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Mathematically, there is a partial order called the happens-before order over all actions performed by the program. The happens-before order subsumes the program order; if one action occurs before another in the program order, it will occur before the other in the happens-before order. In addition, releases and subsequent acquisitions of locks ...
(In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex. [16] This relationship is simply a guarantee that memory writes by one specific statement are visible to another specific statement. A volatile fields are linearizable.
In computer science, the happened-before relation (denoted: ) is a relation between the result of two events, such that if one event should happen before another event, the result must reflect that, even if those events are in reality executed out of order (usually to optimize program flow).
[1] [4] Conversely, the memory order is called weak or relaxed when one thread cannot predict the order of operations arising from another thread. [1] [4] Many naïvely written parallel algorithms fail when compiled or executed with a weak memory order. [5] [6] The problem is most often solved by inserting memory barrier instructions into the ...
Modern programming languages like Java therefore implement a memory model. The memory model specifies synchronization barriers that are established via special, well-defined synchronization operations such as acquiring a lock by entering a synchronized block or method. The memory model stipulates that changes to the values of shared variables ...
The following multi-threaded program, running on a multi-core processor gives an example of how such out-of-order execution can affect program behavior: Initially, memory locations x and f both hold the value 0. The software thread running on processor #1 loops while the value of f is zero, then it prints the value of x. The software thread ...
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Transactional memory model [7] is the combination of cache coherency and memory consistency models as a communication model for shared memory systems supported by software or hardware; a transactional memory model provides both memory consistency and cache coherency. A transaction is a sequence of operations executed by a process that ...