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    <p>@g.inozemtsev is correct. The MPI collective calls -- including those in Open MPI -- are "blocking" in the MPI sense of the word, meaning that you can use the buffer when the call returns. In an operation like MPI_REDUCE, it means that the root process will have the answer in its buffer when it returns. Further, it means that non-root processes in an MPI_REDUCE can safely overwrite their buffer when MPI_REDUCE returns (which usually means that their part in the reduction is complete).</p> <p>However, note that as mentioned above, the return from a collective operation such as an MPI_REDUCE in one process has no bearing on the return of the same collective operation in a peer process. The only exception to this rule is MPI_BARRIER, because barrier is defined as an explicit synchronization, whereas all the other MPI-2.2 collective operations do not necessarily need to explicitly synchronize.</p> <p>As a concrete example, say that all non-root processes call MPI_REDUCE at time X. The root finally calls MPI_REDUCE at time X+N (for this example, assume N is large). Depending on the implementation, the non-root processes may return much earlier than X+N or they may block until X+N(+M). The MPI standard is intentionally vague on this point to allow MPI implementations to do what they want / need (which may also be dictated by resource consumption/availability). </p> <p>Hence, @g.inozemtsev's point of "You cannot rely on synchronization" (except for with MPI_BARRIER) is correct.</p>
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