Introduction to Real-Time Java

I wish I have more time to spend on this. But I am exhausted between my work,  my 3 months old child and this free time project.

I am not an expert on real-time Java. If you send your corrections to denizoguz@denizoguz.com, I will apply them to this presentation and upload it again.

Thanks.

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Introduction to Real Time Java — Presentation Transcript

1. Real Time Java Deniz Oğuz Initial:22.06.2008 Update:05.12.2009
2. • Downside of Using C and Java Together
   • Aim of This Course
3. Aim of This Course Gaining introductory level knowledge about Real-Time Java and evaluate its suitability to your next project.
4. Outline
   • What is Real-time?
   • What are the problems of Java SE?
   • Real-Time Linux
   • Features of RTSJ (Real Time Specification for Java), JSR 1, JSR 282
   • IBM Websphere Real-Time
   • SUN RTS
5. What is Real-time?
   • Real-time does not mean fast
   • • Throughput and latency are important but does not enough
   • Real-time is about determinism
   • • Deadlines must be met
   • Real-time is not just for embedded systems
   • • Financial applications
   • • Telecommunication applications
   • • Network centric systems etc…
6. Categories of Application Predictability Hard Real-Time None Real-Time Soft Real-Time No time-based deadlines Ex:Batch processing, web services Deadlines may be missed occasionally Ex:Router, automated trading systems Deadlines can not be missed Ex:fly-by-wire system, anti-lock brake system, motion control
7. Hard/Soft real-time
   • Hard real-time
   • • A late answer has no value
   • Soft real-time
   • • late answer has still a value but value of answer rapidly degrees
8. Software Complexity vs. Time Taken From IBM WebSphere Real Time: Providing predictable performance by Michael S. Fulton, Java chief architect;Darren V. Hart, Real Time team lead; andGregory A. Porpora, IBM Software Group. December 2006
9. Latency and Jitter
   • Latency is the time between external event and system’s response to that event
   • Jitter is the variation in latency
10. Hardware Architecture & OS
    • Modern processors and operating systems are optimized for throughput
    • • It makes excellent sense for most systems to trade a rare factor of 100 times slowdown for a performance doubling everywhere else
11. Hardware & OS Cont.
    • Worst-case scenario for an instruction
    • • Instruction is not in the cache, processor must read it from memory
    • • An address translation cache miss requires more memory access
    • • Instruction might be in demand paged memory
    • • Data may not be in cache
    • • A large DMA may delay operations
    • • SMI on X86 platforms
    • • …….
12. Worst Case Execution Cont. This example is taken from Real-Time Java Platform Programming by Peter C. Dibble 100101660 Total 10000000 One Big DMA 20000000 Demand paging for instruction read (write dirty page) 500 Dirty data cache write ATC miss 500 Instruction ATC miss 10 Execute Instruction 100000 Interrupts 10000000 Demand paging for data cache read (read page) 10000000 Demand paing for data cache write (read page) 20000000 Demand paging for data cache write (write dirty page) 500 Data cache read ATC miss 50 Instruction cache miss 20000000 Demand paging for data cache read (write page) 10000000 Demand paging for dirty data cache write (read page) 100 Data cache miss Estimate Time (ns) Event
13. How to prevent worst case
    • Disable paging for time critical code
    • Use processor affinity and pin RT Threads to cpus
    • Use tunable DMA
    • Use large pages to reduce load on TLB
    • Disable interrupts or give RT Threads higher priority than some interrupts
    • On x86 architecture pay attention to SMI (System Management Interrupts)
    • • Do not disable it completely , you may burn down your processor.
14. Processor Pinning
    • Less context switching jitter
    • Decreased cache miss
    • Example (Linux and Sun RTS only):
    • • -XX:RTSJBindRTTToProcessors=0,1
    • • -XX:RTSJBindNHRTToProcessors=0,1
    • • Alternatively you can create a cpu set named xx and use /dev/cpuset/xx
15. Large Memory Pages
    • Garbage collectors performs very bad if memory is swaped to disk
    • For increased determinism use large memory pages and pin these pages to memory
    • Example Linux and Sun’s Java SE only:
    • • echo shared_memory_in_bytes > /proc/sys/kernel/shmmax
    • • echo number_of_large_pages > /proc/sys/vm/nr_hugepages
    • • Start JVM using XX:+UseLargePages argument
    • • Verify using cat /proc/meminfo | grep Huge

    Refer following Sun article for large memory pages: Java Support for Large Memory Pages

16. RT-POSIX
    • An extension to POSIX standard to address hard and soft real-time systems (POSIX 1003.1b)
    • • Priority Inversion Control and Priority Inheritance
    • • New schedulers
    • • Asynchronous IO
    • • Periodic, Aperiodic and Sporadic Threads
    • • High Resolution Timers
    • • RT File System
    • • Some others ….
    • For further information refer to RT Posix standard
17. RT Linux
    • Fully preemptible kernel
    • Threaded interrupt handlers for reduced latency
    • High-resolution timers
    • Priority inheritance
    • Robust mutexes and rt-mutexes
    • To install use
    • sudo apt-get install linux-rt
    • in ubuntu. Select rt kernel at next system start-up
18. Why Java?
    • Software (including embedded software) becomes more complex and gets unmanageable with old practices and tools
    • • Ex:Financial systems, Network Centric systems
    • Single language, tools for real-time and non-real time parts
    • Java Platform provides a more productive environment
    • A large set of 3 rd party libraries are available
    • Has a very big community
    • • Large number of developers
    • • Support from big companies like IBM, SUN, Oracle
    • A lot safer when compared to low level languages
19. Downside of Using C and Java Together
    • Same functionality is codded twice
    • • for example coordinate conversion, distance calculations, R-tree etc. are implemented in C++ because they can not reuse PiriMap
    • High amount of integration problems
    • • For example work package is divided between two people
    • Interfaces between C and Java is ugly and introduce overhead
    • • For example DDS transfer Struct but large amount of code is written to convert these structs to Java classes and interfaces accustomed by java developers
    • Communication via JNI can violate safe features of Java
    • The JNI interface is inefficient
    • Increased maintenance cost in the feature due to above problems
20. What are the problems of Java SE?
    • Dynamic Class Loading and Linking
    • JIT (Just in Time Compiler)
    • Thread Handling
    • Garbage Collector
    • No Raw Memory Access
    • No support for real-time operations, like deadline miss handling, periodic scheduling, processor pinning etc.
21. Dynamic Class Loading
    • A Java-conformant JVM must delay loading a class until it’s first referenced by a program
    • • Early loading is not allowed so JVM can not do this for you at application startup
    • This introduce unpredictable latency from a few microseconds to milliseconds.
    • • Depends on from where classes are loaded
    • • Static initialization
    • • Number of classes loaded
22. JIT (Just In Time Compiler)
    • Most modern JVMs initially interpret Java methods and, and later compile to native code.
    • For a hard RT application, the inability to predict when the compilation will occur introduces too much nondeterminism to make it possible to plan the application’s activities effectively.
23. Garbage Collection
    • Pros
    • • Pointer safety,
    • • leak avoidance,
    • • Fast memory allocation:faster than malloc and comparable to alloca
    • • Possible de-fragmentation
    • Cons
    • • Unpredictable pauses:Depends on size of the heap, number of live objects on the heap and garbage collection algorithm , number of cpus and their speed
24. Main Garbage Collection Features
    • Stop-the-world or Concurrent
    • Moving objects
    • Generational
    • Parallel
25. Garbage Collection in HotSpot
    • Serial Collector
    • • -XX:+UseSerialGC
    • Parallel-scavenging
    • • -XX:+UseParallelGC
    • Parallel compacting
    • • -XX:+UseParallelOldGC
    • Concurrent Mark and Sweep (CMS)
    • • -XX:+UseConcMarkSweepGC
26. Garbage First Collector (G1)
    • Planned for JDK 7
    • Low pause and high throughput soft real-time collector
    • It is parallel and concurrent
    • Performs compaction
    • Devides heap to regions and further devides them to 512 bytes cards
    • It is not a hard real-time collector
27. Thread Management
    • Although standard Java allows priority assignment to threads, does not require low priority threads to be scheduled before high priority ones
    • Asynchronously interrupted exceptions
    • • Similar to Thread.stop but this version is safe
    • Priority Inversion may occur in standard Java/Operating systems
    • • RTSJ requires Priority Inheritance
28. What is RTSJ?
    • Designed to support both hard real-time and soft real-time applications
    • Spec is submitted jointly by IBM and Sun
    • Spec is first approved in 2002 (JSR 1)
    • Minor updates started on 2005 (JSR 282)
    • First requirements are developed by The National Institute of Standards and Technology (NIST) real-time Java requirements group
29. Expert Group of RTSJ
30. RTSJ’s Problem Domain
    • If price of processor or memory is a small part of whole system RTSJ is a good choice
    • • If processor and memory price is very significant when compared with the rest of the system RTSJ may not be a good choice because you will need slightly more powerful processor and larger memory
    • • For very low footprint applications with very limited resources you may consider a non RTSJ compliant virtual machine like Aonix Perc Pico or you may even consider a hardware JVM.
31. Implementations
    • IBM Webshere Real Time (RTSJ Compliant)
    • • Works on real-time linux kernel
    • • There is a soft real-time version
    • SUN RTS (RTSJ Compliant)
    • • Works on Solaris x86/Sparc or real-time linux kernel
    • Aonix Perc (Not RTSJ Compliant but close)
    • • Perc-Ultra works on nearly all platforms
    • • They have a safety critical JVM (Perc-Raven)
    • Apogee
    • • Woks on nearly all platforms
32. Main Features of RT Java Implementations
    • Full Java SE compatibility and Java syntax
    • A way to write programs that do not need garbage collection (New API)
    • High resolution timer (New API)
    • Thread priorities and locking (New API)
    • Asynchronous Event Handers (New API)
    • Direct memory access (New API)
    • Asynchronous Transfer of Control (New API)
    • AOT (A Head of Time) compilation (Not in RTSJ specification)
    • Garbage collection (Not in RTSJ specification)
33. Real-Time Garbage Collectors
    • Time based (Metronome of Websphere RT)
    • Work based
    • Henriksson’s GC (Garbage Collector of Sun’s Java RTS is based on this)
34. Sun’s RT Garbage Collector
    • 3 modes of execution
    • Non generational, concurrent and parallel collector
    • In normal mode works with priority higher than non-rt threads but lower than any rt thread.
    • When free memory goes below a certain threshold priority is increased to boosted priority but RT Threads (including the one with priority lower than GC) still continues to work concurently
    • When free memory goes below a critical threshold.All threads whose priority is lower than GC boosted priority are suspended
    • Most of its working can be tuned using command line switches like: NormalMinFreeBytes, RTGCNormalWorkers, RTGCBoostedPriority, BoostedMinFreeBytes etc.
35. AOT, ITC and JIT
    • Nearly all Java SE vendors use JIT (Just in time compiler) due to performance reasons
    • Websphere RT uses AOT (Ahead of Time Compilation)
    • SUN’s Java RTS uses ITC (Initialization Time Compilation)

    Refer following IBM Article for further information: Real-time Java, Part 2: Comparing compilation techniques

36. High-Resolution Timer
    • A more powerful notion of time
    • • AbsoluteTime
    • • RelativeTime
    • 84 bits value
    • • 64 bits milliseconds and 32 bits nanoseconds
    • • Range is 292 million years
    • Every RTSJ implementation should have provide at least one high resolution clock accessible with Clock.getRealTimeClock()
    • Resolution is system dependent
    • • For example it is 200ns for SunFire v240 (Ref:Real-Time Java Programming by Greg Bollella)
    • • Do not expect too much from a laptop
37. HighResolutionTime Base Class
    • Base class for other 3 time classes.
    • Define the interface and provides implementation of some methods
    • This class and its subclass do not provide any synchronization
38. AbsoluteTime and RelativeTime
    • AbsoluteTime represents a specific point in time
    • • The string 03.12.2008 11:00:00.000 AM represents an absolute time
    • • Relative to 00:00:00.000 01.01.1970 GMT
    • RelativeTime represents a time interval
    • • Usually used to specify a period for a schedulable object
    • • Can be positive, negative or zero
39. Priority Scheduler
    • Normal scheduling algorithms try to be fair for task scheduling.
    • • Even lower priority tasks are scheduled some times
    • There are different real-time and non real-time schedulers
    • • Earliest-deadline first, least slack time, latest release time etc.
    • Fixed priority preemptive scheduler is most used real-time scheduler
40. Thread Scheduling in RTSJ
    • Does not specify a scheduling implementation
    • Allow different schedulers to be plugged
    • Required base scheduler should be a priority scheduler with at least 28 unique priorities
    • Most implementations provide more than 28 priorities
    • More on this with Scheduling Parameters Slide
41. What is Priority Inversion ?
    • Priority Inversion is the situation where a higher priority task waits for a low priority task.
    • There are 2 generally accepted solution
    • • Priority Inheritance (required by RTSJ)
    • • Priority Ceiling Emulation (optional for RTSJ)
42. What is Priority Inversion? cont. Time Legend Lock request  Waiting Task T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Med priority executing task High priority executing task Shared Resource         Low priority executing task
43. Schedulable Interface
44. Scheduling Parameters
45. Different Task Types
    • Periodic
    • Aperiodic Tasks
    • Sporadic
46. Release Parameters
47. Sample Periodic Task public class HelloWorld extends RealtimeThread {        public static long[] times = (long[]) ImmortalMemory.instance().newArray(long.class, 100);           public HelloWorld(PeriodicParameters pp) {            super(null, pp);        }           public void run() {            for (int i = 0; i < 100; i++) {                times[i] = System.currentTimeMillis();                waitForNextPeriod(); //wait for next period            }        }           public static void main(String[] argv) {             //schedule real time thread at every 100 milisecond            PeriodicParameters pp = new PeriodicParameters(new RelativeTime(100, 0));            HelloWorld rtt = new HelloWorld(pp);            rtt.start();               //wait real time thread to terminate            try {                rtt.join();            } catch (InterruptedException ex) {                ex.printStackTrace();            }               //print results            for (int i = 0; i < 100; i++) {                System.out.println(times[i]);            }        }   }
48. Periodic Execution vs Thread.sleep Real-time systems do not use a loop with a sleep in it to drive periodic executions While (true) { performPeriodicTask() Thread.sleep(period) } WRONG
49. Deadline Miss!
    • For some applications a deadline should not be missed for any reason
    • For most applications dealine miss is bad, but it is not a total failure and a recovery is possible
    • In RTSJ there are 2 ways to detect a deadline miss
    • • waitForNextPeriod() returns false immediately
    • • Use a deadline miss handler which is an instance of AEH
50. NoHeapRealTime Thread
    • Can not access heap
    • • Can preempt GC immediately
    • • If this rule is violated, MemoryAccessError is thrown
    • Can use ScopedMemory or ImmortalMemory
    • Should be created and started in a scoped or immortal memory
    • • They can not be created from normal Threads
51. WaitFreeWriteQueue
    • Intendent for exchanging data between real-time and non real-time part
    • Real-time producer does not block when queueing data
    • Multiple non-real time consumers may dequeue data
52. WaitFreeReadQueue
    • Intendent for exchanging data between real-time and non real-time part
    • Real-time consumer does not block when read ing data
    • Multiple non-real time producers may queue data
53. Memory Regions
    • Heap Memory
    • • Same as in Java SE
    • • Can be accessed with javax.realtime.HeapMemory
    • Scoped Memory
    • • Created and sized at development time
    • • Can be accessed with javax.realtime.ScopedMemory
    • • Can not be garbage collected; reference count to ScopedMemory object is used. Finalize method of objects are called
    • • Can be stacked
    • Immortal Memory
    • • Can be accessed with javax.realtime.ImmortalMemory
    • • Only one instance exist and size is determined at development time.
    • • All static data and allocations performed from static initializers are allocated in Immortal memory. Interned Strings are also allocated in immortal memory
    • Physical Memory
    • • There are LTPhysicalMemory, VTPhysicalMemory, and ImmortalPhysicalMemory
54. Memory Regions
55. Raw Memory Access
    • Models a range of physical memory as a fixed sequence of bytes
    • Allows device drivers to be writen in java
    • All raw memory access is treated as volatile, and serialized
56. Raw Memory Access Cont private final long DEVICE_BASE_ADDRESS = xxxxxx ; private final long CTRLREG = 0; private final long STAT REG = 4 ; ……… p ublic void init() { RawMemoryAccess device = new RawMemoryAccess(type, DEVICE_BASE_ADDRESS); device.setInt(CTRL_REG, MY_COMMAND); //send command to device while (device.getInt(STATREG) != 0); //wait for device to response }
57. Async Events
    • Many of the processing in RT systems are triggered by internal or external events
    • You don’t need to manage threads
    • Hundreds of AEHs may share a pool of threads
    • BoundAsyncEventHandler has always bounded to a dedicated thread
58. Handling Posix Events
    • Use POSIXSignalHandler
    • • Ex :

    ….. class SigintHander extends AsynchEventHandler { public SigintHandler() { //set it to highest priority setSchedulingParameters(new PriorityParameters(RTSJ_MAX_PRI); } public void handleAsynchEvent() { //handle user specified signal } } …… //add handler to posix predefined posix signal POSIXSignalHandler.addHandler(PosixSignalHandler.SIGUSR1, sigintHandler) Use kill -s SIGUSR1 PID to test

59. Time Triggered Events
    • OneShotTimer : execute handleAsyncEvent method once at the specified time
    • PeriodicTimer : execute handleAsyncEvent method repeatedly at specified interval. A periodicTimer and a AEH combination is roughly equivalent to Periodic Threads.
    • Enable/Disable Timer : A disabled timer is still kicking. But it does not generate events. When enabled again, it continues like never disabled.
60. Javolution Library (http://javolution.org)
    • High performance and time deterministic (util/lang/text/io/xml)
    • Struct and Union base classes for direct interfacing with native applications
    • NHRT Safe
    • Pure Java and less than 300Kbytes
    • BSD License
61. IBM WebSphere Real Time
    • Runs on linux with real time patches applied to linux kernel on x86 platforms
    • Has AOT and JIT Compilers
    • Shared classes support
    • Contains Metronome: a real-time garbage collector
    • Full RTSJ 1.0.2 and Java SE 6.0 support
    • A well defined list of NHRT safe classes
62. Metronome Garbage Collector
    • Uses a time based method for scheduling
    • Applications threads are given a minimum percentage of time (utilization)
    • • User supplied at startup
    • Uses a new two-level object model for arrays called arraylets
63. Metronome Garbage Collector Sample Execution
    • GC Pause Time Histogram
    • Utilization Graph
64. Websphere AOT
65. SUN RTS
    • Achieves maximum latencies of 15 microseconds, with around 5 microseconds of jitter.
    • Runs on real-time linux and Solaris 10
    • Has a real-time garbage collector
66. IBM WebSphere Real Time
    • Runs on linux with real time patches applied to linux kernel
    • Has AOT and JIT Compilers
    • Contains Metronome real-time garbage collector
    • RTSJ and Java SE 5.0 Compliant
67. Real World Projects:J-UCAS X-45C
68. Real World Projects:FELIN
69. Real World Projects:DDG-1000
70. Real World Projects:ScanEagle
    • This milestone marked the first flight using the RTSJ on an UAV and received the Java 2005 Duke’s Choice Award for innovation in Java technology.
71. Is RTSJ Required for You?
    • If your application can tolerate some degree of indeterminism use standard JVM and tune it to milliseconds level
    • Only if you fail the first approach use RTSJ.
    • • Try to meet your timing constrains with real-time garbage collector (if available) without using advance/complex features like scoped memory
    • • If first approach fails make use of NonHeapRealTimeThread, ImmortalMemory, ScopedMemory
72. A comparison of the features of RTSJ with the increased predictability From IBM WebSphere Real Time Manual
73. Safety Critical Java (JSR 302)
    • A subset of RTSJ that can be certified DO-178B and ED-128B.
    • Most probably garbage collector will not be available (not needed)
    • Will be targeted to Java ME platform, because Java SE and Java EE are too complex to be certified.
74. Expert Group of JSR 302
    • Specification Lead: C. Douglass Locke (POSIX 1003.4 Real‑Time Extensions Working Group)
    • Expert Group
75. Resources
    • Real-Time Java Platform Programming by Peter C. Dibble
    • Real-Time Java Programming with Java RTS by Greg Bollea
    • RTSJ Main Site ( www.rtsj.org )
    • IBM’s Developerworks Articles ( http:// www.ibm.com/developerworks/ )
    • SUN RTS ( http:// java.sun.com/javase/technologies/realtime/reference.jsp )
    • Deniz Oğuz’s blog ( www.denizoguz.com )
76. Resources Cont. (JavaOne Presentations in 2008)
    • TS-4797 Fully Time-Deterministic Java Technology
    • • Explains Javalution Library
    • TS-5767 Real-Time Specification for Java (JSR 1)
    • • Explains status of RTSJ. Join presentation from SUN, IBM, Locke Consulting (JSR 302 spec lead)
    • TS-5925 A City-Driving Robotic Car Named Tommy Jr.
    • • An autonomous ground vehicle fully powered by Java.
    • TS-5609 Real Time: Understanding the Trade-offs Between Determinism and Throughput