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Subject: [OMPI users] Problems with computation-communication overlap in non-blocking mode
From: Velickovic Nikola (nikola.velickovic_at_[hidden])
Date: 2014-03-07 09:49:20

Dear all,

I have a simple MPI program with two processes using non-blocking communication illustrated bellow:

process 0: process 1:

MPI_Isend MPI_Irecv

compute stage compute stage

MPI_Wait MPI_Wait

Actual communication is performed by offloading it to another thread, or using DMA (KNEM module is used for this).
Ideally what should happen is that process 0 issues a non-blocking send, process 1 receives the data
and in the meantime (in parallel) the CPU cores where the processes run are doing the compute stage.
When compute stage is completed calling MPI_Wait wraps up the communication.

When I profile my application it turns out that actual communication is initiated with MPI_Wait (significant amount of time is spent there) and hence disables overlapping
communication and computation since MPI_Wait is called after the compute stage.
Computation in my test case takes more time than communication so MPI_Wait should not be consuming significant amount of time since the communication should be over by then.

This I confirmed also by using MPI_Test instead of MPI_Wait.
MPI_Test has the same effect as MPI_Wait (to the best of my knowledge) but is non-blocking.
When placing MPI_Test strategically in the compute stage it initiates the communication and a certain communication-computation overlap is achieved.

Could you please shed some light for me if I am doing something wrong with the library?
Is it the way it should behave (MPI_Wait initiates the actual transfer)?
How to achieve communication-computation overlap?