[berkut]

But that doesn't really scale - unless the per-thread stuff is small (and then you'd have to page on a sub-mesh level - i.e. split up the mesh spatially and page it as you traverse through it for internal scattering or something), if you double the number of threads, the memory used for threads by doubling the number of threads will obviously double as well.

For stuff like hair where you need loads of scattering events or for SSS doing that (paging subsections of shapes) is just going to thrash any geometry cache you have unless they're quite big.

And I'm still not convinced by ray sorting: it means your light integration is extremely segmented into artificial progressions as you expand down the ray tree, and makes anything regarding manifold exploration (MLT or MNEE) close to impossible to do well.