Comment by jasonwhite

Since CPU is a "compressible" resource, system load doesn't affect the determinism. It'll make it slower of course. Since memory is a non-compressible resource, things can start getting killed by the OOM-killer and there's nothing we can do about it. There are also certainly things like external network communication and file system access that are non-deterministic that must be handled at a higher level (e.g., with a reproducible FS image or by recording & replaying network traffic).