![octree scaling gpu vs cpu](octree_scaling.png)

The `hotwheels` tree build can run either in serial or in parallel. The parallel version can run both on multiple cores and or on a GPU using OpenMP+Target directives.

Here above there is the scaling test for insterting `1e7` particles into the tree. As you can see the algorithm scales very well with openmp threads. The GPU code scales as a CPU code with 4-8 threads. Therefore it is suggested to use this setup in situations where the number of cores per MPI rank is limited. Otherwise, for OpenMP-dominated runs, the CPU tree build scales much better than the GPU.

Improvement is in progress and things may vary in the future. Here below a job script for running the scaling test on Leonard BOOSTER machine.

```bash
::include{file=leonardo_booster.sh}
```