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Data sharing among tasks is fast (speed of memory access).
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Attractive feature of shared memory is that the time to communicate among the tasks is effectively a factor of a single fixed value, that being "the time it takes a single task to read a single location."
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Disadvantage: scalability is limited by number of access pathways to memory.
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If there are more tasks than connections to memory, you have contention for access to the desired locations, and this amounts to increased latencies while all tasks obtain the required values. So the degree to which you can effectively scale a shared memory system is limited by the characteristics of the communication network coupling the processors to the memory units.
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