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yrkumar

Just to make sure - the reason why the ocean simulation speedup actually decreases after ~16 processors is due to the communication costs increasing enough for it to overwhelm any speedup in computation (i.e. the communication-to-computation ratio is large because the size of the simulation is small and the number of processors is large). Am I explaining this correctly?

kayvonf

Correct. The overhead of communication grows as processor count increases. After 16 processors the extra cost of this overhead overcomes the benefits of additional parallel execution. This is the reason we observed a slowdown between 16 and 32 processors.