[Bug] Render thread freeze in RayTracing_FinishGatherInstances - lost task wakeup with 50+ players (UE 5.7.4)

[Mouloud95]

Summary

Summary

Random render thread freeze (4+ seconds) in multiplayer with 50+ skeletal mesh characters
and Hardware Ray Tracing enabled (Lumen HW RT, MegaLights, Nanite, DX12).

Details

The render thread hangs at FinalizeGatherRelevantPrimitivesTask.Wait() inside
RayTracing_FinishGatherInstances (RayTracing.cpp:1974).

Unreal Insights profiling shows:

• Foreground workers are NOT saturated — idle gaps between tasks during the entire
  4.2s freeze
• TryRetractAndExecute fails in 300ns — prerequisites appear incomplete
• Oversubscription detected 3 times (4.4ms)
• The dependent task never starts despite available workers
• This points to a lost completion signal in the task scheduler — not starvation

The game thread also freezes (blocked in Slate::DrawWindows → GatherWindowElements)
waiting for the render thread.

Please select what you are reporting on:

Creative

What Type of Bug are you experiencing?

Stability

Steps to Reproduce

1. UE 5.7.4, Windows 11, DX12, Hardware Ray Tracing enabled
2. Lumen HW RT + MegaLights + Nanite + VSM + DLSS
3. 50+ players with skeletal meshes in the same area
4. Play normally — random freeze after 20s to 5+ minutes
5. Dev builds freeze less frequently (timing-dependent)

Expected Result

FinalizeGatherRelevantPrimitivesTask should complete and not block the render thread.

Observed Result

Disabling RT entirely avoids the codepath but removes all RT visual features.

Platform(s)

PC

Upload an image