What is the difference between isend and issend?
mpi
Solution
Both `MPI_Isend()` and `MPI_Issend()` return immediately, but in both cases you can't use the send buffer immediately.
Think of the difference that there is between `MPI_Send()` and `MPI_Ssend()`:
`MPI_Send()` can be buffered or it can be synchronous if the buffer is too large to be buffered locally, and in this case it waits to complete sending the data to the corresponding receive operation.
`MPI_Ssend()` is always synchronous: it always waits to complete sending the data to the corresponding receive operation.
The inner working of the corresponding "I"-operations is very similar, except for the fact that they both don't block (return immediately): the difference is only when the MPI library signals to the user program that you can use the send-buffer (that is: `MPI_Wait()` returns or `MPI_Test()` returns `true` - the so called send-complete operation of the non-blocking send):
with `MPI_Isend()` this can happen either when the data has been copied locally in a buffer owned by the MPI library, if below the "synchronous threshold", or when the data has been actually moved to the sibling task: the send-complete operation can be local, in case the underlying send operation is buffered.
With `MPI_Issend()` MPI doesn't ever buffer data locally and the "buffer-free condition" is returned only after the data has been actually transferred (and probably ack'ed, at low level): the send-complete operation is non-local.
The MPI standard document is quite pedantic on these aspects. See section 3.7 Nonblocking Communication.
Problem
Need clarification to my understanding of isend and issend as given in Send Types My understanding is that isend will return once the send buffer is free, i.e. when all the data has been released. Issend on the other hand returns only when it receives an ack from the receive of getting/not getting the entire data. Is this all there is to it?