I want to implement (in C++) a feature, using MPI, in an existing (non-MPI) application. I am thinking of using mpich-3.4.1 for this.
I am planning to create a .so file for that feature, which the original application can link to. I initially thought to have a function in the .so file that starts with an MPI_Init() and ends with MPI_Finalize() and, in between, calls all required MPI apis to do the parallel job. As part of the MPI job, the new feature makes the current application an MPI server by calling APIs like 'MPI_Open_port' and 'MPI_Comm_accept'. Other worker processes (possibly running on different machines) connect to this server, send/receive messages, and complete a heavy computation in parallel. The application then resumes its other non-mpi work.
It seems to me that Singleton MPI_INIT mechanism will be useful for this. I found the following page on Singleton Init: https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node254.htm
This page says, "A high-quality implementation will allow any process (including those not started with a ``parallel application'' mechanism) to become an MPI process by calling MPI_INIT. Such a process can then connect to other MPI processes...".
However, the comments in mpich-3.4.1/src/mpi/init/init.c says, "The MPI standard does not say what a program can do before an 'MPI_INIT' or after an 'MPI_FINALIZE'. In the MPICH implementation, you should do as little as possible. In particular, avoid anything that changes the external state of the program, such as opening files, reading standard input or writing to standard output."
Based on the above comments, it seems we should not have MPI_Init(NULL, NULL) and MPI_Finalize() as part of any implementation in a library. In that case, I am thinking to have the init and finalize APIs in the original application's main function, and have rest of the API calls made from the .so file. My original application is a working large software, and may not need to execute my mpi feature at all, in some situations.
My questions are:
(1) Does it make sense to have MPI_Init(NULL, NULL) and MPI_Finalize() called in the main function of this application, and rest of the MPI functionalities in a .so file?
(2) Once MPI_Init(NULL, NULL) is called in the main, would it interfere with the normal execution of the software in any way? Would there be any performance impact on the existing application?
(3) Is there an MPI implementation that handles this better?
(4) Is MPI a good approach to handle this requirement, or other mechanisms like ZeroMQ better? In the comments made by Wesley Bland in the following link, he says that "MPI may not be right for you if you're looking for a client/server model. Yes, it's possible, but it's not really optimized for that use case and you might have better luck using a different communication mechanism". Is that true in 2022? client relationship within MPI server
