Hacker News Reader

Comment by kllrnohj | Hacker News Reader

> Technically there is nothing I don't like about JVM right now, everything that seems impossible 15 years ago is now solved.

Value types are a major missing piece in the JVM stack right now. It's at least on the roadmap, but it keeps getting pushed back and back and back. I'd also argue runtime generics is another one, and perhaps more depressingly one that is unlikely to ever get fixed.

.NET has both of them and also has the same core strengths JVM does, so given the choice I'd go with .NET over JVM 100% of the time as a result. JVM's GC & JIT seem to be on a never ending improvement cycle, but the actual language & core libraries are incredibly slow to react to anything.

View on HN

> .NET has both of them

I'll give you value types, but reified generics in .NET were a mistake. It really makes interop and code sharing among languages hard, in exchange for a rather slight added convenience. This means that if you're a language implementor and you're targeting .NET, you'll get much less from the platform than you would if you target Java, which makes .NET not a very appealing common language runtime. And not only is Java a good platform for Kotlin and Clojure, but thanks to Truffle/Graal it's becoming a very attractive, and highly competitive, platform for Ruby, Python and JavaScript. All of that would have been much more difficult with reified generics.

Also, I don't think value types in Java are being "pushed back." The team is investing a lot of work into them as it's a huge project, but AFAIK no timeline has been announced.

> and also has the same core strengths JVM does

I don't think so. Its compilers are not as good, its GCs are not nearly as good, and its production profiling is certainly not as good (have you seen what JFR can do?); and its tooling and ecosystem are not even in the same ballpark.

I see it as a double-edged sword.

On the one hand, reified generics means that it's .NET's object model or the highway.

On the other hand, .NET maintains a much higher standard of inter-language interoperability. When I was on .NET, I didn't have to worry much about the folks working in F# accidentally rendering their module unusable from C#. Now that I'm on the JVM, I've accepted that it's just a given that the dependency arrows between the Java and Scala modules should only ever point in one direction.

It's not .NET vs Java so much that the development of the two languages you mention are coordinated. For Kotlin and Clojure interop works both ways; Scala doesn't care much about that, so Scala developers write special facades as Java language API. There are lots of different languages on Java, and they care about different things. The Java team itself develops only one language, although at one time there was another (JavaFX Script). Some language developers (Kotlin, JRuby) work more closely with the Java team, and others (Scala) hardly ever do.

Is a value type different from a value class?

https://github.com/google/auto/blob/master/value/userguide/i...

Yes, those value classes are still heap/GC allocated objects.

Value types are a generalisation of `int`, `long`, `float` (etc), where values are stored inline, not allocated on the heap. For instance, a `Pair<long, double>` that isn't a pointer, but is instead the exactly the size of a long plus a double, and is the same as writing `long first; double second;` inline.

Dumb question. Why do reified generics make interop challenging? Or is it reified generics plus value types that don't inherit system.Object? Couldn't the language implementations basically pass around ICollection<Object> in .NET somewhat similar to how they do in Java?

The problem is that your GenericClass<T1> and GenericClass<T2> are really more like GenericClass_T1 and GenericClass_T2 with their own distinct type definitions and interfaces. From the perspective of a different runtime/language trying to interop with these types, you have to somehow understand and work with this mapping game. It's much easier from inside the .NET runtime than outside.

The general solution is, like you suggested, to avoid using reified generics in the module interface where the interop happens.

The solution I remember from the last time I dealt with Python on .NET (which was admittedly a long time ago) was the opposite - you did use the reified generics, and there were facilities to create an instance of GenericClass<TWhatever> from within Python. There's a whole dynamic language runtime that is purpose-built for smoothing over a lot of that stuff.

What wouldn't work would be to, e.g., create a Python-native list and try to pass it into a function that expects a .NET IList<T>. Which doesn't feel that odd to me - they may have the same name, but otherwise they're very different types that have very different interfaces.

That said, the Iron languages never took off. My personal story there is that all the new dynamic features that C# got with the release of the DLR pretty much killed my desire to interact with C# from a dynamic language. The release that gave me Python on .NET also turned C# itself into an acceptable enough Python for my needs.

> Why do reified generics make interop challenging?

Suppose you have types A and B, such that A <: B. What is, then, the relationship between List<A> and List<B>? This question is called variance, and different languages have very different answers to this, but once generics are reified, the chosen variance strategy is baked into the runtime.

> Couldn't the language implementations basically pass around ICollection<Object> in .NET somewhat similar to how they do in Java?

They could, but then this adds significant runtime overhead at the interop layer. For example, a popular standard API may take a parameter of type List<int>. How do you then call it from, say, JavaScript, without an O(n) operation (and without changing JS)?

> This question is called variance, and different languages have very different answers to this, but once generics are reified, the chosen variance strategy is baked into the runtime.

Which, realistically, is probably the only principled way to do things if you want to be doing much with variant generics in a cross-language way.

The Java way, "I pick my variance strategy, you pick yours, and we'll both pass everything around as a List<Object> at runtime and just hope that our varying decisions about what their actual contents are allowed to be never cause us any nasty surprises for each other at run time," is not type-safe and can lead to nasty surprises at run time. It's easier, sure, but easier is not necessarily better.

Except, realistically, of all the polyglot runtimes, the ones that have good interop erase and the one that doesn't reifies.

.NET Value types do inherit from System.Object.

See: https://docs.microsoft.com/en-us/dotnet/api/system.valuetype...

.NET is tied to Microsoft, so I'd avoid it 100% of the time.

Yes, yes. I know that Microsoft theoretically open sourced and ported it. However the way that this always works is that there is a base that can be written in, but anything non-trivial will have pulled in something that, surprise surprise, is Windows only.

Otherwise I agree that it is a better Java.

I don’t think is true anymore.

The .net 5 announcement was very clear that .net core is the future and its been a while since you’ve had needed things that are windows only to build a non-trivial .net core application.

Please double check my wording.

Yes, you can write a non-trivial .NET application on Linux. But if you take a non-trivial .NET application that runs on Windows, the odds are low that it can easily be ported to Linux. And there are almost no non-trivial .NET applications that weren't originally written for Windows.

The result is that if you work with .NET, you're going to be pushed towards Windows.

It the wording of the announcement (taking them in good faith) that applies to applications using .NET Framework. .NET Core should be 100% portable to Linux/Mac/wasm.

.NET 5 should supersede both Core and Framework IIRC

I have been hearing announcements about how Microsoft was working to make .NET code portable ever since Mono was first started in 2001.

In the years since, I've encountered many stories that attempted to make use of that portability. All failed.

I've seen the promise of portability with other software stacks, and know how hard it is. I also know that taking software that was not written to be portable, and then making it portable, is massively harder than writing it to be portable from scratch.

So, based on both the history of .NET and general knowledge, I won't believe that .NET will actually wind up being portable until I hear stories in the wild of people porting non-trivial projects to Linux with success. And I will discount all announcements that suggest the contrary.

I have been hearing announcements about how Microsoft was working to make .NET code portable ever since Mono was first started in 2001.

Microsoft didn’t have anything to do with Mono until 2016.

If you start from scratch with a greenfield .Net Core project there aren’t really any issues getting it to work cross platform.

You are no more “pushed toward Windows” with new code you do with .Net Core than you are if you use any other cross platform language.

While I’ll agree that anything that uses any of the advanced features of Entity Framework and MVC is not trivially ported.

When you're trying to write new code, you run into a problem and look for a library that solves it. But all of the libraries that you find are Windows First, and it is not until after you're committed to them that you sometimes discover how they are Windows Only.

So yes, even in a new project, there will be a pull back to Windows. Because virtually nothing is truly written from scratch.

It’s no more of a problem with .Net Core than it is with Python modules that have native dependencies, or Node modules with native dependencies.

You’re not going to mistakenly add a non .Net Core Nuget package to a .Net Core project. It won’t even compile.

Of course you can find Windows only nuget packages for Windows only functionality like TopShelf - a package to make creating a Windows Service easy. But even then, I’ve taken the same solution and deployed it to both a Windows server and an AWS Linux based lambda just by configuring the lambda to use a different entry point (lambda handler) than the Windows server.

You can even cross compile a standalone Windows application on Linux and vice versa.

I use a Linux Docker container to build both packages via AWS CodeBuild.

Would you also criticize Python for not being cross platform because there are some Windows only modules?

https://sourceforge.net/projects/pywin32/

Looking at the announcement it seems they're basically folding all of the Windows-specific stuff back into .net core. Isn't that just going back to a compatibility minefield?

They didn't "theoretically" open source it - they actually open sourced it.

I get why people used to shit on Microsoft, but Microsoft has demonstrated over a number of years that its changed under Satya.

> However the way that this always works is that there is a base that can be written in, but anything non-trivial will have pulled in something that, surprise surprise, is Windows only.

Outside of desktop GUIs, this is simply not true. I'm writing complex, cross-platform systems that work just fine on Windows and Linux (and would on MacOS if I chose to target it).

Hell, even a lot of the tooling is now cross-platform: Visual Studio Code, Azure Data Studio, even Visual Studio and Xamarin run on MacOS!

No, Visual Studio does notrun on macOS. Visual Studio for macOS is a fork of MonoDevelop.

So because it’s not the same code base but it is produced by the same company with the same purpose? It’s not “Visusl Studio Code”? Was Photoshop not Photoshop on Windows when the assembly language optimizations were different between PPC and x86?

> the actual language & core libraries are incredibly slow to react to anything.

async/await being the obvious example. Still no sign of it in the Java language, nor any expectation of it, unless I missed something.

https://wiki.openjdk.java.net/display/loom/ which is superior to async/await, if I may say so myself (I'm the project lead)

> Fibers are user-mode lightweight threads that allow synchronous (blocking) code to be efficiently scheduled, so that it performs as well as asynchronous code

Sounds a lot like Erlang BEAM processes.

Yep; or Go's goroutines. Except that the fibers are implemented and scheduled in the Java libraries; the VM only provides continuations.

How does it compare to Windows fibers?

I find this really interesting. Care to provide some comparisons?

Great article, thanks. Some perhaps-silly questions:

1. Is it possible to inspect the state of a 'parking' operation, the way you can in .Net with Task#Status?

2. So fibers run in 'carrier threads'. Is there a pool of carrier threads, or can any thread act as a carrier? I'm thinking of .Net's where this is configurable (ignoring that .Net 'contexts' aren't exactly threads), by means of Task#ContinueWith() and the Scheduler class. I take it from the following snippets that fibers can only run on the thread where they were created:

starting or continuing a continuation mounts it and its stack on the current thread – conceptually concatenating the continuation's stack to the thread's – while yielding a continuation unmounts or dismounts it.

And also:

Parking (blocking) a fiber results in yielding its continuation, and unparking it results in the continuation being resubmitted to the scheduler.

On a non-technical note, how do OpenJDK projects feed back into the Java spec and Oracle Java?

1. Yes.

2. Configurable. A fiber is assigned to a scheduler that is, at least currently, and Executor (so you can implement your own).

> I take it from the following snippets that fibers can only run on the thread where they were created

No. A fiber has no special relationship to the thread (or fiber) that created it, although now there can be a supervision hierarchy thanks to structured concurrency: https://wiki.openjdk.java.net/display/loom/Structured+Concur...

> OpenJDK projects feed back into the Java spec and Oracle Java?

OpenJDK is the name of the Java implementation developed by Oracle (Oracle JDK is a build of OpenJDK under a commercial license). Projects are developed in OpenJDK (for the most part by Oracle employees because Oracle funds ~95% of OpenJDK's development, but there are some non-Oracle-led projects from time to time[1]) and are then approved by the JCP as an umbrella "Platform JSR" for a specific release (e.g. this is the one for the current version: https://openjdk.java.net/projects/jdk/12/spec/)

[1]: E.g. the Shenandoah GC (https://wiki.openjdk.java.net/display/shenandoah) is led by Red Hat, TSAN (https://wiki.openjdk.java.net/display/tsan) is led by Google, and the s390 port (http://openjdk.java.net/projects/s390x-port/) is led by SAP.

Very neat. So it preserves the virtues of .Net's task-based concurrency, but is even less intrusive regarding the necessary code-changes to existing synchronous code.

Does it impact things from the perspective of the JNI programmer?

> OpenJDK is the name of the Java implementation developed by Oracle (Oracle JDK is a build of OpenJDK under a commercial license).

Ah, of course. I'd missed that.

> there are non-Oracle-led projects from time to time[1], and are then approved by the JCP as an umbrella "Platform JSR" for a specific release

How do they handle copyright?

> but is even less intrusive regarding the necessary code-changes to existing synchronous code.

Yes. All existing blocking IO code will become automatically fiber-blocking rather than kernel-thread-blocking, except where there are OS issues (file IO; Go has the same problem). Fibers and threads may end up using the same API, as they're just two implementations of the same abstraction.

> Does it impact things from the perspective of the JNI programmer?

Fibers can freely call native code, either with JNI or with the upcoming Project Panama, which is set to replace it, but a fiber that tries to block inside a native call, i.e., when there is a native frame on the fiber's stack, will be "pinned" and block the underlying kernel thread.

> How do they handle copyright?

Both the contributors and Oracle own the copyright (i.e. both can do whatever they want with the code). This is common in large, company-run open source projects.

> a fiber that tries to block inside a native call, i.e., when there is a native frame on the fiber's stack, will be "pinned" and block the underlying kernel thread.

Doesn't this boil down to the native function blocking the thread?

How about the C API/ABI of JNI? Will there be additions there for better supporting concurrency (i.e. not simply blocking)? Or can that be handled today, with something akin to callbacks?

If the native routine blocks the kernel thread, it blocks, and if not, it doesn't. While something could hypothetically be done about blocking native routines, we don't see it as an important use case. Calling blocking native code from Java is quite uncommon. We've so far identified only one common case, DNS lookup, and will address it specifically.

Nice. So going 100% async is a real possibility?

I'm not sure what that means.

That's very nice, but saying it's strictly superior to async/await is a stretch. Fibers/stackful coroutines are a different approach with its own tradeoffs.

On the plus side, fibers offer almost painless integration of synchronous code, while async/await suffer from the "colored functions" problem[1].

The price you pay for that, is the higher overhead of having to allocate stacks. If you don't support dynamic stacks which can be resized, you basically don't have a much better overhead than native threads. There are two solutions I'm aware of, both of them have been done by Go at different times: segmented stacks and re-aligning and copying the stack on resize. Both carry some memory overhead (unused stacks) and computational overhead (stack resizing).

[1] http://journal.stuffwithstuff.com/2015/02/01/what-color-is-y...

> Fibers/stackful coroutines are a different approach with its own tradeoffs.

The only tradeoffs involved, as far as I'm aware, are effort of implementation. There are no runtime tradeoffs.

> The price you pay for that, is the higher overhead of having to allocate stacks.

You have to allocate memory to store the state of the continuation either way. Some languages can choose not to call the memory required for stackless continuations "stacks" but it's the same amount of memory.

> Both carry some memory overhead (unused stacks) and computational overhead (stack resizing). Their "advantage" is that, because they're inconvenient, people try to keep those stacks shallow.

Stackless continuations have the same issue. They use what amounts to segmented stacks. "Stackless" means that they're organized as separate frames.

I think that the approach done in https://wiki.openjdk.java.net/display/loom/Main is better than the async/await infrastructure.

Well, other than the fact that it only supports Linux and MacOS.

Is that true? The build instructions are for a Posix-like evironment, but I haven't actually looked to see if the actual implementation supports Windows yet.

As someone who runs Windows and Linux about equally, in differing proportions over time, I do find it disappointing that some (b)leading edge JVM and Java features don't support Windows yet.

> I do find it disappointing that some (b)leading edge JVM and Java features don't support Windows yet

Seems understandable though. Java is primarily a tool for heavyweight Unix servers, after all. (This is of course an empirical claim, and I have no source, but I'd be surprised if I turn out to be mistaken.)

Makes good sense to go with the strategy of building an industry-strength technology before investing the time to handle Windows.

It's a prototype. It will support Windows when it's released, and probably sooner. We're literally changing it every day, and it's hard and not very productive to make these daily changes on multiple platforms, especially as none of the current developers use Windows (this is changing soon, though).

Reply on news.ycombinator.com