Hhi @motorsep, I have looked into this, basically CPUL and LGPUL use Swarm. Swarm clears the baking cache, so everything rebuilds every time.
Epic GPU Lightmass is built on DXR which uses .ush (Unreal Shaders a dervative of HLSL). Directx 12 MS examples uses HLSL and the MS driver code clears the Shader Cache for HLSL .
I cannot see anywhere in Epic GPUL the cache being cleared except when you first go into the plugin. This imples you have to close the Epic GPUL plugin and restart the Plugin. Therefore you cannot do multiple builds as the Shader Cache is not cleared.
LGPU In Swarm on the menu bar you can clear cache manually, which messes up bake picture!.
Epic GPUL never tests for Directx 12 Tier 1 needed for Shader 6!, but they bypass all this by just call it anyway.
I found this easy description of the tiers
DXR 1.0 with separated ray generation and hit point shaders is preferred for NV and highly recommended for Intel.
DXR 1.1, meaning the inline tracing feature, where we put a traceRay function in a compute shader and then process the result in the same shader, is often faster for AMD.
But inline tracing prevents any reordering optimizations the HW might do (traversal reordering), or actually does for sure (hit point sorting for coherent shading).
In the description above the use of DXR 1.1 is unclear
This all made very clear to me in the post
Solved : Cant Enable RayTracing in UE5 (gtx 1060) patch
UE_LOG(LogD3D12RHI, Log, TEXT("Ray tracing emulation is now enabled. Use at own risk. D3D12 tier 1.1 check ignored. Tra-La-La!"));
GRHISupportsRayTracing = RHISupportsRayTracing(GMaxRHIShaderPlatform);
GRHISupportsRayTracingShaders = GRHISupportsRayTracing && RHISupportsRayTracingShaders(GMaxRHIShaderPlatform);
This emulation ensures the every NVIDA graphics card works the same.
MSI RTX 3060 needs a manual firmware upgrade to support D3D12 tier 1.1
Nvidia RTX 3060 is automatically tests and asks user permission to upgrade firmware.
This probably applies to other RTX manufacturers as well