Java Real-Time Distributed Processing over Chorus/OS

Transcription

1 Java Real-Time Distributed Processing over Chorus/OS Christophe Lizzi CS Technologies Informatiques CNAM, CEDRIC lab.

2 Outline Motivations Our vision Real-time Java Operating system and network supports Java real-time extensions Further work and conclusion

3 Motivations Current industrial control systems various networks - fieldbuses, control buses, LANs, analog links - QoS, performances, service model various environments - hardware: workstations, embedded systems, PLCs - software: operating systems, APIs, programming languages various bandwidth and QoS demanding applications - command/control data, reporting, audio/video streaming for quality control or failure localization Heterogeneity is the key problem scalability, proprietary, fixed QoS, integration, maintainance, cost

4 Our vision An open, integrated, homogeneous distributed real-time environment How? by using ATM and Java as the core technologies of the system Homogeneous networking infrastructure: ATM integrated, scalable, open, flexible QoS, easy integration Homogeneous distributed processing environment: Java binary portability, high-level abstractions, security, modularity Challenges end-to-end performance guarantees over ATM Java real-time processing integrated approach of networking, scheduling and synchronisation

5 Java What is Java? an object-oriented programming language a runtime system an application programming interface (JDK classes) Why Java? the Java language and the Java platforms support application portability higher level of abstraction security mechanisms Java is designed to support component integration and reuse Beneficts portability, reliability, high level application design Drawback runtime efficiency

6 Real-time Java Why Java in real-time systems? Industrial applications run on the entire spectrum of system platforms - from workstations with standard operating systems to microcontrollers without an OS in embedded systems - lots of specialized and costly developement systems - various abstractions, services, semantics impacts portability, integration, development costs of applications Real-time Java retain the benefits of the Java language - portability, high level of abstraction add deterministic temporal behavior - real-time scheduling, synchronization, communications enable access to hardware resources

7 Problems raised by Java in a real-time environment Thread scheduling scheduling policies are JVM-dependent need for a high level model of tasks and temporal constraints Thread synchronization Code interpretation dynamic class loading, verification and link edition delays are not predictable Memory management interferences with the garbage collector Performances just-in-time compiling, dynamic optimization run-time analysis

8 PERC from NewMonics Related work and products specific compiler, JVM and core classes uses an extended dialect of Java new keywords: atomic and timed - enable to express required temporal behavior rate monotonic scheduling real-time incremental garbage collection - can be interrupted by real-time threads proprietary classes to access to physical memory, I/O ports and interrupt handlers EmbeddedJava and JavaOS for Consumers from Sun, JBED from Oberon, Kaffe... subsets of Java proprietary packages Standardization (?) efforts NIST, Real-Time Java Working Group, J Consortium, Java Experts Group...

9 Our approach Enabling end-to-end deadline inheritance uniform model of temporal constraints - message and task scheduling Adding real-time capabilities to our JVM virtual machine support for Java real-time scheduling and synchronization enhanced Java Native Interface access to real-time communications over ATM access to hardware resources through native methods Underlying real-time distributed operating system Chorus/OS from Sun Microsystems extended to support deadline-based scheduling and communications

10 Overview end-to-enddeadline inheritance EDF-scheduled Java thread message handler EDF-scheduled Java thread shared buffers shared buffers GNU Japhar JVM GNU Japhar JVM ChorusOS RT-IPC server ATM driver RT-IPC server ATM driver ChorusOS RT-binding

11 Operating System Support Chorus/OS microkernel-based distributed real-time system now commercialized by Sun Microsystems foundation of the JavaOS for Consumer product Modularity made of building blocks selected according to application requirements selected components determine the core features of the system memory model, scheduling policies, available APIs Customization support of ATM networks deadline-based scheduling real-time IPC enhanced Java virtual machine

12 ATM support ATM networking a set of switches connected by point-to-point links cell based switching and asynchronous multiplexing Why ATM? high bandwidth connection oriented native end-to-end QoS support scalability - flexible topology, distance (LAN, MAN, WAN), bandwidth open technology Why ATM in real-time systems? integrated traffic transport one network technology - virtual, not physical isolation among connections

13 Enforcement of QoS CBR and rt-vbr service categories are suitable for time sensitive traffic open loop control: congestion avoidance via cell scheduling Traffic contract traffic specification: PCR, SCR, MBS QoS specification: maxctd, CDV, CLR The network commits to support the QoS for all compliant connections Separation of the network provider and the user ATM comes from the telco world protect the network and its performance Leads to traffic contract, admission control, shaping, policing and scheduling never heard of these concepts in LANs designed by the computer industry

14 Enforcement of QoS (cont.) Traffic contract user description of traffic and requested QoS network connection admission control require traffic models and conformance definitions Shaping make sure the traffic passes the policer s conformance test Policing Usage Parameter Control at UNI = test for compliance GCRA algorithm = leaky bucket Scheduling which cell to send? decision based on QoS parameters

15 Java real-time support Thread scheduling and synchronization Earliest Deadline First scheduling - release time, absolute deadline, worst-case execution time, period support for basic priority inheritance protocol Communications location-transparent real-time IPC service - basic concepts inherited from the regular Chorus IPC service binding between RT-IPC ports is subject to network admission control Enhanced Java Native Interface Java objects may be moved by the garbage collector, so copies are supplied - data copies limit the JNI performances enabling the sharing of data between Java and native methods byte arrays may not be allocated in the Java heap, but using native memory

16 Thread scheduling Real-time extensions class SampleThread extends PeriodicEdfThread { public SampleThread() { setreleasetime( 0 ); setabsolutedeadline( * ); setworstcaseexectime( 100 * ); setperiod( 100 * ); } setmaxreleasecount( 10 ); public void edfrun() { dosomethinghere(); } } public static void main(string[] args) { new SampleThread().start(); }

17 Real-time extensions (cont.) Communications similar to the standard Chorus IPC framework exchange of RT-messages through RT-ports Service RT-messages convey a deadline required explicit binding of RT-ports between any interactions binding is subject to negociation and admission control deadline-scheduled handlers are associated to RT-ports End-to-end deadline inheritance an absolute deadline is propagated from the sender to the receiver of an RT-message RT-message deadline is set by the user or inherited from the sender thread integration with the real-time synchronization mechanisms

18 Real-time extensions (cont.) Java Native Interface real-time communication relies on native methods heavy transfers of serialized data accross the JNI interface JNI design the JNI spec prohibits memory sharing between native and Java objects Java objects may be moved by the garbage collector During JNI invocations involving arrays: pin down the Java object during the execution of the native code, or supply a non-moveable copy to the native side - if updated data are returned, then another copy is performed Limits JNI bandwidth

19 Enhanced JNI interface Real-time extensions (cont.) enable the sharing of data between Java and native methods, boosting performances Java heap public class SampleClass { public byte shared[]; SampleClass instance shared variable } public void samplemethod() { shared = nativeallocmethod(); shared[ 5 ] = 123; } jbytearray attribute Java virtual machine native code 123 JNI interface native body of shared byte array ChorusOS memory pages arrray body not allocated in the Java heap, but using native DMA able pages - not touched by GC operations

20 Memory Management Garbage collection Java exempts the programmer to explicitly release the allocated memory done by the garbage collector GC techniques reference counting mark and sweep copying Real-time Java requires incremental GC techniques memory is collected while the reference graph is explored by the GC - the graph can be altered by the application while it is being explored - the GC can be interrupted Dedicated non-copying GC algorithms Treadmil algorithm from Baker refined version from WiIson & Johnsone

21 Memory Management (cont.) The overhead induced by garbage collection can be unacceptable for some classes of real-time applications real-time GC is not included in the current draft specs of the RTJWG - treated as and optional, vendor-specifc, extension Allocation contexts partition of the memory heap expresses the lifetime of the objects it contains - objects are automatically reclaimed when their allocation context is destroyed default per-thread allocation context a class may implement the following interface to indicate that all objects instantiated by its methods must be allocated in a specific context interface CoreAllocator { public CoreAllocationContext allocationcontext(); } - the allocating context being used is represented by the object returned by allocationcontext() Stackable objects

22 Further Work RMI are not yet supported serialisation of composite objets is partially missing stream-based interactions only Higher abstraction model the Sun JINI environment the Jonathan flexible ORB - J.B. Stefani s team at France Telecom CNET Towards an extensible Java-based SCADA environment Java virtual manufacturing devices (VMD) as defined by the MMS model

23 Conclusion Heterogeneity of industrial systems is the key problem various computing environments connected by various networks and running various QoS-demanding distributed applications negative impact on scalability, integration, maintenance, cost Our proposal homogeneous virtual computing environments connected by one network core technologies: Java and ATM Benefits integrated, scalable networking with native QoS support high-level, portable Java computing environment Promising approach (?), but these technologies are still in infancy required support for real-time communications over ATM what real-time Java really means is still discussed - RTJWG, J Consortium, NewMonics vs. Java Experts Group, Sun, IBM