•Non-blocking routines return immediately .
•They do not wait for the actual arrival of the message
nor for the sending protocols to be
finished.
•The MPI library performs these tasks whenever it is able
to with the user having no control.
•These routines are intrinsically unsafe as there is
the danger that the application buffers
may be modified by the program without
ascertaining whether the non-blocking
operation has been completed or not.
•Non-blocking routines allow communications and computations to overlap .