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## Re: max number of CPU's

 From: Jean-Noël Grad Subject: Re: max number of CPU's Date: Fri, 07 Jan 2022 18:37:47 +0100 User-agent: Roundcube Webmail/1.3.17

```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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```
```

```