Asynchronous building of acceleration structures.

Is it possible to build acceleration structures asynchronously so when a bottom level structure is marked dirty it does not introduce delay to the render time ? Would it be possible to build the structure in CPU and upload the results to GPU when they are done ?

No, the OptiX API is not thread safe.

See caveats in the OptiX Programming Guide: http://raytracing-docs.nvidia.com/optix/guide/index.html#caveats#caveats