[gmx-users] MPI runs on a local computer

[Xu, Jianqing]

Dear all,

I am learning the parallelization issues from the instructions on Gromacs website. I guess I got a rough understanding of MPI, thread-MPI, OpenMP. But I hope to get some advice about a correct way to run jobs.

Say I have a local desktop having 16 cores. If I just want to run jobs on one computer or a single node (but multiple cores), I understand that I don't have to install and use OpenMPI, as Gromacs has its own thread-MPI included already and it should be good enough to run jobs on one machine. However, for some reasons, OpenMPI has already been installed on my machine, and I compiled Gromacs with it by using the flag: "-DGMX_MPI=ON". My questions are:


1.       Can I still use this executable (mdrun_mpi, built with OpenMPI library) to run multi-core jobs on my local desktop? Or the default Thread-MPI is actually a better option for a single computer or single node (but multi-cores) for whatever reasons?

2.       Assuming I can still use this executable, let's say I want to use half of the cores (8 cores) on my machine to run a job,

mpirun -np 8 mdrun_mpi -v -deffnm md

a). Since I am not using all the cores, do I still need to "lock" the physical cores to use for better performance? Something like "-nt" for Thread-MPI? Or it is not necessary?

b). For running jobs on a local desktop, or single node having ...  say 16 cores, or even 64 cores, should I turn off the "separate PME nodes" (-npme 0)? Or it is better to leave as is?

3.       If I want to run two different projects on my local desktop, say one project takes 8 cores, the other takes 4 cores (assuming I have enough memory), I just submit the jobs twice on my desktop:

nohup mpirun -np 8 mdrun_mpi -v -deffnm md1 >& log1&

nohup mpirun -np 4 mdrun_mpi -v -deffnm md2 >& log2 &

Will this be acceptable ? Will two jobs be competing the resource and eventually affect the performance?

Sorry for so many detailed questions, but your help on this will be highly appreciated!

Thanks a lot,

Jianqing



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