TARDIS: Affordable Time-Travel Debugging in Managed Runtimes Earl T. Barr Mark Marron University College London Microsoft Research
[email protected] [email protected] Abstract 1. Introduction Developers who set a breakpoint a few statements too late or Developers spend a great deal of time debugging. Skilled de- who are trying to diagnose a subtle bug from a single core velopers apply the scientific method: they observe a buggy dump often wish for a time-traveling debugger. The ability program’s behavior on different inputs, first to reproduce the to rewind time to see the exact sequence of statements and bug, then to localize the bug in the program’s source. Re- program values leading to an error has great intuitive appeal producing, localizing, then fixing a bug involves forming but, due to large time and space overheads, time-traveling and validating hypotheses about the bug’s root cause, the debuggers have seen limited adoption. first point at which the program’s state diverges from the in- A managed runtime, such as the Java JVM or a JavaScript tended state. To validate a hypothesis, a developer must halt engine, has already paid much of the cost of providing core program execution and examine its state. Especially early in features — type safety, memory management, and virtual debugging, when the developer’s quarry is still elusive, she IO — that can be reused to implement a low overhead time- will often halt execution too soon or too late to validate her traveling debugger. We leverage this insight to design and current hypothesis.