Re: max number of CPU's

[Jean-Noël Grad]

Hi,

For domain decomposition, there must be at least two cells per 
direction, and at most 32 cells per direction (for a cubic box). The 
number of cells per direction depends on the interaction range. You can 
find the algorithm implementation in [1]; it solves up to 9 equations (3 
for each direction), and there is not always a solution.

As a rule of thumb, for a cubic box without NpT, the number of cells per 
direction is often `floor(l_box / (l_cut + l_skin))`. For example, in a 
system with length l_box=12, LJ cutoff l_cut=2.5 and Verlet skin 
l_skin=0.4, the number of cells is 4 in each direction, which limits the 
number of MPI threads to 64.

For N-square, since the pair search algorithm considers all particles, 
there is no constraint from the interaction range. There are however 
special considerations regarding the number of MPI threads [2]. I'm only 
mentioning this for completeness, domain decomposition is usually the 
preferred cell system.

Best,
JN

[1] 
https://github.com/espressomd/espresso/blob/7e32e60f7789156683040c29c6333b736e3c5b4c/src/core/DomainDecomposition.cpp#L305-L360
[2] https://espressomd.github.io/doc4.1.4/system_setup.html#n-squared

On 2022-01-07 02:34, Dusan Racko wrote:
> Hi,
>
> please, is there a way to determine the maximum number of CPU's for a
> parallel job
> for a given box side length l_box, with LJ potential truncated at
> l_cut and skin set to l_skin?
> How can I estimate the number of processors based on these parameters?
>
> Dusan
>
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