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For the execution of a single thread, the rules are simple. The Java Language Specification requires a Java virtual machine to observe within-thread as-if-serial semantics. The runtime (which, in this case, usually refers to the dynamic compiler, the processor and the memory subsystem) is free to introduce any useful execution optimizations as ...
The Java programming language and the Java virtual machine (JVM) is designed to support concurrent programming. All execution takes place in the context of threads. Objects and resources can be accessed by many separate threads. Each thread has its own path of execution, but can potentially access any object in the program.
A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation.
Since CAS operates on a single pointer-sized memory location, while most lock-free and wait-free algorithms need to modify multiple locations, several extensions have been implemented. Double compare-and-swap (DCAS) Compares two unrelated memory locations with two expected values, and if they're equal, sets both locations to new values.
Free GPL version 2 or later JamVM: Robert Lougher 13 March 2003 2.0.0 30 July 2014 Free GPL version 2 or later Jato VM: Pekka Enberg and contributors [9]? 0.3 [10] 4 January 2012 [10] Free GPL version 2 only [9] JC virtual machine: Archie L. Cobbs ? 1.4.7 13 November 2005 Free LGPL version 2.1 or later Jikes RVM: IBM: 14 October 2001 3.1.4 [11 ...
Multiple threads can interfere with each other when sharing hardware resources such as caches or translation lookaside buffers (TLBs). As a result, execution times of a single thread are not improved and can be degraded, even when only one thread is executing, due to lower frequencies or additional pipeline stages that are necessary to accommodate thread-switching hardware.
In Java 1.1, green threads were the only threading model used by the Java virtual machine (JVM), [8] at least on Solaris. As green threads have some limitations compared to native threads, subsequent Java versions dropped them in favor of native threads.
The performance of a Java bytecode compiled Java program depends on how optimally its given tasks are managed by the host Java virtual machine (JVM), and how well the JVM exploits the features of the computer hardware and operating system (OS) in doing so.