title: sending and receiving multiple elements via MPI_Send/recv
body: <p>I am starting to learning MPI. I am just starting to do simple example: I have a rectangle which is divided into two sub-boxes. I have lets say N/2 particles in each box (summing to N particles as whole). I am using two process/cores to do this job. First core handles the first sub box and second core handles second sub-box. During time, the particles cross from one sub-box to other. I am sure I have to use MPI_Send and MPI_Recv to do this job. But what I dont understand is if I have multiple particles crossing (from one core to other) each time, how do I use efficiently MPI_Send/Recv or MPI_Sendrecv ? I forgot to say I dont know the number of particles crossing each time. Sorry If the question is too simple/silly.</p>