Try to parallel computing, but the CPU utilization is very low.

vienloer · April 19, 2021, 23:48

I tried to use 20 cores for simulation, but my cpu utilization was just 5%. what should I do to increase CPU usage? My cpu has 16 logical cores and 32 physical cores.

File "decomposeParDict",

numberOfSubdomains 20;

method simple;

coeffs
{
n (2 10 1);
}

The code,
~$ mpirun -np 20 icoFoam -parallel

Thank you for help!

LeeRuns · April 20, 2021, 09:23

Try to have number of cores same as number subdomains. Reduced cross core communication.

cyss38 · April 20, 2021, 19:06

How many cells do you have in the mesh?
The recommendation and the tests that I have been able to perform show that a good choice is to have about a minimum of 15,000 cells per process to be efficient.
Thus for 20 cores, the mesh would have to consider 300,000 cells.
In addition, I recommend the scotch mesh decomposition method.

LeeRuns · April 20, 2021, 22:23

Also pretty important to have cells evenly distributed. If one section is way complex for solver the use more cells there, but make domain smaller. Sorry about wording typing fast between tasks....try to think about your geometry and make sure that each section of the domain has a similar workload (complexity, number of cells)

Also is it really cpu that is holding you back I often find that goal io is pretty tough on machines with OF

vienloer · April 29, 2021, 08:30

Thank you for your reply. So the total mesh number is important，right? I just have 10,000 meshes, the core number can not be 20 or more, if I still try to use too many cores, the system will break, right?

vienloer · April 29, 2021, 08:38

Thank you! I will try more cases to learn more.

April 19, 2021, 23:48	Try to parallel computing, but the CPU utilization is very low.	#1
vienloer New Member Join Date: Apr 2021 Posts: 10 Rep Power: 5	I tried to use 20 cores for simulation, but my cpu utilization was just 5%. what should I do to increase CPU usage? My cpu has 16 logical cores and 32 physical cores. File "decomposeParDict", numberOfSubdomains 20; method simple; coeffs { n (2 10 1); } The code, ~$ mpirun -np 20 icoFoam -parallel Thank you for help!

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April 20, 2021, 09:23		#2
LeeRuns Member Join Date: Feb 2016 Posts: 41 Rep Power: 10	Try to have number of cores same as number subdomains. Reduced cross core communication.

April 20, 2021, 19:06		#3
cyss38 Member Cyrille Bonamy Join Date: Mar 2015 Location: Grenoble, France Posts: 86 Rep Power: 11	How many cells do you have in the mesh? The recommendation and the tests that I have been able to perform show that a good choice is to have about a minimum of 15,000 cells per process to be efficient. Thus for 20 cores, the mesh would have to consider 300,000 cells. In addition, I recommend the scotch mesh decomposition method.

April 20, 2021, 22:23		#4
LeeRuns Member Join Date: Feb 2016 Posts: 41 Rep Power: 10	Also pretty important to have cells evenly distributed. If one section is way complex for solver the use more cells there, but make domain smaller. Sorry about wording typing fast between tasks....try to think about your geometry and make sure that each section of the domain has a similar workload (complexity, number of cells) Also is it really cpu that is holding you back I often find that goal io is pretty tough on machines with OF

April 29, 2021, 08:30		#5
vienloer New Member Join Date: Apr 2021 Posts: 10 Rep Power: 5	Thank you for your reply. So the total mesh number is important，right? I just have 10,000 meshes, the core number can not be 20 or more, if I still try to use too many cores, the system will break, right?

April 29, 2021, 08:38		#6
vienloer New Member Join Date: Apr 2021 Posts: 10 Rep Power: 5	Thank you! I will try more cases to learn more.