How to perform background tasks using MPI_Irecv and be notified as an interrupt?

Question

My pseudo code has something like this:

//Some work
MPI_Send(....)
MPI_Irecv(&yesno, 1, MPI_INT, 0, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
//Do some WORK1 (background work) till MPI_Irecv completes and when receive completes do WORK 2.
--WORK1
--WORK2

Where do I put WORK1 and WORK2 and how to redirect control to WORK2 just when receive completes?

MORE INFO: I am trying to implement a master-slave program where the master issues work to slaves. The master continues to work when it has issued work to say slave 1 and needs to be interrupted by slave 1 when done. When this interrupt happens some code needs to be executed by the master. Then the master needs to continue from where it left off before interrupt.

Answer 1

If WORK1 is a loop, then use MPI_Test() at the start/end of each iteration to see if the request posted by IRecv has finished. If it has, then do WORK2, post a new IRecv, and continue with the next iteration of WORK1.

If WORK1 is a sequential task, then you still have two options. You could simply pepper it with MPI_Test() calls as described above, but that's not very elegant. Instead, you might want to spawn a new thread where you post your initial IRecv, and then simply use MPI_Wait() to wait for it to complete while the original thread is doing WORK1. If your MPI implementation is halfay decent, MPI_Wait() should only block the thread that calls it, so WORK1 will proceed just fine.