brians

EP nineteenma
Every time I hit render in Karma, I need to wait ~15–20 seconds of black screen
before seeing the first pixel.

This is via MPlay? Or does this happen in IPR (ie the viewport) too?
If only with MPlay try setting this envvar and let us know if the stall still happens.

KARMA_XPU_OPTIX_FAST_COMPILE_OVERRIDE=1

Try tweaking the variance-oracle settings (or disable it completely)

smbell
I'm not sure what it does, or if there are any downsides to this?

Optix has two different paths for compilation (PTX vs OptixIR). Each has different bugs in different drivers, so we internally choose one or the other depending on driver version. However, it's helpful to also have it on an environment variable for testing/debug purposes.

Any downside is that you might notice a performance difference (either faster or slower, depending on the scene).

I guess that you might have some cache corruption.
Could you try this?

remove the KARMA_XPU_OPTIX_FORCE_OPTIXIR envvar

delete these two directories
C:\Users\USERNAME\AppData\Local\NVIDIA\OptixCache
C:\Users\USERNAME\AppData\Local\Temp\houdini_temp\VEX_CodeCache

then try again. I guess that things should work again for you.
I've seen this cache corruption elsewhere, so we are starting to look into it.

Thanks