UndoDB – The interactive time travel debugger for Linux C/C++ for debugging
50 comments
·May 23, 2025ognarb
What's the difference with RR?
mark_undoio
Undo (where I'm CTO) has existed for longer than RR and its real benefit is that it scales to use cases where RR (for one reason or another) isn't a fit.
Technically:
* Doesn't need hardware performance counters - runs on more CPUs and on cloud systems (where performance counters are often blocked).
* Can attach and detach at any time - means you get to record just a subset of program execution that's interesting.
* You can our ship recording tech with your application and control it by API, so you can grab crash recordings on customer systems.
* Supports programs that share memory with non-recorded processes.
* Supports direct device access (e.g. DPDK).
* Accelerated debugging features - searching with recordings using parallel processing, accelerated conditional breakpoints a few thousand times faster than native GDB.
* We provide a stable, patched fork of GDB that we're occasionally told is more stable than the default.
For many people's use cases none of these really matter - they should use RR if they're not already.
But if you need any of these things then Undo can give you time travel debugging. In practice, it's usually big software organisations that we deal with because they have development pain and the extreme requirements we can match.
db48x
> * You can our ship recording tech with your application and control it by API, so you can grab crash recordings on customer systems.
That’s actually pretty neat.
roca
[rr developer here]
Undo has cool features like Live Recording that we don't have in rr. They don't need access to the hardware PMU which is a big advantage in some situations. They can handle accesses to shared memory in cases where rr can't. https://undo.io/resources/undo-vs-rr/ is a good resource.
sidkshatriya
If you don't have access to the hardware PMU then you can try https://github.com/sidkshatriya/rr.soft (which is a modification of the rr debugger).
It may not be commercial quality but its open source and free :)
[I built rr.soft]
null
hugograffiti
Undo also support Java and Scala: https://docs.undo.io/java/index.html
leni536
AFAIK it records multithreaded applications on multiple threads and CPU, rr records them on a single OS thread, AFAIK. Not sure about replay. Never used undo though, so not sure how much better it is.
dzaima
rr does support multithreaded and multi-process applications, via, like Undo[1], allowing only a single thread to run at a time. (edit note - that's only about multithreading; Undo might have parallel multi-process recording)
[1]: https://undo.io/resources/undo-performance-benchmarks/ - "Undo serializes their execution"
leni536
I stand corrected, not sure where I heard this then.
kristopolous
[flagged]
gregthelaw
Undo founder here. I just got a slack from one of our marketing team who is thrilled you appreciate their work. :-)
Our customers are top tier tech companies and our users are among the smartest engineers on the planet. I'm proud of our marketing, but no-one is going to spend a bunch of money with us just because of that.
So there must be something else in it I guess ;-)
delta_p_delta_x
Free Windows equivalent: WinDbg Time Travel Debugging (https://learn.microsoft.com/en-gb/windows-hardware/drivers/d...).
mark_undoio
WinDbg's time travel debug is really cool and more people should know about it. I'm always a little sad that it's not (so far!) officially integrated in something like VS Code.
Before it was released publicly I believe Microsoft had been using it internally to share recordings on bug reports against massive pieces of software like Office. So it's a serious piece of tech.
senderista
I used it (iDNA) on the Windows team starting around 2006 or so and we were able to resolve bugs in minutes that had been open for years. It was absolute magic.
minamoto625
gdb should already have a similar feature?
mark_undoio
GDB does:
https://sourceware.org/gdb/current/onlinedocs/gdb.html/Proce...
But it's limited. It's really cool that it's integrated by default but it doesn't scale to big applications / workloads.
RR and Undo both use GDB as a user interface, though, so any skills you have there will carry over.
ho_schi
A lot people don't know and don't use GDBs reverse debugging. It is an awesome and hidden feature, which more developers should know :)
All these Oh wait. I missed it...debugging sessions. and these What exactly changed over there? are answerable.
db48x
It does, but it is really sad by comparison with rr and UndoDB. You could use it to record a few function calls or perhaps if you’re lucky a whole frame of your game but not a whole program.
drawnwren
if you could get this working on embedded arm cpus, I think you'd be surprised how many customers there would be.
Veserv
Time travel debugging on embedded ARM has been available for over 20 years via trace probes [1].
The category namer of time-travel debugging, TimeMachine, (hence time-travel debugging in contrast to other attempted names such as reversible, bidirectional, record-replay, etc.) was available in 2003 and supports/supported the ARM7 [2]. Note, that is not ARMv7 architecture, that is the ARM7 chip [3] in use from 1993-2001.
From what I know, the ARM7 was one of the first ARM designs implementing the Embedded Trace Macrocell (ETM) which could output the instruction and data trace data used to support trace probe-based time travel debugging.
[1] https://jakob.engbloms.se/archives/1564
mark_undoio
What's limiting us is that Undo does need a Linux kernel - so traditional embedded programming wouldn't be a fit. Embedded Linux could work and we do support ARM64.
I've thought I bit about how you might support time travel on bare metal embedded - but actually there are hardware-assisted solutions (Lauterbach's Trace32 was one we came across) there sometimes.
schaefer
Let me save you a click:
Pricing & Licensing
A UDB floating license costs $7,900 per year.
db48x
If you’re going to spend money, then you would be better off using rr and paying for Pernosco. Pernosco is amazing.
dima55
rr is awesome and is free and open and all that. How much better could this possibly be?
gregthelaw
Undo co-founder here. rr is indeed awesome. If it works for your use-case, you should use it!
Undo is mostly used by companies whose world is complex enough that rr doesn't work for them, and they understand how powerful time travel debugging is.
There has now been a LOT of engineering invested by a lot of very smart people into Undo, so it does also have a lot of polish and nice features.
But honestly, if rr is working for you, that's great. I'm just glad you're not doing printf debugging the whole time :)
AlotOfReading
They have a comparison page: https://undo.io/resources/undo-vs-rr/
I was in talks with them recently because I kept running into limitations with rr. The main advantages for my use case were that undo doesn't have the same dependency on hardware timers, which means the ARM support is much better, you can run it in a VM (e.g. a cloud machine) and you can do replays on different systems.
dzaima
A couple minor notes:
- If your program is very light on syscalls (i.e. basically entirely in-memory computation), rr can go to a basically 1.0x slowdown. In particular this means you can run benchmarks in it at full capacity, provided that I/O is outside of the repeated part (e.g. if sometimes the bench is noticably slower, you can replay and see if some important loads/stores crossed a cacheline/page). You can even "perf record" / "perf stat" a replay if you want to! (none of this is too useful, but it's fun! Gathering repeated stats over the same execution for more resolution might be useful with proper tooling though)
- rr does have an in-memory buffer of recording data.
- rr recordings should be portable within the architecture, as long as the replay hardware has the extensions the recorder did (or if replayer-unsupported features are disabled at record-time).
Veserv
Well, if you have a Google L5 making ~365k [1] then it would need to make them ~2.2% more productive overall to be worth it when just considering direct pay. If we consider a Google L3 at ~187k then it would need to make them ~4.2% more productive overall.
This, of course, ignores employee benefits and overhead which usually amount to ~100% extra costs over direct pay. So that is now ~1.1% and ~2.1%, respectively.
And that ignores the fact that you need to pay people less than they produce to be profitable which probably drops us down to ~0.5% and ~1.0%, respectively.
[1] https://www.levels.fyi/companies/google/salaries/software-en...
edit: Incorrectly linked to product designer instead of software engineer levels.
dima55
OK... Most of us don't know what a "google l5" is, so I guess we can safely ignore this. Heh.
eviks
The major fail of such "just a 1% / cup of coffee" is that there is an infinite number of things you could pay for with the same potential productivity promise without any hard data on whether those are true, so just the fact the you can use a calculator and divide to get to a low % doesn't help you much if at all
esafak
No-one is going to spend $8K out of pocket to A/B test this on themselves. Of all the things you could be doing to improve your productivity, this is some high hanging fruit.
null
FOSS alternative: https://rr-project.org/