Does MPI_Ssend/MPI_Issend use a system buffer?

1
votes

According to the documentation, MPI_Ssend and MPI_Issend are a blocking and a non-blocking send operations, both synchronous. The MPI specification says that a synchronous send completes when the receiver has started to receive the message and after that it is safe to update the send buffer:

The functions MPI_WAIT and MPI_TEST are used to complete a nonblocking communication. The completion of a send operation indicates that the sender is now free to update the locations in the send buffer (the send operation itself leaves the content of the send buffer unchanged). It does not indicate that the message has been received, rather, it may have been buffered by the communication subsystem. However, if a synchronous mode send was used, the completion of the send operation indicates that a matching receive was initiated, and that the message will eventually be received by this matching receive.

Bearing in mind that a synchronous send is considered to be completed when it's just started to be received, I am not sure of the following:

  • It is possible that only a part of the data has been read from the send buffer at the moment when MPI_Ssend or MPI_Issend signal about send completion? For example, the first N bytes have been sent and received while the next M bytes are still being sent.
  • How can the caller be safe to modify the data until the whole message is received? Does it mean that the data is necessarily copied to the system buffer? As far as I understand, the MPI standard permits the use of a system buffer but does not require it. Moreover, from here I read that MPI_Issend() doesn't ever buffer data locally.
1
mpi

1 Answers

2
votes

MPI_Ssend() (or the MPI_Wait() related to MPI_Issend()) returns when :

  1. the receiver has started to receive the message
  2. and the send buffer can be reused

the second condition is met if the message was fully received, or the MPI library buffers the data locally.

I did not read that the MPI standard prohibits data buffering.

From the standard, MPI 3.1 chpt 3.4 page 37

A send that uses the synchronous mode can be started whether or not a matching receive was posted. However, the send will complete successfully only if a matching receive is posted, and the receive operation has started to receive the message sent by the synchronous send. Thus, the completion of a synchronous send not only indicates that the send buffer can be reused, but it also indicates that the receiver has reached a certain point in its execution, namely that it has started executing the matching receive. If both sends and receives are blocking operations then the use of the synchronous mode provides synchronous communication semantics: a communication does not complete at either end before both processes rendezvous at the communication. A send executed in this mode is non-local.