR-282 (RTSJ 1.1)• RTSJ offers two types of real-time threads:

• RealtimeThread (RtThs) and NoHeapRealtimeThreads (NhRtThs)

• Reducing unnecessarily the language expressiveness– RtThs suffer the garbage collector whereas NhRtThs cannot use

the heap– A general purpose application may require both types of support

• Why don’t a simplification in this model? – One unique type of thread that decides its relationship with the

garbage collector and the heap at runtime– Benefits: a more flexible specification and better synchronization

models.


Real-Time Java

Centralized concurrent entities

RTSJ Thread

RealtimeThread

NoheapRealtimeThread

AsyncEventHandler

BoundAsyncEventHandler

Distributed

concurrent entities

DRTSJ DistributedRealtimeThread

NoHeapDistributedRealtimeThread

DistributedEventHandler

Synchronization RTSJ Synchronized

RealtimeQueues

Signals


Real-Time Java with RealtimeThread++

Centralized concurrent entities

RTSJ Thread

RealtimeThread

NoheapRealtimeThread

AsyncEventHandler

BoundAsyncEventHandler

Distributed

concurrent entities

DRTSJ DistributedRealtimeThread

NoHeapDistributedRealtimeThread

DistributedEventHandler

Synchronization RTSJ synchronized

RealtimeQueues

Signals


RealtimeThread++: changes

• One thread is initially running in either heap or noheap environments– By default, in its instantiation, NhRtThs and NhDRtThs run in

noheap memory whereas RtThs run in heap

• One thread may change at runtime its relationship with the garbage collector– We may change it, temporally, nesting a heap (with enterHeap) or a noheap (with enterNoHeap) runnable object

01: Runnable r1 = new Runnable(){02: public void run(){03: System.out.println(“Running in heap”);04: }05: };06:07: Runnable r2 = new Runnable(){08: public void run(){09: System.out.println(“Running in Noheap”);10: RealtimeThread.enterHeap(r1)11: }12: };13: RealtimeThread.enterNoHeap(r2);


RealtimeThread++: full API

package javax.realtime;public class RealtimeThread extends Thread{

public static void enterHeap(Runnable r){. . .}public static void enterNoHeap(Runnable r){. . .}public static boolean isRunningInHeap(){. . .}

. . .}



RealtimeThread++ in action


One task printing out two messages: the first in noheapand the second in heap



RealtimeThread++ in action

Evolution of the system, the active environment is the one on the top of the environment stack



Required underlying support

• Activation and deactivation of reading barrier typically associated with a NhRtThs– Low level interaction with the virtual machine required

• The check of local variables (for NH to H changes)– Easy to obtain in a portable way.

• Modification of the relationship maintained with the garbage collector– Sometimes, NhRtThs have privileges when garbage collector scans their

frame stack

#Turning off the read barrierRTJVM.disableNoHeapReadBarrier();


Runtime checking barriers (examples)

#Checking local variablesif r allocated in HeapMemory throws new IllegalAssignmentException();

#Turnning on the read barrierRTJVM.enableNoHeapReadBarrier();

Runtime barrier that changes the behavior from heap to no-heap.


Runtime barrier that changes the behavior from o no-heap to heap



Integration with the garbage collector

NoHeapRealtimeThreadqueue

RealtimeThreadqueue

(i) moving from noheap to heap

(ii) moving from heap to noheap



Taking advantage of the extension

– Sharing data • Among heap and no-heap entities using enterNoHeap

– Enrichment of the current RTSJ• Giving more flexibility to event-handler programming

– Enrichment of the ongoing DRTSJ• Eases some architecture implementations


Garbage collector priority inversion

01: public class Table {02: private long count=0;03: private Object lock=new Object();04: public void inc(){05: synchronized(lock){ 06: count++; //May cause the GC inversion propagation07: }08: } 09: }

ImmortalMemoryScopedMemoryHeapMemory

rt-threadtable

nhrt-threadinc inc

heap noheap


Garbage collector priority inversion (current RTSJ approach)

00: public class Table{01: private long count=0;02: private Object lock=new Object();03: private WaitFreeReadQueue wfq=04: new WaitFreeReadQueue(5,false);05: public Counter(){06: th.start();07: }08: private NoHeapRealtimeThread th=new 09: NoHeapRealtimeThread(){10: public void run(){11: do{ 12: wfq.waitForData();13: wfq.read();14: count++;15: }while(true);16: }};17: public void inc(){18: wfq.write(lock);//OK 19: }20: } 21: }

ImmortalMemory ScopedMemory

HeapMemory

rt-thread table

nhrt-thread

inc inc

heap noheap


Garbage collector priority inversion (with RealtimeThread++)

ImmortalMemory

ScopedMemoryHeapMemory

rt-threadtable

nhrt-thread

inc inc

heap noheap

01: public class Table{

02: private long count=0;

03: private private Object lock=new Object();

04: private Runnable r=new Runnable(){

05: public void run(){

06: synchronized(lock){ //GC OK

07: count++;

08: }

09: };

10: public void inc(){

11: RealtimeThread.enterNoheap(r); //Ok

12: }

13: }


Enhanced RTSJ event model

ImmortalMemory

ScopedMemory

heap noheap

HeapMemory

Heap

Externalevent

NoHeap

NoHeap

21

NoHeap

enterHeap

Currently event handler decide at creation time itsrelationship with the garbage collector


Session-level isolation for DRTSJ(with real-time thread++ extension)

Remote ObjectStub

enterNoHeapenterNoHeapenterHeap

Programmer space

DistributionMiddleware

heap or noheap

noheap

client server


Absolute and relative performance


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Basanta jtr2009

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