There are some talks in the archive (by yours truly) that discuss the RTX - SM round trips and how it relates to anyhit invocations. See the OptiX Tools & Tricks, for example: OptiX talks from Siggraph 2019 There might be some more recent ones as well. It’s not super detailed, but I talk about the existence of the round-trip overheads, and how to think about and evaluate them, as well as a little about how to optimize (reduce) their use. The relative cost is unique to every application and every GPU model. It depends on choices the application makes, on how much shading compute there is, how large the payload is, what the caching behavior is, scene size & complexity, etc., etc. And so you really have to profile your code to find out.
As far as doing computation on the SM while the RT cores are busy, that does happen already, and it happens automatically as long as there is work to do. The SMs are pretty good about switching to work on other threads as needed anytime a thread has to wait for resources like the RT cores or VRAM or PCI transfers or anything else.
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David.