Document Geant4 RunManager performance issue

*Drift and diffusion visualization of charge carrier groups being transported through a high-resistivity CMOS silicon sensor.

The plot shows the situation after an integration time of 20 nanoseconds, only charge carrier groups which reached the

implant side of the sensor are drawn.*

#### Why does GeometryBuilderGeant4 warn me about reduced performance with disabled multithreading?

You might have see this log message:

```

Using Geant4 modules without multithreading might reduce performance when using complex geometries, please check the documentation for details

```

You might want to set `multithreading=true` and `workers=1` instead of instead of `multithreading=false` if this is allowed

by the module configuration.

The reason behind message this is explained more detailed in [Section 14.1](../14_additional/01_tools.md#geant4-interface).

benefiting from the multithreading feature while the VisualizationGeant4 module uses `RunManager` to be able to visualize the

particles passage through the detectors.

{{% alert title="Note" color="info" %}}

The `MTRunManager` significantly reduces Geant4's run initialization time (this happens before every event in Allpix Squared)

compared to Geant4's stock run managers (see [Bugzilla/Geant4 2527](https://bugzilla-geant4.kek.jp/show_bug.cgi?id=2527) for

details).

It is not feasible to implement this improvement in the single-threaded `RunManager` since it directly inherits from Geant4's

stock `G4RunManager`. With this run manager, the run initialization time scales with the complexity of the geometry and can -

*in the worst case scenario* - take significantly more time than the actual simulation itself.

Thus it is recommended to use multithreading when using Geant4 in Allpix Squared if allowed by the module configuration.

Allpix Squared allows to use multithreading with only one worker as alternative to `multithreading=false`, though it is

suggested to benchmark the performance for both cases to find the optimal setting for the given geometry.

{{% /alert %}}

## Runge-Kutta integrator

A fast Eigen-powered \[[@eigen3]\] Runge-Kutta integrator is provided as a tool to numerically solve differential equations