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Data is propagated between processors via messages which can be divided into packets but at MPI level we only see logically single complete messages
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The building block is Point to Point Communication with one processor sending information and one other receiving it
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Collective communication involves more than one message
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Broadcast of one processor to group of other processors (call a multicast on Internet when restricted to group)
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Synchronization or barrier
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Exchange of Information between processors
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Reduction Operation such as a Global Sum
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Collective Communication can ALWAYS be implemented in terms of elementary point to point communications but is provided
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for user convenience and
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often the best algorithm is hardware dependent and "official" MPI collective routines ought to be faster than portable implementation in terms of point to point primitive routines.
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Secs 649.4
