To add to this, yes, we recommend that the CUDA context exists prior to a call to MPI_Init. That is because a CUDA context needs to exist prior to MPI_Init as the library attempts to register some internal buffers with the CUDA library that require a CUDA context exists already. Note that this is only relevant if you plan to send and receive CUDA device memory directly from MPI calls. There is a little more about this in the FAQ here.
From: Matthieu Brucher [mailto:firstname.lastname@example.org]
Sent: Wednesday, December 14, 2011 10:47 AM
To: Open MPI Users
Cc: Rolf vandeVaart
Subject: Re: [OMPI users] How "CUDA Init prior to MPI_Init" co-exists with unique GPU for each MPI process?
Processes are not spawned by MPI_Init. They are spawned before by some applications between your mpirun call and when your program starts. When it does, you already have all MPI processes (you can check by adding a sleep or something like that), but they are not synchronized and do not know each other. This is what MPI_Init is used for.
2011/12/14 Dmitry N. Mikushin <email@example.com>
For GPU Winter School powered by Moscow State University cluster
"Lomonosov", the OpenMPI 1.7 was built to test and popularize CUDA
capabilities of MPI. There is one strange warning I cannot understand:
OpenMPI runtime suggests to initialize CUDA prior to MPI_Init. Sorry,
but how could it be? I thought processes are spawned during MPI_Init,
and such context will be created on the very first root process. Why
do we need existing CUDA context before MPI_Init? I think there was no
such error in previous versions.
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