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[1] [2] [3] 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.
Program order guarantees that each process issues a memory request ordered by its program and write atomicity defines that memory requests are serviced based on the order of a single FIFO queue. In relaxing program order, any or all the ordering of operation pairs, write-after-write, read-after-write, or read/write-after-read, can be relaxed.
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 only need to be made visible to other threads when such a synchronization ...
For example, a second CPU may see memory changes made by the first CPU in a sequence that differs from program order. A program is run via a process which can be multi-threaded (i.e. a software thread such as pthreads as opposed to a hardware thread). Different processes do not share a memory space so this discussion does not apply to two ...
[1] [2] [3] This introduces write-after-write (WAW) hazards because the second instruction to write the value to a memory location needs to wait until the first instruction finishes writing data to the same memory location or else when the memory location is read at a later time it will contain the wrong value. [2] An example of an output ...
This can be used to model stack-based memory allocation (contrast with linear types which can be used to model heap-based memory allocation). [ 1 ] : 30–31 Without the exchange property, an object may only be used when at the top of the modelled stack, after which it is popped off, resulting in every variable being used exactly once in the ...
function simple memory bounded A *-star (problem): path queue: set of nodes, ordered by f-cost; begin queue. insert (problem. root-node); while True do begin if queue. empty then return failure; //there is no solution that fits in the given memory node:= queue. begin (); // min-f-cost-node if problem. is-goal (node) then return success; s:= next-successor (node) if! problem. is-goal (s ...
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.