-`magnetic_initial_condition(x, y, z)`: if defined, the solver calls this function at the beginning of the simulation in order to initialize the magnetic field. The parameters `x`, `y` and `z` represent the point at which the field needs to be evaluated. The function has to return a triple specifying the three field components, i.e. `return Hx, Hy, Hz`.
### Considerations on the evaluation of sources and the computation on GPU
Currently sources for the timestep `t+1` are evaluated asynchronously on the CPU while the GPU is computing the timestep `t`. When the GPU is done, the updated sources are uploaded from the CPU to the GPU and the cycle restarts. This usually works relatively well on big domains with high approximation order, however on small domains at low approximation order does not give any speedup. For this reason, if the sources do not need to be evaluated along the whole simulation, it is suggested to turn them off at the appropriate timestep by using the `on_timestep()` callback and the appropriate functions described below. A future version of the solver will implement repetitive boundary sources, that will be precomputed on the CPU and uploaded on the GPU only once.
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Currently sources for the timestep `t+1` are evaluated asynchronously on the CPU while the GPU is computing the timestep `t`. The sources at `t+1` are then uploaded asynchronously from the CPU to the GPU. This usually works relatively well, especially on big domains with high approximation order, however on small domains at low approximation order you can notice slowdowns. For this reason, if the sources do not need to be evaluated along the whole simulation, it is suggested to turn them off at the appropriate timestep by using the `on_timestep()` callback and the appropriate functions described below. Despite of that, I am not sure that implementing repetitive boundary sources will be necessary.
