It seems to me the engine always processes a fixed amount of work and does not react to outer actions until that package is done. Don't know if including a more direct communication within the tracing would cause much unwanted overhead.
If it's negligible, i want this as well.
Sometimes when i'm impatient and the render was for preview purposes, i close the command line window (mxcl...), that's the quickest way to kill everything.


- By Jochen Haug