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Comment by killingtime74 | Hacker News Reader

I'm not sure what "solved" would mean for GC. Can't there always be improvements in maintainability, pause duration, correctness, throughput etc. Many dimensions to continually improve on.

Unless someone comes up with a no pause, no compute power no code fully correct GC?

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There are some dimensions yes, but I do wonder if GC research is starting to approach the point of being "solved" well enough that we'll see a drop in investment. Not yet, clearly, but approaching that territory.

If you look at the very latest JVMs you have three GCs which a spread over throughput-optimized (parallel), a middle ground jack of all trades (g1), and latency-optimized (zgc). They're a lot more self configuring in the past, so no matter what needs your app has it's usually just one CLI switch to get something fairly optimal.

But ZGC is going generational, at which point it will have significantly higher throughput than before. Also, all the JVM GCs can handle massive heaps these days (like, terabyte scale). So at that point the choice might become even simpler: just use ZGC all the time unless you can tolerate higher pauses and want to reclaim some CPU for the app, in which case go with parallel. And in many cases the right tradeoff between latency and performance could be inferred automatically from the frameworks and libraries that are on the classpath, meaning that for most apps even needing to make that choice could go away.

If you have a GC that doesn't pause, can handle terabyte sized heaps, has great throughput, is largely or completely self-configuring, is portable, open source, supports dozens of languages and has clean code, there are really very few metrics left to optimize after that. It seems like HotSpot is nearly there, especially when combined with GraalVM for language support.

> If you have a GC that doesn't pause, can handle terabyte sized heaps, has great throughput, is largely or completely self-configuring, is portable, open source, supports dozens of languages and has clean code

Yeah. If.

That's basically ZGC Generational + GraalVM. So, code that's available now albeit you'd have to compile it from version control (graalvm stable releases don't support zgc yet, released zgc isn't generational yet). Give it another year or so and that'll be shipping out of the box in ordinary JVMs.

No pauses and high throughput? Do you have any links to how it does this? I'd really like to know.

That's the subject of the video being discussed. More technical info here and in the links: https://openjdk.org/jeps/439

Appreciated.

The hardware still evolves, so at the least, garbage collectors will have to be tuned or self-tune themselves for that.

I also think it’s unlikely that a GC can be written that handles both terabyte sized heaps on machines with hundreds of cores and megabyte sized heaps on single-CPU systems well.

Single GC, no. Single VM, yes. HotSpot serial GC is designed for the latter use case. I forgot to mention it before but it's still there and supported, I think even selected automatically in some cases.

But there aren't so many single core systems with megabyte sized heaps anymore. Even watches are multi-core these days.

Are there any production-ready GCs that have strictly bounded latency? I'm not aware of any, and that means that GC is still too immature for use in about 95% of the computers in my house, the deeply embedded ones.

Metronome did this for embedded hard realtime use cases. I don't know much about it. ZGC has bounded pause latency at least but it's not really meant for very small machines. Android has used a pauseless GC for a while.

Strictly bounded anything (hard real-time) is an entirely different category, with very few uses for the average person - and contrary to what one might think from its name, it is not “fast” at all. It’s mostly about “calculating the trajectory of that incoming projectile should definitely happen in 0.3s”, which is many orders of magnitude more than what a modern computer would take to do 3 multiplications.

But to answer your question, there is JamaicaVM and many niche JVM implementations with hard real time GCs — but it’s simply false that a GC would be somehow a problem for most of your devices.

Hard real-time has a huge amount of uses for the average person. I just made toast -- the software-controlled thermostat in my toaster oven has (slow) hard bounds on its sense-to-control latency to avoid dangerous overheating. Before that, I brushed my teeth -- the brushed motor controller detects a motor stall by increased current if I brush too hard and stall it, shuts down the motor to avoid overheating (with a hard real-time sense-to-actuate constraint), and beeps to inform me. Then I spent some time on HN, with a laptop with PMICs and other microcontrollers that have hard real-time constraints all over the place. By far the majority of my computer interactions so far today have been with computers that have hard real-time constraints, and I haven't even gotten in my car yet!

Java already has that GC, it's called Epsilon https://blogs.oracle.com/javamagazine/post/epsilon-the-jdks-...

Epsilon doesn't GC at all?

I think it was a tongue in cheek reply, as it is indeed a no-code, no-pause, no-compute power GC that does eventually reclaim the garbage (at process exit).

Correct me if I am wrong but does it really GC at all if you receive an OutOfMemoryError?

It’s meant for short lived apps where you simply wouldn’t get OOM either way. I don’t know about an objective definition for what a GC is, one I quite like is “gives the illusion of infinite memory”, based on that it isn’t a GC.

But there are also conservative GCs, RCs, etc, that fail different definitions also (leaving behind circular dependencies, having false positives, in reverse order), like in case of many things in CS, there isn’t an objective definition.

Thank you for the explanation, I think I get it now!

Theoretically someone could perhaps one day discover the theoretically optimal GC algorithm, and someone else could write the TeX of GCs using that algorithm, and then perhaps we'd have the final GC.

Extremely unlikely that such an algorithm exists of course, and even if it did, looking at sorting will quickly suggests that improvements will keep being found even if the best asymptotic complexity had been achieved.

> Unless someone comes up with a no pause, no compute power no code fully correct GC?

Functional programming languages can be: https://www.reddit.com/r/haskell/comments/d5d13i / https://archive.is/4iyaG

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