CODEWITHSUNDEEP
c++cc++11mpi
B. Vandenbroucke
B. Vandenbroucke

Reputation: 126

Detecting not using MPI when running with mpirun/mpiexec

I am writing a program (in C++11) that can optionally be run in parallel using MPI. The project uses CMake for its configuration, and CMake automatically disables MPI if it cannot be found and displays a warning message about it.

However, I am worrying about a perfectly plausible use case whereby a user configures and compiles the program on an HPC cluster, forgets to load the MPI module, and does not notice the warning. That same user might then try to run the program, notice that mpirun is not found, include the MPI module, but forget to recompile. If the user then runs the program with mpirun, this will work, but the program will just run a number of times without any parallelization, as MPI was disabled at compile time. To prevent the user from thinking the program is running in parallel, I would like to make the program display an error message in this case.

My questions is: how can I detect that my program is being run in parallel without using MPI library functions (as MPI was disabled at compile time)? mpirun just launches the program a number of times, but does not tell the processes it launches about them being run in parallel, as far as I know.

I thought about letting the program write some test file, and then check if that file already exists, but apart from the fact that this might be tricky to do due to concurrency problems, there is no guarantee that mpirun will even launch the various processes on nodes that share a file system.

I also considered using a system variable to communicate between the two processes, but as far as I know, there is no system independent way of doing this (and again, this might cause concurrency issues, as there is no way to coordinate system calls between the various processes).

So at the moment, I have run out of ideas, and I would very much appreciate any suggestions that might help me achieve this. Preferred solutions should by operating system independent, although a UNIX-only solution would already be of great help.

Upvotes: 5

Views: 2399

Answers (1)

Zulan
Zulan

Reputation: 22670

Basically, you want to run a a detection of whether you are being run by mpirun etc. in your non-MPI code-path. There is a very similar question: How can my program detect, whether it was launch via mpirun that already presents one non-portable solution.

Check for environment variables that are set by mpirun. See e.g.: http://www.open-mpi.org/faq/?category=running#mpi-environmental-variables

As another option, you could get the process id of the parent process and it's process name and compare it with a list of known MPI launcher binaries such as orted,slurmstepd,hydra??1. Everything about that is unfortunately again non-portable.

Since launching itself is not clearly defined by the MPI standard, there cannot be a standard way to detect it.

1: Only from my memory, please don't take the list literally.

From a user experience point of view, I would argue that always showing a clear message how the program is being run, such as:

Running FancySimulator serially. If you see this as part of mpirun, rebuild FancySimuilator with FANCYSIM_MPI=True.

or

Running FancySimulator in parallel with 120 MPI processes.

would "solve" the problem. A user getting 120 garbled messages will hopefully notice.

Upvotes: 0

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