Optimising for Concurrency: Comparing the BEAM and JVM virtual machines(erlang-solutions.com) |
Optimising for Concurrency: Comparing the BEAM and JVM virtual machines(erlang-solutions.com) |
There's also the Quasar library that adds fiber support to existing Java projects, but its mostly unmaintained since the maintainers were pulled in to work on Project Loom.
Then there's Project Loom, an active branch of of OpenJDK with language support for continuations and a fiber threading model. The prototype is done and they're in the optimization phase. I expect fibers to land in the Java spec somewhere around JDK 17.
I figure its fair to mention these as the authors criticisms are somewhat valid but will not be for very long (few years max?)
In summary: Java will have true fiber support "soon". This will invalidate the arguments for Erlang concurrency model. They are already outdated if you are okay using mixed java/kotlin coroutines or Quasar library
The newer Java GC's Shenandoah and ZGC address authors criticisms of pause times. They already exist, are free, and are in stable releases. Dare I say they are almost certainly better than Erlang's GC. They are truly state of the art, arguably far superior to the GC's used in Go, .NET, etc. Pause times are ~10 milliseconds at 99.5%ile latency for multi terabyte heaps, with average pause times well below 1 millisecond. No other GC'ed language comes close to my knowledge. His points 1 and 2 no longer exist with these collectors. You don't need 2X memory for the copy phase and the collectors quickly return unused memory to the OS. This has been the case for several years.
Hot code reloading. JVM supports this extensively and its used all the time. Look into ByteBuddy, CGLIB, ASM, Spring AOP if you want to know more. Java also supports code generation at build time using Annotation Processors. This is also extensively used/abused to get rid of language cruft
What about failure domains? As far as I'm concerned, this is the strongest reason for actor-based concurrency. I can design my architecture so that groups of processes that need to die together die together. And it's usually one or two lines of code, if any.
Here's a real life example. I have a process that maintains an SSH connection to a host machine, and that ssh connection is used to query information about running VMs on that host machine. If the SSH connection dies, it kills the process that is tracking the host machine, which in turn kills the processes tracking the associated VMs, without perturbing any of the other hosts' processes or vms. This triggers the host process to be restarted by a supervisor, which then creates a new SSH connection to query for information (possibly repopulating VM processes for tracking information). All of this I wrote zero lines of code for (which, importantly, means I made no mistakes), just one or two configuration options. More importantly, the system doesn't get stuck in an undefined state where complex query failures can cause logjams in the running system.
In Java you would create a thread pool and configure it to restart the threads if they die. Each thread would wake up every so often to query SSH and dump their results into a queue. If the query threads die, the processes reading the queue at the other end have nothing to do so they won't execute. Its easy to make a consumer queue that executes some code on another thread whenever data arrives.
Java's exposure of the underlying OS threads and cheap transfer of data between threads lets people build libraries on top that offer memory models used by Erlang and others. Its not built in or quite as convenient, but you can use actors and fibers in Java if you want to.
The failure domain here isn't precisely defined because shared data is allowed (but not required). You could define it as "anything reachable from the thread/fiber stack".
It would allow you to write something like:
try (var scope = FiberScope.open(Option.PROPAGATE_CANCEL)) {
var fiber1 = scope.schedule(() -> sshKeepAlive());
var fiber2 = scope.schedule(() -> trackHost());
var fiber3 = scope.schedule(() -> trackVMs());
}
Does Java's Hot code reloading support data migration? One benefit of Erlangs model is that you can execute hooks when HCR is performed to make sure your data in memory is migrated to a new format.
But really, the most important thing about Erlangs actor model is error handling. If I spin up a process in Erlang and it fails, it won't corrupt the state of my other processes. In Java this can only be attained through disipline since all memory is shared. Also, I can very easily specify which processes should work together as units, such that if one fails, they all fail, and can be restarted together from a known working state. This, again, requires discipline in Java.
Not sure what you mean by data migration on code reloading. I suspect the mechanisms are different enough that it can't be compared. With Java you can load arbitrary new code, but changes to existing code are limited in ways that prevent data incompatibilities. For example you can add fields to existing object but you can't change the type of existing ones.
Data corruption from threading is rare in Java. I can't remember the last time I ran into it. Its easy to do but everyone is used to threads and the concurrency implementation is one of the best I've used. Java also supports thread groups to ensure that threads die and get restarted together. Its not automatic, you need to manage the groups, but I think it achieves the same.
As a counter-point, I've been working on a platform for the last few years which uses Kotlin and Quasar in production. Quasar was cool at first but now it's just a nightmare and I wish we never opted to use it. It leaks abstractions all over the place with @Suspendable annotations and users of the platform find the quasar related errors super confusing. Debugging is also very difficult because of Quasar. On the other hand, Kotlin is great!
If I could turn back the time, I'd build the messaging/async workflow part of the platform using Erlang. I've mentioned this to a few people but they all think I'm mad... "Erlang... are you on drugs?!", which is disappointing because it's literally perfect for our use case.
Why didn't you use Kotlin coroutines? My understanding is that they achieve the same as Quasar without the insanity.
You may also want to look at Vert.X. Its evolved into a lot more than a REST framework. It uses thread-per-core and nonblocking to achieve high performance instead of green threads. It theoretically performs better because there's not a lot of stacks hanging around and only 1 thread per core. There's a lot of callbacks though, so if you're not used to RxJava style chaining its hard to get used to. Its very much like Node.
Erlang or Go would be the easiest if you need a lot of threads. If you just need high performance with a lot of connections, Vert.X may suffice. Java IO in recent years is fully non-blocking so you don't need a lot of threads for high concurrency. Vert.X can handle millions of concurrent clients, enough that you will need to adjust your kernel to hit its limits. And its built on Netty which is rock solid.
Java's new garbage collectors, ZGC and Shenandoah, have average pause times of 0.3 milliseconds on heaps less than 4GB. I find it unlikely that another language has pause times shorter than that given the sheer amount of work put into Java GC over the years
The biggest problem with Erlang is that hardly anything out there needs this level of concurrency and robustness in a single system - in the new world of microservices and serverless architectures there are other ways to cope with scaling. This is the main selling point, and unfortunately in all other areas Erlang is significantly outdated and refuses to evolve - even less so than the Java language which is a dinosaur in itself.
Having said that I think Erlang is a fantastic teaching tool and should be on everyone's bucket list of "things to learn in this life as a software engineer".
I wondered about this myself before I started using Elixir. In practice, it turns out when it's cheap to make things concurrent more services take advantage of this feature.
Tests and the elixir compiler are extremely fast because of this, and it makes the whole development experience better.
Because the primitives are so simple, people experiment more which makes better software. Nobody would come up with phoenix live view for Play framework in their spare time because play framework is so overly complicated.
I did an interview at a major streaming company and one critical part was written in Erlang. It has been working great but the guy who wrote it had left for some time and nobody knew Erlang there, so they would have to rewrite it if an update was needed.
I never understood this often repeated point. As junior / mid-level developer I had the privilege to run self written .jar files on government scale systems with more than 50 cores. I used Java thread pools and concurrent data structures to do heavy cross thread caching.
It was all pretty simple and concurrency & parallelism were never an issue but simply a necessity to make things run fast enough.
Am I a concurrent programming genius? Were the types of problems/challenges I was solving too simple? When is concurrency in Java ever hard+?
+ I know about Java masterpieces like the LMAX Disruptor that are mostly beyond my skill level, but those are low level writte-once libraries you wouldn't write yourself.
The article states benchmark of 5000% speedup on floats when switching from BEAM to the JVM. I would like to offer $100 as a gift incentive to anyone here who wants to work on optimizing BEAM math.
I'm trying to learn Elixir and being a systems thinker so before I (can) get too comfortable I'm gonna want to dive into origin stories to build up my holistic map of why things are the way they are, what can be done and what can't be done, and understanding bottlenecks in the BEAM seems like it's gonna have to be part of that (the way I studied JVM tech documentation when I did perf and architecture work in Java)
As I understand it, it is feature complete and actually runs Erlang pretty well. Could be interesting to see some benchmark testing.
Potentially-racey stuff:
* Synchronized primitives don't compose. You can safely `synchronized get(...)` and safely `synchronized put(...)`. But their composition put(get(...)+1) isn't synchronized. And it's hard to mentally revisit it at the end of the day: if you have a class with some methods marked synchronized, nothing will tell whether you've synchronized the right methods. You just have to think it through again and hope you reach the same conclusions as before.
Other (non-racey) stuff:
* Threads are heavy, CompletableFutures are light. But CFs lack the functionality of Threads. A CF can't decide to sleep for a while, nor can it be cancelled. (As an aside, BEAM threads are super light).
You can achieve arbitrary non-blocking delays by using the cruft scheduled thread executor or doing it sanely with RxJava. Really its just dangerous to do nonblocking stuff in Java without a wrapper like RxJava. That's not a good thing, I look forward to the day there's real fibers
Since JDK8: https://docs.oracle.com/javase/8/docs/api/java/util/concurre...
Clojure Concurrency - Rich Hickey https://www.youtube.com/watch?v=dGVqrGmwOAw
Even though the talk is called clojure concurrency, first half of the talk is about the problems clojure solving in traditional concurrency.
one my favorite talks I ever went to.
A lot of developers are not aware of what thread is going to execute their code, or of what that implies (I think it takes practice, at least it did for me), and in my experience it often leads to shared mutable state without proper guards, or deadlock hell from locks being created all over the place in hope to make things safe, or other nightmares.
>I know about Java masterpieces like the LMAX Disruptor that are mostly beyond my skill level
Both the basic idea of the Disruptor, and its simplest implementation (mono publisher, mono subscriber), are pretty simple: just using minimal memory barriers to write and read data cycling on an array, and (busy-)wait whenever you bump into whoever is ahead (the publisher if you're the subscriber, or the subscriber if you're the publisher).
Quoting one of its authors:
« Sometimes we have absolutely no choice and we need to go parallel and use a lot of concurrency. If you do, get people in who are good at it. And actually, I found most of the people who are really good at it, their instinct is they'll do it as an absolute last resort, because they know how complicated it actually gets. There is a scottish comedian called Billy Connolly [who said]: "people who want to own a gun, or be a politician, should be automatically barred from either of them." And I think it's the same with concurrency: anybody who just wants to do it should not be allowed. » (https://www.infoq.com/presentations/top-10-performance-myths)
Take a look at dated, but still relevant book by Brian Goetz - Java Concurrency in Practice - many problems are illustrated with a code section.
yes and no. Yes - in the sense that pretty equivalent things can be done in different languages. No - i'm in the same group of "geniuses" as GP, and i see for example on our current huge C++ platform project the highly technical people struggle with and do the wrong things with concurrency/multithreading that i don't remember seeing the even mildly technical people doing on various large Java projects.
Without more information this is the likely scenario, going by my own experience.
BTW, if it turns out you are a concurrent programming genius please write about it, eh? (Like a blog or book or something.)
But why can't it be both? Why can't you do everything that BEAM does... and then also have an optimising JIT for the straight line maths code? Couldn't you leave all the other parts of the system the same and keep all the existing benefits? Improving one doesn't damage the other does it?
The problem with number crunching or maths is that it is very difficult to cut the whole computation into smaller units and pre-emptively schedule it. If it is possible for a specific use case, then it is moderately easy to replace that part with NIFs. For effective maths you need to convert the internal tagged number representation to machine native code that is also expensive. Solving these two things in the generic case is very difficult while preserving all the good parts.
Perhaps there are some easy wins, but JIT is not an easy thing. Depending on your needs, pushing math onto a port, or a nif is probably a quicker win than trying to make it fast in Erlang. However, I wonder if the single static assignment optimizer would offer a path towards recognizing 'straight line math code' and potentially running things much faster. But there's still an issue of potential mismatch between the very general number format with automatic bignum promotion and whatever the underlying machine provides.
I love this attitude. BEAM is something novel and special, and I think it's important to think of how to incrementally address its current shortcomings instead of throwing our hands up. I find GHC is another place where incrementalism on top of novelty is resulting in a lot of people's wishlists to be fulfilled.
Make it 10M and we can make it work for one version of OTP in a few years.
Make it 100M and recurring for 25 years and we can make it so it is in the ecosystem. This problem is hard and a lot of people have tried over the years. It always break down by not being able to deliver or noone wanting to maintain it.
From my experience, the gotchas tend to hit with emergent behavior, which is hard to benchmark, and may be repeatable in production, but is hard to model in a testing framework.
I'm not sure how much impact off-heap messaging has had, but the basic gotcha is that as a process gets bigger, it tends to run slower (because GC over more memory takes longer), and develop a larger message queue, which makes it slower. You need to have backpressure in your system, or small blips in procesing can blow up to huge messaging queues that can't be processed. Monitoring for overall queue size and maximum queue size is an important health indicator.
The other basic gotcha is that Erlang/OTP tends to default to 'unlimited' resource limits and 'infinity' time outs. You often want to have limits, and timeouts, but a general system doesn't know what you want. Sometimes, the unlimited settings result in terrible system behavior if you hit larger numbers than anyone else tested, but if you hit this, it's usually easy to fix.
A good thing about OTP is that they've written as much as possible of the environment in Erlang itself, so it's easier to change things when needed than a system where most of the provided apis are implemented in C.
The BEAM Book [1] is a good, though unfinished resource talking in general about the implementation - the memory model and the interpreter.
If you're interested in some very low-level details of the runtime, the internal documentation [2] also holds a lot of interesting details.
There are also some additional details on internals at Spawned Shelter [3].
[1]: https://blog.stenmans.org/theBeamBook/ [2]: https://github.com/erlang/otp/tree/master/erts/emulator/inte... [3]: http://spawnedshelter.com/#erlang-design-choices-and-beam-in...
I get mostly false positives trying to find those sorts of discussions or metrics.
However, that's not quite the same thing. The JVM allows you to change instructions, but not -data-. That is, in between versions you change what data a class contains, there is no way to change it out from the running instance. The JVM either has one version of the bytecode loaded, or the other; it has no concept of transitioning between them.
The BEAM has a mechanism to do that. It can have both loaded. And you can write transformation functions to allow the internal process state to transform from one to the other.
Per the article, "Hot code loading means that the application logic can be updated by changing the runnable code in the system whilst retaining the internal process state" - emphasis added. That's the key bit for maintaining uptime during an upgrade. Honestly, I don't think it's used that often, but it's there.
I want to make sure you're talking about the same thing.
To be clear: JVM enables the feature, so “technically” JVM allows hot code reload. Not sure how useful this is in practice for non-Clojure JVM users.
[1] https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoa...
There are limits to how much you can change in existing loaded class code, but if you are just loading up new dynamically generated code you can do pretty much anything.
Its a big reason why some of these Java frameworks are so fast. They can generate highly optimized code on the fly, load it, and have it running alongside the existing app code within a few hundred milliseconds. And the Java JIT will optimize it as if the code was there the whole time.
This makes performance optimizations easy that would be impossible in AOT languages like Go, C, C++, Rust, etc
Ultimately, systems like akka are extremely complicated to get right, even for experts, because you have to think about all of the vm bits underneath. I can (and have) teach a junior programmer basic OTP concepts with the confidence that they can't mess things up. Now, they wouldn't be able to come up with the architecture I designed as a good idea, but I could tell them to implement it (with tests!) and expect them to get it right.
The answer is that one user thread would have a lot less memory than 4GB, and the GC only needs to work on heap sizes of KBs to MBs for most cases.
There is no shared memory(well, mostly).
It's comparing apples and oranges.
> How can't a CF be cancelled?
So glad you brought up the docs. CF implements cancel(boolean mayInterruptIfRunning)... which does nothing ;)
More precisely, it will not cancel a running CF. If the CF hasn't started yet, by all means cancel it. But if it's running, that cancel method does nothing.
Debuggers also use the functionality to allow live code editing and expression evaluation when paused on a breakpoint
Erlang, due to how actors encapsulate state, and dynamic typing, allows you to do those things pretty trivially.
One of the main problems with the Quasar/coroutine based model is that the semantics are quite hard to undersand for developers who are not very familiar with concurrency. They write code that _looks_ synchronous but is actually async. We get a lot of support tickets claiming there's a bug in the platform when the reality is that they don't understand what's going on. I sympathise with them and we probably need to do a better job of hiding the complexities. As you note, the bytecode instrumentation is a bit of a pain but not only that... It also has quite a big impact on performance!
There has been talk of doing some experiments with Akka and that's something I'm interested in exploring. But I think, hypothetically, that writing parts of the platform again in Erlang/OTP would yield huge productivity benefits... gen_fsm offers exactly what we need out of the box. From the little playing around I've done with Erlang, it feels like you can get a small, competent team, up to speed fairly quickly.
I know there's plugins for Kotlin support in other IDE's, have you used them and if so are they any good?
Since Erlang has one GC per process, you can create garbage in one process without triggering GC if that process is short lived. Once the process dies, the entire heap for that process is returned to memory. So in Java, you'd have to write code in a special way to avoid GC, but in Erlang that happens automatically if either your process exits before the heap for that process needs GC. And in Erlang it's pretty normal to run one process per http request, so this does happen in practice, without requiring anything of the programmer.
When it comes to hot code reloading and data migration. When you hot load code into an Erlang vm, a hook will be called if defined which allows you to migrate all data that is in memory into new format. So, you're not restricted by data-incompatibility.
Your last paragraph is what I referred to by required discipline. Everyone that touches the code is required to understand what causes corruption and what doesn't. It also requires that you know which classes are thread safe and which arent, which is hopefully documented somewhere. Thread groups need to be understood (I work in Java/Kotlin every day, and I didn't know what thread groups were before today). In Erlang, data corruption due to multiple processes doesn't happen, and grouping processes together (supervision trees) is so common I can't remember the last time I saw an Erlang program without one.
Which of course doesn't mean that Erlang is superior to Java. But when you're working on something highly concurrent which needs to be fault tolerant, I'd argue that you'd get a better result with less effort than in Java. But of course, if you know Java really well and don't know Erlang at all, YMMW.
Erlang's model with fibers and message passing sounds close to Golang. Java has decent support for immutable objects with immutable collections, Lombok, the FreeBuilder library, both build-time code generators, and Java 14 record types. Automatic passing between machines is unique to Erlang
Per process GC isn't anything like Java does, but the new GC's are probably fast enough that it doesn't matter in practice. For any sane sized heaps the GC pauses are around 0.5 millisecond. This wasn't true until a few years ago, and in production most people don't know or care enough to use the new GC's.
You are right about thread safety in objects. Thankfully the JDK surface is fully documented. Third party libraries usually are. Internal code is a crapshoot. It requires discipline, but I still find it rare in practice because the normal patterns lend themselves to thread safety.
I think its safe to say that Java is a lower level language than Erlang which enables many of the same patterns with less convenience. You can probably get better performance with Java, but your fault tolerance completely depends on how good your coders are. Java will not save you from doing stupid things between threads.
Just wanted to touch on one point. Golang also has shared memory, even though it encourages sharing by communicating. In Erlang you don't have a choice. Golang also doesn't have something like supervision trees (threads that die and restart together). So in practice golang and erlang concurrency is very different.
Prior to finding this document (http://www.cs-lab.org/historical_beam_instruction_set.html) I had no idea whether you could actually do computation on the BEAM. I was starting to wonder if they had misappropriated the term VM, and some sort of inline assembly trick was being used for everything but control flow and IPC.
Interpreted code has very, very real computational constraints and you can't assume people will know this, even now. Especially if your system is noteworthy for how it is not like other systems. Where does it stop being 'weird' and start being conventional? The boundaries describe both sides of a distinction. Even if you're only interested in the exotic part, leave some breadcrumbs for others.
After watching a bunch of videos on BEAM/erlang/elixir I came to the conclusion that it isn't a platform for computation, it's a platform for communication. The best video (by far) was The Soul of Erlang and Elixir • Saša Jurić https://www.youtube.com/watch?v=JvBT4XBdoUE
Two shallow benchmarks:
https://benchmarksgame-team.pages.debian.net/benchmarksgame/...
https://www.techempower.com/benchmarks/ (phoenix is at 175)
They cannot be pre-empted, but they must also return quickly, or risk causing lots of problems (see https://erlang.org/doc/man/erl_nif.html for slightly more detail on what this means). As such you can't just write some big function in C to do number crunching.
The NIF documentation mentions some ways around the problem, but all of them take some effort, or have tradeoffs of some sort. I was really excited when “dirty” NIFs were introduced, which can tell the BEAM that they'll run for a while, thus appearing to allow for long-running NIFs with no extra work other than setting a flag. However, it turns out that the BEAM just spins up N threads for scheduling dirty NIFs, and if you have too many such NIFs, too bad, some won't get scheduled till the others have completed. In retrospect it should have been obvious that there couldn't be a silver bullet for this problem, because it really isn't easy.
Erlang may well be my favorite language, but as you imply, it's just not going to be the right approach for everything: in my experience, it's absolutely fantastic in its niche but that niche is quite small. I think that's fine, though. For me, where Erlang does make sense, its concurrency approach makes it unbeatable, and I'll live with the performance tradeoffs. It turns out that basically all the NIFs I've had to write were just to gain access to functionality that Erlang doesn't expose (e.g. network namespaces on Linux, which are supported now, but weren't when I needed them).
It's actually worse than that; as I recall, the internal numerical representations of numbers do not necessarily map to the CPU's (for instance, there is no byte sizing; you have integers and floats, and they can be arbitrarily large). The work to perform that conversion, do the math, and convert back, would almost assuredly make it so that a single calculation takes more time than just doing it within the BEAM. The only way to save time would be to convert once, do a bunch of math, and convert back. Which would, yes, prevent pre-emption, AND require indication of intent (so brand new language constructs, minimally).
That's a lot to expect of the user, and a lot to implement in the language...all to avoid just writing a NIF.
There wouldn't need to be an indication of intent, other than writing the math separate from any function calls. I don't know how much code fits this pattern, but it's an idea that could be explored. I think that's part of what hipe is supposed to do, but I haven't looked into hipe in a long time.
I don't understand why. If you have a maths-intensive operation like matrix-multiplication using untagged maths, why does that prevent pre-emption? Why does it require indication of intent?
And there's already a basically zero-overhead way to implement pre-emption - safepoints - that's what the JVM does when it wants to pre-empt in user-space.
I envy Rust and its borrow checker. Its a pain to get used to, but enables "shared when you say it is" concurrency model with no overhead. No message passing overhead, optional but safe mutability, no data corruption possibility, zero copy basically everywhere
Erlang for instance scopes gc pools per process so short lived processes just drop the pool. Also GC of one worker doesn't stop any others. Can't remember if it even needs to be generational because the heaps are already sliced by process. It's the closest thing to heap arenas I've seen in a VM based language.
Or take Lua which is single threaded and doesn't require VM safepoints since everything is done via cooperative coroutines.
Java needs to assume worst case and as such has to be conservative in some of it's approaches.
And given that you didn't know that, you really need to study them.
java.util.concurrent is one of the greatest gems of software ever written.
Is this method one of its gems? A cancel() which doesn't cancel? https://docs.oracle.com/javase/8/docs/api/java/util/concurre...
> Synchronized primitives don't compose. You can safely `synchronized get(...)` and safely `synchronized put(...)`. But their composition put(get(...)+1) isn't synchronized.
So is there anything in java.util.concurrent that does compose? put(get()) has exactly the same problem up here at the 'high level' (of CountdownLatches and Semaphores) as it does at the 'low level' (of synchronized methods.)
Which is the point. You can't preempt crunching those numbers if it's not within the BEAM. Which might be fine. Or it might not. Making it invisible to the user is not really a good idea when going for soft realtime properties; at least with a NIF and dirty scheduler you're being explicit about it.
And having blocked them, you can then pre-empt them.
> You can't preempt crunching those numbers if it's not within the BEAM.
I still don't see why sorry. If you had a JIT and you compiled maths intensive code to native code, it could run efficiently in BEAM and still be pre-emptible by having a safepoint in the generated code.
How do you think Java is doing optimised numerical code that is pre-emptible from user-space? Safepoints! BEAM could do the same thing.
What did we want to achieve? We wanted to be able to run a tight loop of highly optimised, untagged numerical code but still be able interrupt it to switch threads on demand from user-space if needed.
Safepoints let us do that.
What else did we need that this doesn't cover?
You can get a lot better nonblocking support with third party libraries. Like RxJS in javascript, RxJava is almost a requirement when doing non-blocking code.
You are right that true green threads would allow thread cancellation one day. Right now, Java can't because it relies on OS threads which aren't safe to cancel. Userspace threads don't have that problem
I can't follow the reasoning here. To refute it, you'd only need to show a language with shared memory and futures with cancel(), right? Aside from that, a second shortcoming isn't a good excuse for the first.
> You can get a lot better nonblocking support with third party libraries.
I don't doubt this. Another example is vavr.io, which gives much better Lists/Optionals/Streams than the standard Java 8 ones. And Joda-Time before that.
> RxJava is almost a requirement when doing non-blocking code.
Why can't CompletableFutures do what RxJava did?
> it relies on OS threads which aren't safe to cancel. Userspace threads don't have that problem
This bit was the most confusing to me. In my shitty understanding of the model, Java Threads are based on OS threads, which is why they're relatively heavy. CFs on the other hand are are lighter and managed by the Java runtime. Why is it then that I can cancel Threads, but I cannot cancel CFs. Your explanation would seem to justify the opposite.
Notably C lets you do it with the big warning that it can crash everything. C# made the same decision as Java and doesn't allow it.
Java and pretty much all OS thread-using languages DO allow you to end threads by nicely asking them to stop. This is not the same as forcing them to. In all cases, if a thread is stuck in an infinite loop or a blocking call, it probably won't cancel if you ask it to. It depends on whether the thread is written to handle cancellation properly.
This is also a limitation of some languages with fibers, including Erlang. You can't force a thread thats stuck in certain states to stop running in Erlang even though it uses fibers. Some languages with fibers do allow it though.
I'm not 100% sure on what you mean by an optional interrupt, but I'm guessing it's the boolean flag on the cancel(); Let's look!
public boolean cancel(boolean mayInterruptIfRunning) {
boolean cancelled = this.result == null && this.internalComplete(new CompletableFuture.AltResult(new CancellationException()));
this.postComplete();
return cancelled || this.isCancelled();
}
mayInterruptIfRunning - this value has no effect in this implementation because interrupts are not used to control processing.
Try it.
@Test
public void cannotCancelARunningFuture() {
// Future that just sleeps
CompletableFuture<Void> sleeping =
CompletableFuture.supplyAsync(() -> {
int i = 0;
while (true) {
System.out.println("zzzz: " + i++);
threadSleep(300);
}
});
// Make sure it started
threadSleep(1000);
sleeping.cancel(true);
System.out.println("Sleep was cancelled");
threadSleep(1000);
System.out.println("Haha no it wasn't");
}
zzzz: 0
zzzz: 1
zzzz: 2
zzzz: 3
Sleep was cancelled
zzzz: 4
zzzz: 5
zzzz: 6
Haha no it wasn'tNot at all. Thanks for your patience.
You've laid out really good arguments as to why Java Threads cannot be cancelled, and suggested that userspace threads -- which I (perhaps mistakenly) interpreted as CompletableFutures -- should be able to be cancelled.
But the thing is, I can cancel (request) Java Threads. And I cannot cancel CompletableFutures.
From my original comment:
> But CFs lack the functionality of Threads. A CF can't decide to sleep for a while, nor can it be cancelled.