Unity has posted a technical deep dive for the GPU Lightmapper offered as a preview in the 2018.3 release. They've partnered with AMD and chosen to use RadeonRays, an open source ray tracing library from AMD, to implement power sampling, rays compaction, and custom BVH traversal.
The deep dive goes into the design decisions, tradeoffs, pipeline design, and general architecture for the Lightmapper, including discussion of its control flow:
This producer/consumer approach allows the Scene to be continuously edited while the GPU Lightmapper is working asynchronously. The results can then be displayed in the editor when ready.
Scenes can potentially be very large and contain many lightmaps. To ensure that work is spent where it offers the most benefit to the user, it is important to focus baking on the currently visible area. To do this, we first detect which of the lightmaps contain most unconverged visible texels on a screen, then we render those lightmaps and prioritize the visible texels (off-screen texels will be baked once all the visible ones have converged).
Check out the full deep dive here: GPU Lightmapper: A Technical Deep Dive.