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There is tradeoff between parallelism and communication
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Programmer defines the data mapping and compiler uses this to assign processing
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Underlying assumptions are that:
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An operation on two or more data object is likely to be carried out much faster if they all reside in the same processor,
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And that it may be possible to carry out many such operations concurrently if they can be performed on different processors
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This is embodied in "owner computes" rule -- namely that in for instance
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A(i,j)= .....
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One brings everything on right hand side to process "owning" A(i,j) and performs computation in this processor
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Owner computes algorithm is usually good and often best
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