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cluo1

Even in some work such as summing numbers that appears to be not independent between threads, we could still find a way to split the work and achieve substantially speedup by utilizing local variable.

chenh1

We need N >> P to get near-P speedup because when N is small the overhead of threads will offset the benefits get from parallelism. Only when we save a relative large amount of execution time, the speedup will be obvious.

pk267

This overhead includes not just the computation cost, but also the communication cost which could be non-trivial if the nodes are spread out over large geographical distance.