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Distributed operating systemsDistributed shared memory is a technique for making multicomputers easier to program by simulating shared memory on them. Small multiprocessors are often bus based, but large ones are switched. The protocols the large ones use require complex data structures and algorithms to keep the caches consistent. NUMA multiprocessors avoid this complexity by forcing the software to make all the decisions about which pages to place on which machine. A straightforward implementation of DSM, as done in IVY, is possible, but often has substantially lower performance than a multiprocessor. For this reason, researchers have looked at various memory models in an attempt to obtain better performance. The standard against which all models are measured is sequential consistency, which means that all processes see all memory references in the same order. Causal consistency, PRAM consistency, and processor consistency all weaken the concept that processes see all memory references in the same order. Another approach is that of weak consistency, release consistency, and entry consistency, in which memory is not consistent all the time, but the programmer can force it to become consistent by certain actions, such as entering or leaving a critical region ...» |
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