[gregthelaw]

Co-founder of Undo here. This is a common misunderstanding, and just not true -- neither for Undo nor rr. Most races will reproduce at least as easily in Undo, especially if you use our "thread fuzzing" feature (rr has something similar, called chaos mode).

Sure, there will always be some races/timing issues that just won't repro under recording (Heisenberg principal and all that), but in fact most races are _more likely_ to occur under recording. Part of this is because you slow down the process being recorded, which is equivalent to speeding up the outside world.

And of course, when you do have your gnarly timing issue captured in a recording, it's usually trivial to root-cause exactly what happened. Our customers tell us that races and timing issues are a major use-case.

[matu3ba]

Thanks for clarification. I have not been aware of thread fuzzing and chaos mode before.