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Comment by cesaref | Hacker News Reader

I've worked in low latency trading, and with large distributed back testing setups with hundreds of high core count machines, and not once felt the need to disable exceptions, or for that matter, felt the impact of them happening. A fair bit of noexcept stuff was useful to improve runtime performance, but that was about it.

I would suggest you need to address that 1% of failing tasks and determine what the issue is, as frankly, you are solving the wrong problem. If I might suggest a solution, it sounds like you are using threading when process farms might be a better solution.

(and if i'm way off the mark with my suggestion, apologies, i'm trying to help and have little information to work with).

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Even reducing it to .1% we'll still have 1000 cores or at least threads before too long. GPUs already have thousands. Prosumer CPUs already have 128 threads. Epyc supports SMT, and will be 512 threads on dual socket Bergamo in 2023 I think on the roadmap. There is an intermediate at 384 threads dual socket Genoa in 2022.

Low latency is not the issue per se. It’s more like being able to guarantee a worst case upper bound runtime. Maybe your system can tolerate indeterminate but rare worst case run times but many useful systems cannot. E.g. flight control systems but also ideally any interactive system.

Well i'm now working in a hard realtime domain (audio, noone dies when it goes wrong, but it's still hard realtime) and i've still got exceptions turned on. But no exceptions are thrown during runtime, they're there for exceptions. Yes, there's overhead in places, but we noexcept some code paths to reduce that overhead in some very hot code paths.

If you allow exceptions in your real-time audio loop then when an exception occurs the only useful thing to do would be to take down the entire process. Handling exceptions as a norm in your audio loop would be unacceptable because it could never be done in a way that would always meet the deadline. In that case, if you cannot usefully handle the exception, why have your exceptional situation cause an exception in the first place? You’re just as well off calling abort().

> If you allow exceptions in your real-time audio loop then when an exception occurs the only useful thing to do would be to take down the entire process.

as a user of audio software, I would really prefer them to have the occasional glitch instead of crashing, potentially loosing work. Like, how it is even a discussion ?

Sure but that isn’t the issue. We’re discussing the architecture of an audio loop, e.g. handling exceptions as a norm in the codebase. If it is avoided then there is no large behavioral difference between calling abort() and throwing an exception.

... I hope I just don't understand. Say you have some audio node in your engine which at some point fails because of some error condition, for instance it tried to load a preset in an invalid format which was supposed to be sanitized beforehand, the preset tried to load a sample and the OS returned -ENOPERM, whatever.

Surely you agree that it's better to have this throw an exception, and a big try.. catch around your audio callback so that just this tick, which loads the preset, fails, and then the program continues normally at the next tick, just with the wrong preset for a node, rather than calling abort() which entirely kills the program and looses what the user was working on ?

> Surely you agree that it's better to have this throw an exception, and a big try.. catch around your audio callback so that just this tick

Yes I do agree but my point is that if any exception is being thrown for the sake of being handled then your Audio code is bad.

Generally your Audio loop should encounter no errors at all (it should be a pure process) so what the OP is saying is that excepting/aborting is fine in that case because it would truly be exceptional. I can’t imagine what the exceptional situation would be but to the extent one exists, I agree with that. My only point is that throwing an exception when there is no handler is no better than calling abort(), obviating the need for enabling exceptions.

> Yes I do agree but my point is that if any exception is being thrown for the sake of being handled then your Audio code is bad.

thank god ! then we both agree.

> Generally your Audio loop should encounter no errors at all (it should be a pure process)

and audio handlers should never lock mutexes yet I have seen my fair share of them in the wild.

> My only point is that throwing an exception when there is no handler is no better than calling abort()

every thread or event loop should always have a catch-all handler at top level.

> and audio handlers should never lock mutexes yet I have seen my fair share of them in the wild.

They are incorrect. Only bounded-time synchronization primitives can be used. E.g. lock-free queues.

> every thread or event loop should always have a catch-all handler at top level.

Two things. 1) There should be no reason for a properly coded real time audio loop to have a top level handler because exceptions should not be thrown because they cannot be guaranteed to be handled within the necessary time constraints.

2) top level exception handlers divorced from context cannot be handled, only ignored. Ignoring exceptions is very dangerous because there is generally no way for the top level handler to be sure that the rest of the program is in a correct state (unless it knows all code is exception-safe, which is rare). Continuing to run can result in data corruption. Exceptions leaving the context that has the information to handle them is a programming error, just like a failed assertion.

> They are incorrect. Only bounded-time synchronization primitives can be used. E.g. lock-free queues.

yes, my point is that incorrect code exists in the wild, that's the world we have to live in.

> properly coded real time audio loop

most things aren't properly coded, and yet still useful

I see a difference between bugs and design flaws.

I'm not sure I understand the appeal of C++ exceptions. For years I've been told that Rust and Go's lack of advanced exception support is a crutch; and that error returns or sum types were unimaginative.

Now, in this thread, exceptions are supposed to be used rarely. I don't see the difference between an exception and Rust/Go's `panic`. If the error is rare enough, then chances are you cannot gracefully recover. If the error isn't rare; then why are you using exceptions for control flow?

Exceptions have always supposed to be used rarely - that's the point! The most obvious example is 'new' failing and returning std::bad_alloc. Given that new is implicit all over the place (pushing to a vector for example) the code would get very cluttered if you had to cope with this everywhere, so the pragmatic decision was taken to use exception handling to give apps that cared the chance to deal with them, and those that don't a sensible error message by default when the exception escapes main.

It's interesting that the lack of enforced exception handling was seen as a strength when exceptions were added to the language, compared to now when the lack of enforced exception handling is sometimes seen as a weakness. Programming language design and expectations are a moving target, so it'll be interesting to see how these things develop over time.

And yes, Rust and Go take a different approach. Is there right and wrong? No, i'd say just different approaches which will play to the strengths of some problem domains and coding styles.

> If the error is rare enough, then chances are you cannot gracefully recover.

that does not match my experience. Generally, rolling back to the state at the beginning of whatever user interaction caused the exception is good enough in a large amount of cases and a much better experience for end-users than abort()

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