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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 form edges in the happens-before graph.
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).
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 ...
(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 computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. This typically means that operations issued prior to the ...
The memory order is said to be strong or sequentially consistent when either the order of operations cannot change or when such changes have no visible effect on any thread. [1] [4] Conversely, the memory order is called weak or relaxed when one thread cannot predict the order of operations arising from another thread.
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Memory model (programming) describes how threads interact through memory Java memory model; Consistency model; Memory model (addressing scheme), an addressing scheme for computer memory address space Flat memory model; Paged memory model; Segmented memory; One of the x86 memory models