The distinction that we often have trouble getting across is between eliminating/controlling nondeterminism, vs recording what happens to occur.

Undo, like rr, and Microsoft TTD, records basically all syscalls, but doesn’t determinize anything in the original execution, only in the replay. A “hermit run” call is like a 0-byte recording —- no nondeterminism so you can “replay” by just running again.

On the overhead, the largest factor is the means of program instrumentation. I don’t know where rr sits, but I’ve heard Microsoft’s solution is quite performant on Windows.