Context: I'm debugging a simulation code that requires that the number of MPI threads does not change when continuing the simulation from a restart file. This code was running on a large cluster, but I'm debugging it on a smaller local machine so that I don't have to wait to submit the job to a queue. The code requires 72 threads, which is more than the number of cores on the local machine. This is not a problem in itself - I can run with more threads than cores, and just take the performance hit, which is not a major issue when debugging.
The Problem: I want to leave some cores free for other tasks and other users. For instance, if my small local computer has 48 cores, I want to run my 72 threads on, say, 36 cores, and leave 12 cores free. I want to debug my large code locally without completely taking over the machine.
Assuming I'm willing to deal with the memory and performance issues of running on more threads than cores, how do I actually do this? Do I have to get into the back-end of the scheduler somehow? Does it depend on whether I'm using MPICH or Open-MPI etc?
I'm essentially looking for something like mpirun -np 72 --cpus-per-proc 0.5, if that were possible.
taskset -c 0-35 mpiexec -np 72 ./a.out should do the trick if the process are to be launched all on the same host and should work with basically all MPI distributions (Open MPI, MPICH, Intel MPI, etc.). Also, make sure to disable any process binding by the MPI library, i.e. pass --bind-to none for Open MPI 1.8+, -bind-to none for MPICH with Hydra or -genv I_MPI_PIN=0 for Intel MPI.