before I ask my question and describe my problem, I want to summarize what I did so far and what I experienced, so that it might make more sense.
I installed LIGGGHTS-Public and Paraview according do the Tutorials and Guides in this forum and the doccumentation (using make auto). Everything works fine, I can run simulations and they look good so far.
So I wanted to go a step further and ran the simulations in parallel (using -np numberofparalleljobs) this kind of worked as well. The more I incresed the number of parallel jobs the faster the simulation became until n-1 cores (I use a Intel Core i5 @ 2.80GHz × 6 ). After that the calculation slows down again.
The next step was to use the cluster provided by my lab. I installed openmp on another PC in the network and ran some mpirun commands to see if I could remotely control the other PC and everything was fine. I created a shared folder on our Public Server where I placed the executable lmp_auto and all the necessary files and setup a HOSTFILE with the IP adresses of my PC (the master PC) and the other PC. I then ran the command
mpirun --hostfile HOSTFILE ./lmp_auto < in.sand
It worked but was not very fast. The simulation ran on 10 procs without me defining -np XX. I tried increasing the proc count with -np to 16 (the other PC is a Intel Core i7 @ 3.40GHz × 12) and it slowed down quiet a bit.
mpirun -np 16 --hostfile HOSTFILE ./lmp_auto < in.sand
I cannot make out what the problem here is and this is where I am stuck right now.
So my questions are*
Why are my parallel jobs on two PC slower with more procs even though there are still cores that are not used?
How do I fix the problem?
Is there anything I am doing wrong here?