Every noncollective data access routine MPI_FILE_XXX has a collective counterpart. For most routines, this counterpart is MPI_FILE_XXX_ALL or a pair of MPI_FILE_XXX_BEGIN and MPI_FILE_XXX_END. The counterparts to the MPI_FILE_XXX_SHARED routines are MPI_FILE_XXX_ORDERED.
The completion of a noncollective call only depends on the activity of
the calling process.
However, the completion of a collective call
(which must be called by all members of the process group)
may depend on the activity
of the other processes participating in the collective call.
See Section 9.6.4,
page ,
for rules on semantics of collective calls.
Collective operations may perform much better than their noncollective counterparts, as global data accesses have significant potential for automatic optimization.
MPI-Standard for MARMOT