Java Real-Time System (RTS) 2.0 is Sun's fully compliant implementation of the industry standard set of extensions for the Java platform. It helps you set process priorities according to importance (this is typically not supported in Java software applications).
If you run a normal Java application under Java RTS 2.0, nothing really changes. You'll start to notice the differences when you need to develop a real-time application. There are, however, significant improvements in the latest version of Java RTS.
Features and enhancements in Java RTS 2.0
A key feature of Java RTS 2.0 is a very predictable method for recycling memory within Java software programs. Using this method, you can determine how and when functions should be executed -- down to a millisecond or less.
The Java RTS 2.0 provides the following enhancements to the previous version:
- Improved determinism for long-running applications.
- Application monitoring, management, and troubleshooting functionality.
- A new DTrace probe provider for Java RTS.
- New MXBean to toggle between deterministic and debugging modes.
- Modified acquisition of inherited access control for no-heap real-time threads (NHRTs).
- Minor bug fixes.
Two enhancements in this new version that I think deserve calling special attention to are: its real-time garbage collector and the ability to create and assign a processor set.Tune the real-time garbage collector The main feature of Java RTS 2.0 is its real-time garbage collector (RTGC), which makes it fundamentally easier to approach real-time solutions and to control and tune applications. You can convert your existing Java threads to real-time threads (RTTs) by renaming them and yet still having the same semantics. Now you can start using the RTGC; you can set the priority of the thread even higher than the priority of the garbage collector and have no more suffering from the garbage collector. This used to be a very annoying problem for time-critical applications.
Java RTS 2.0 supports two garbage collectors: the RTGC and the non-real-time serial garbage collector. The RTGC, which is the default, might exhibit lower throughput than non-real-time serial garbage collectors, particularly on uniprocessors. For applications that are more sensitive to the collector's throughput than to the pause times induced by the collector's execution, the RTGC can be turned off with the -XX:-UseRTGC option. In this case, the non-real-time serial garbage collector is used, and all threads except NHRTs might suffer from pause times caused by garbage collection.
The RTGC is truly concurrent, thus it can be preempted at any time. There is no need to run the RTGC at the highest priority, and there is no stop-the-world phase, where all the application's threads are suspended during the garbage collection execution. On a multiprocessor, one CPU can be doing some garbage collection work, while an application thread is making progress on another CPU.
The only time RTGC prevents a thread from running is when it has to look at a particular thread's Java stack, where the local variables and operands are stored; hence, the main potential source of pause time for a given thread is caused by the scanning of its stack. Since a thread is not impacted by the scanning of the other thread stacks, the pause time for a thread is smaller than with non-concurrent garbage collectors.
For additional information, check out the Garbage Collection Guide for Java RTS 2.0.Create and assign processor sets For better temporal behavior on a multiprocessor machine, you can create a processor set devoted to the exclusive use of Java RTS 2.0. You can also dedicate a separate processor set to the exclusive use of the NHRTs and the RTTs of Java RTS 2.0. This partition of the available processors ensures the best temporal behavior for these threads by reducing cache thrashing effects.
To request Java RTS 2.0 to assign the NHRT threads to an existing dedicated processor set, use the -XX:RTSJBindNHRTToProcessorSet=<processor_set_id> option. The processors assigned to this processor set should all be set to no-intr to minimize latency and jitter. If this option is set, calling Runtime.availableProcessors from an NHRT will return the number of processors that are available in the processor set that has been assigned to NHRTs. You can use a similar option, RTSJBindRTTToProcessorSet, to bind RTTs to an existing dedicated processor set.
The Solaris operating system allows a machine's processors to be partitioned into a number of smaller, nonoverlapping processor sets. Processors assigned to a processor set are reserved for the processes explicitly bound to that set. Processes bound to that processor set cannot use those processors. You can create a new processor set with the following command (run with the superuser privileges):
# /usr/sbin/psrset -c created processor set <pset id>
Then, you can assign processors to this new processor set with the following command:
# /usr/sbin/psrset -a <pset id> <cpu id>
where <cpu id> refers to one of the on-line processors displayed by the following command:
<cpu id> on-line since mm/dd/yyyy hh:mm:ss
<cpu id> on-line since mm/dd/yyyy hh:mm:ss
<cpu id> on-line since mm/dd/yyyy hh:mm:ss<cpu id> on-line since mm/dd/yyyy hh:mm:ss
Now you can assign the Java RTS Virtual Machine to run in the new processor set, as follows:
# /usr/sbin/psrset -e <pset id> <Java RTS command line>
How Java RTS differs from the Java HotSpot VM
Java RTS differs in functionality from Java SE and the Java HotSpot VM in a number of ways; it has clocks, timers, class initialization, memory management, and scheduling. In addition, certain issues are only applicable to Java RTS; this includes programming considerations, limitations, bugs, and workarounds.
There are also general differences in the way that Java RTS behaves in comparison to the Java HotSpot VM, which include the following:
- The default values for the command-line parameters have been tuned for Java RTS and might differ from the default values in the Java HotSpot VM. For instance, the default values for -Xms and -Xmx have been increased for Java RTS.
- All the memory that is specified by the -Xms and -Xmx parameters is acquired and locked at initialization of the Java HotSpot VM.
- Java RTS has limited support for the monitoring, management, and troubleshooting tools.
- Since NHRTs exist, classes and interned strings are never garbage collected. The Real-Time Specification for Java (RTSJ) also prevents objects allocated in static initializers from being garbage collected. You should be aware that such objects are allocated in immortal memory and survive until the Java HotSpot VM exits.
- Java RTS only provides is the client compiler. The server compiler depends on nondeterministic optimization; therefore, it has been deactivated.
- Just-In-Time (JIT) compilation is asynchronous by default in Java RTS.
- Java RTS provides the implementation of variants of inline caches and virtual method inlining. These variants are especially intended for use with the RTGC.
- Java RTS only provides the 32-bit "client" version of the Java HotSpot VM; the 64-bit version is not supported and using the -server option has no effect.
- Java RTS 2.0 is currently supported on an UltraSPARC processor or on a Sun x86/x64 platform running Solaris 10 (update 3 or 4 only); Java RTS 2.1 for Linux is available as early access now (real-time Linux versions only): SUSE Linux Enterprise Real Time 10 and Red Hat Enterprise MRG 1.0.
- It is highly recommended to have a minimum of two processors.
Additional resources about Java RTS
- Real-Time Java: An Introduction
- Real-time Java specification (RTSJ 1.0) under JSR-001
- Real-time Java specification updated (RTSJ 1.1) under JSR-282
Peter V. Mikhalenko is a Sun certified professional who works as a business and technical consultant for several top-tier investment banks.
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