Refactore Plugin `SumCurrents`

[psychocoderHPC]

• the plugin SumCurrents is wrong named and should be called SumCurrentChange invalid request, the current name is absolutely correct since the result is multiplied with the according area of the cell-plane facing it (ax3l)
• use reduce for sum current (see plugin EnergyFields)
• add MPI_Reduce (or libPMacc's version of it) and write only from one rank (in a text file) (ax3l)

[ax3l]

What is a SumCurrentChange?

I thought this plugin just adds up J which is a SumCurrentDensity.

[psychocoderHPC]

Might be my suggested name is not good, but is it not the change of J

[ax3l]

Hm, from checking the source on my cell phone, it looks like a simple globsl reduce of fieldJ:

https://github.com/ComputationalRadiationPhysics/picongpu/blob/dev/src/picongpu/include/plugins/SumCurrents.hpp

That means the current name "SumCurrent" (or what I proposed "SumCurrentDensity" to be more precise but dbl check factor for that) is valid ;)

[psychocoderHPC]

You are right but our FieldJ is not the current, it's the change of the current in one time step. Is it right? Maybe I am wrong.

[ax3l]

Close. Our FieldJ (as I documented inline long time ago in it's header file) is a current density which is the change of the charge over time and divided by the area it went through :) (See Ampère's circuital law in the Maxwell's equations.)

That's why I proposed to either call it "SumCurrentDensity" or to multiply it with the cell Volume (if it isn't already) - which makes it a "SumCurrent".

Update: I just opened my laptop and checked the sources. Since the result of the currentDensity-sum of FieldJ is multiplied with the area of the cell plane normal to the component, the current name is absolutely valid. I took the freedom to update your description.

[ax3l]

I think adding a MPI_Reduce is already sufficient, so only rank 0 creates output. Of course, we can also use the libPMacc reduce to reduce the code further.

[psychocoderHPC]

OK that's fine, than there is only the refactor part open. :+1: