Java Virtual Threads: A Case Study
infoq.com
infoq.com
So it looks like their goal was: try adopting a new technology without changing any of the aspects designed for an old technology and optimised around it.
For example, in Erlang the entire VM is built around green threads with a huge amount of guarantees and mechanisms: https://news.ycombinator.com/item?id=40989995
When your entire system is optimized for green threads, the question of "it still needs to map onto OS threads" loses its significance
And yes, those semantics are important, but sadly most people stop at "yay we have green threads now" and then a null pointer exception kills their entire app, or the thread that handles requests, or...
How's that? What about parallelism?
Virtual threads are therefore useful if you're writing something like a proxy server, where you want to allow lots of concurrent connections, and you want to use the familiar thread-per-connection programming model.
The advice "don't use virtual threads for that, it will be inefficient" really does need some evidence.
Mildly infuriating though that people may read this and think that somehow the JVM has problems in its virtual thread implementation. I admit their 'Unexpected findings' section is very useful work, but the moral of this story is: don't use virtual threads for this that they were not intended for. Use them when you want a very large number of processes executing concurrently, those processes have idle stages, and you want a simpler model to program with than other kinds of async.
Your observation that computing architectures have chased fast server for decades is apt. There's a truism in computing that those who build systems are doomed to relearn the lessons of the early ages of networks, whether they studied them in school or not. But kudos to whoever went through the exercise again.
What "CPU-intensive apps" did they test with? Surely not acmeair-authservice-java. A request does next to nothing. It authenticates a user and generates a token. I thought it at least connects to some auth provider, but if I understand it correctly, it just uses a test config with a single test user (https://openliberty.io/docs/latest/reference/config/quickSta...). Which would not be a blocking call.
If the request tasks don't block, this is not an interesting benchmark. Using virtual threads for non-blocking tasks is not useful.
So, let's hope that some of the tests were with tasks that block. The authors describe that a modest number of concurrent requests (< 10K) didn't show the increase in throughput that virtual threads promise. That's not a lot of concurrent requests, but one would expect an improvement in throughput once the number of concurrent requests exceeds the pool size. Except that may be hard to see because OpenLiberty's default is to keep spawning new threads (https://openliberty.io/blog/2019/04/03/liberty-threadpool-au...). I would imagine that in actual deployments with high concurrency, the pool size will be limited, to prevent the app from running out of memory.
If it never gets to the point where the number of concurrent requests significantly exceeds the pool size, this is not an interesting benchmark either.
Unfortunately the slides from that presentation were not uploaded to the conference site, but this article summarizes [2] the most significant metrics. The Oracle guy claimed that by using Virtual Threads Oracle was able to implement, using imperative Java, a new engine for Helidon (called Nima) that had identical performance to the old engine based on Netty, which is (at least in Oracle's opinion) the top performing reactive HTTP engine.
The conclusion of the presentation was that based on Oracle's experience imperative code is much easier to write, read and maintain with respect to reactive code. Given the identical performance achieved with Virtual Threads, Oracle was going to abandon reactive programming in favor of imperative programming and virtual threads in all its products.
[1] https://www.eclipsecon.org/2022/sessions/helidon-nima-loom-b...
[2] https://medium.com/helidon/helidon-n%C3%ADma-helidon-on-virt...
Go's goroutines are preemptive (and Go's development team went through a lot of pain to make them such).
Java's lightweight threads aren't.
Java's repeating the same mistakes that Go made (and learned from) 10 years ago.
That means that Java programmers have to be very careful when writing code, lest they block the entire underlying (OS) thread!
Again, Go already went through that experience. It was painful. Java should have learned and implemented it from the start
Still, an annoying gotcha if it hits you unexpectedly.
From JEP 444:
The scheduler does not currently implement time sharing for virtual threads. Time sharing is the forceful preemption of a thread that has consumed an allotted quantity of CPU time. While time sharing can be effective at reducing the latency of some tasks when there are a relatively small number of platform threads and CPU utilization is at 100%, it is not clear that time sharing would be as effective with a million virtual threads.
Also, in this scenario, i think the current scheduler (ForkJoin Pool) will use managed blocker to compensate those pinned carrier threads.
https://docs.oracle.com/en/java/javase/21/core/virtual-threa...
> Like a platform thread, a virtual thread is also an instance of java.lang.Thread. However, a virtual thread isn't tied to a specific OS thread. A virtual thread still runs code on an OS thread. However, when code running in a virtual thread calls a blocking I/O operation, the Java runtime suspends the virtual thread until it can be resumed. The OS thread associated with the suspended virtual thread is now free to perform operations for other virtual threads.
It's not been hidden at all in their presentation on virtual threads.
The OS thread that the virtual thread is mounted to can still be preempted, but that won't free up the OS thread for another virtual thread. However, if you use them for what they're intended for this shouldn't be a problem. In practice, it will be because no one can be bothered to RTFM.
> what they're intended for
Java prides itself on careful and deliberate changes to eliminate foot guns, but this seems like a pretty major restriction. Usually these kinds of cooperative threads are called fibers or something else to distinguish them from truly preempt-able threads.
Expecting developers to read the minutiae of documentation (there's another restriction around synchronized blocks) is a fool's errand TBH. Principle of least surprise, etc.
The best info I could find was this blog post:
https://blogs.oracle.com/javamagazine/post/going-inside-java...
"Virtual threads, however, are handled differently than platform threads. None of the existing schedulers for virtual threads uses time slices to preempt virtual threads."
The next handful of paragraphs are also interesting.
This is crucial, because Java wouldn't necessarily require the same optimizations Go needed.
Making Virtual Threads fully preemptive could be useful, but it's probably not as crucial as it was for Go.
Go does not have a native mechanism to spawn OS threads that are separate from the scheduler pool, so if you want to run a long CPU-heavy task, you can only run it on the same pool as you run your I/O-bound Goroutines. This could lead to starvation, and adding partial preemption and later full preemption was a neat way to solve that issue.
On the other hand, Java still has OS threads, so you can put those long-running CPU-bound tasks on a separate thread-pool. Yes, it means programmers need to be extra careful with the type of code they run on Virtual Threads, but it's not the same situation as Go faced: in Java they DO have a native escape hatch.
I'm not saying a preemptive scheduler won't be helpful at Java, but it just isn't as direly needed as it was with Go. One of the most painful issues with Java Virtual Threads right now is thread pinning when a synchronized method call is executed. Unfortunately, a lot of existing Java code is heavily using synchronized methods[1], so it's very easy to unknowingly introduce a method call that pins an OS thread. Preemeptive could solve this issue, but it's not the only way to solve it.
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[1] One of my pet peeves with the Java standard library is that almost any class or method that was added before Java 5 is using synchronized methods excessively. One of the best examples is StringBuffer, the precursor of StringBuilder, where all mutating methods are synchronized, as if it was a common use case to build a string across multiple threads. I'm still running into StringBuffers today in legacy codebases, but even newer codebases tend to use synchronized methods over ReentrantLocks or atomic operations, since they're just so easy to use.
A number of years ago I remember trying to have a sane discussion about “non blocking” and I remember saying “something” will block eventually no matter what… anything from the buffer being full on the NIC to your cpu being at anything less than 100%. Does it shake out to any real advantage?
With virtual threads, you can limit the damage by using a semaphore, but you still need to tune the size. This isn't much different than sizing a traditional thread pool, and so I'm not sure what benefit virtual threads will really have in practice. You're swapping one config for another.
Memory utilization & performance is going to be similar to the async callback mess.
Yes. The JVM has far more opportunities for optimizing threads because it doesn't need to uphold 50 years of accumulated invariants and compatibility that current OSes do, and JVM has more visibilty on the application internals.
Because evented/callback-driven code is a nightmare to reason about and breaks lots of very basic tools, like the humble stack trace.
Another big thing for me is resource management - try/finally don't work across callback boundaries, but do work within a virtual thread. I recently ported a netty-based evented system to virtual threads and a very long-standing issue - resource leakage - turned into one very nice try/finally block.
Also the virtual threads run on a "traditional" thread pool to my understanding, so you can just tweak the number of worker threads to cap the total concurrency.
The benefit is it's overall more efficient (in the general case) and lets you write linear blocking code (as opposed to function coloring). You don't have to use it, but it's nice that it's there. Now hopefully Valhalla actually makes it in eventually
I've not yet seen a study that shows that virtual threads offer a huge benefit. The Open Liberty study suggests that they're worse than the existing platform threads.
Ideally carrier threads would be pinned to isolated cpu cores, which removes most aspects of OS scheduler from the picture
Not exactly Java virtual threads, but a study on how userland threads beat kernel threads.
https://cs.uwaterloo.ca/~mkarsten/papers/sigmetrics2020.html
For quick results, check figures 11 and 15 from the (preprint) paper. Userland threads ("fred") have ~50% higher throughput while having orders of magnitude better latency at high load levels, in a real-world application (memcached).
> Context switching cost due to CPU cache thrashing doesn't go away regardless of which type of thread you're using.
Except it's not a context switch? You're jumping to another instruction in the program, one that should be very predictable. You might lose your cache, but it will depend on a ton of factors.
> there's a new cost caused by allocating the internal continuation object, and copying state into it.
This is more of a problem with the implementation (not every virtual thread language does it this way), but yeah this is more overhead on the application. I assume there's improvements that can be made to ease GC pressure, like using object pools.
Usually virtual threads are a memory vs CPU tradeoff that you typically use in massively concurrent IO-bound applications. Total throughput should take over platform threads with hundreds of thousands of connections, but below that probably perform worse, I'm not that surprised by that.
Java virtual threads are stackful; they have to save and restore the stack every time they mount a different virtual thread to the platform thread. They do this by naive[0] copying of the stack out to a heap allocation and then back again, every time. That's clearly a context switch that you're paying for; it's just not in the kernel. I believe this is what the person you're replying to is talking about.
[0] Not totally naive. They do take some effort to copy only subsets of the stack if they can get away with it. But it's still all done by copies. I don't know enough to understand why they need to copy and can't just swap stack pointers. I think it's related to the need to dynamically grow the stack when the thread is active vs. having a fixed size heap allocation to store the stack copy.
The key with virtual threads is they are so light weight that you can have thousands of them running concurrently: even when they block for I/O, it doesn't matter. It's similar to light weight coroutine in other language like Go or Kotlin.
This is a common problem with people using Java parallel streams, because they by default share a single global thread pool and the way to use your own thread pool is also extremely counterintuitive, because it essentially relies on some implicit thread local magic to choose to distribute the stream in the thread pool that the parallel stream was launched on, instead of passing it as a parameter.
It would be best if people came up with more dynamic back pressure strategies, because this is a more general problem that goes way beyond thread pools. In fact, one of the key problems of automatic parallelization is deciding at what point there is too much parallelization.
To put it shortly: Writing single-threaded blocking code is far easier for most people and has many other benefits, like more understandable and readable programs: https://www.youtube.com/watch?v=449j7oKQVkc
The main reason why non-blocking IO with it's style of intertwining concurrency and algorithms came along is that starting a thread for every request was too expensive. With virtual threads that problem is eliminated so we can go back to writing blocking code.
I’d say that writing single-threaded code is far easier for _all_ people, even async code experts :)
Also, single-threaded code is supported by programming language facilities: you have a proper call stack, thread-local vars, exceptions bubbling up, structured concurrency, simple resource management (RAII, try-with-resources, defer). Easy to reason and debug on language level.
Async runtimes are always complicated, filled with leaky abstractions, it’s like another language that one has to learn in addition, but with a less thought-out, ad-hoc design. Difficult to reason and debug, especially in edge cases
Async runtimes themselves are simply attempts to bolt-on green threads on top of a language that doesn't support them on a language level. In JavaScript, async/await uses Promises to enable callback-code to interact with key language features like try/catch, for/while/break, return, etc. In Python, async/await is just syntax sugar for coroutines, which are again just syntax sugar for CPS-style classes with methods split at each "yield". Not sure about Rust, but it probably also uses some Rust macro magic to do something similar.
Haskell supports async code while also supporting green threads on a language level, and the async code has most of the same issues as async code in any other languages.
- All IO is non-blocking by default.
- FFI support for interruptible.
- Haskell threads can be preempted externally - this allows you to ensure they never leak. Vs a goroutine that can just spin forever if it doesn't explicitly yield.
- There are various stdlib abstractions for building concurrent programs in a compositional way.
Goroutines are preemptible by the runtime (since https://go.dev/doc/go1.14#runtime) but they're still not addressable or killable through the language itself.
Async exceptions as a way to pass messages (and kill threads!)
Allocation limits for threads.
Software Transactional Memory.
Its attraction for non-blocking coding is that it allows hiding the multi-threaded event dispatching loop. But as the parent comment suggests, this abstraction is extremely leaky. And in addition, CPS in non-functional languages or without syntactic sugar has poor readability. Improving the readability requires compiler changes in the host language - so many languages have added compiler support to further hide the CPS underpinnings of their async model.
I've always felt this was a big mistake in our industry - all this effort not only in compilers but also in debuggers/IDE - building on a leaky abstraction. Adding more layers of leaky abstractions has only made the issue worse. Async code, at first glance, looks simple but is a minefield for inexperienced/non-professional software engineers.
It's annoying that Rust switched to async style - the abstraction leakiness immediately hits you, as the "hidden event dispatching loop" remains a real dependency even if it's not explicit in the code. Thus libraries using asycn cannot generally be used together although last time i looked, tokio seems to have become the de-facto standard.
[1] https://en.wikipedia.org/wiki/Continuation-passing_style
Much the same as JavaScript I understand, but no macros; the compiler turns them into Futures that can be polled
While 'async' is just a name, underneath it's epoll - and the virtual threads would not perform better than a proper NIO (epoll) server. I dont consider myself an 'async expert' but I have my share of writing NIO code (dare say not terrible at all)
I think you're missing the whole point.
The reason why so many smart people invest their time on "virtual threads" is developer experience. The goal is to turn writing event-driven concurrent code into something that's as easy as writing single-threaded blocking code.
Check why C#'s async/await implementation is such a huge success and replaced all past approaches overnight. Check why node.js is such a huge success. Check why Rust's async support is such a hot mess. It's all about developer experience.
How do you developers spew such strong opinions without taking a moment to think about what you're about to say. Rust cannot be directly compared to C#, Java or even Go.
You don't get a runtime or a GC with rust. The developer experience is excellent, you get a lot of control over everything you're building with it. Yes it's not as magical as languages and runtimes like you've mentioned, but the fact that I can at anytime rip those abstractions off and make my service extremely lightweight and performant is not something those languages will allow you to do.
And this is coming from someone who's written non blocking services before Async rust was a thing with just MIO.
The very fact Rust gets mentioned between these languages should be a tribute to the efforts of it's maintainers and core team. The amount of tooling and features they've added into the language gives developers of every realm liberty to try and build what they want.
Honestly, you can hold whatever opinion you want on any language but your comparison really doesn't make sense.
This is the core misunderstanding/dishonesty behind the Loom/Virtual Threads hype. Single-threaded blocking code is easy, yes. But that ease comes from being single-threaded, not from not having to await a few Futures.
But Loom doesn't magically solve the threading problem. It hides the Futures, but that just means that you're now writing a multi-threaded program, without the guardrails that modern Future-aware APIs provide. It's the worst of all worlds. It's the scenario that gave multi-threading such a bad reputation for inscrutable failures in the first place.
So far, they're not quite there yet: the issue of "thread pinning" is something developers still have to be aware of. I hear the newest JVM version has removed a few more cases where it happens, but will we ever truly 100% not have to care about all that anymore?
I have to say things are already pretty awesome however. If you avoid the few thread pinning causes (and can avoid libraries that use them - although most of not all modern libraries have already adapted), you can write really clean code. We had to rewrite an old app that made a huge mess tracking a process where multiple event sources can act independently, and virtual threads seemed the perfect thing for it. Now our business logic looks more like a game loop and not the complicated mix of pollers, request handlers, intermediate state persisters (with their endless thirst for various mappers) and whatnot that it was before (granted, all those things weren't there just because of threading.. the previous version was really really shitily written).
It's true that virtual threads sometimes hurt performance (since their main benefit is cleaner simpler code). Not by much, usually, but a precisely written and carefully tuned piece of performance critical code can often still do things better than automatic threading code. And as a fun aside, some very popular libraries assumed the developer is using thread pools (before virtual threads, which non trivial Java app didn't? - ok nobody answer that, I'm sure there are cases :D) so these libraries had performance tricks (ab)using thread pool code specifics. So that's another possible performance issue with virtual threads - like always with performance of course: don't just assume, try it and measure! :P
Virtual threads usually require significantly fewer resources to spawn and run. And, if the underlying system is implemented with them in mind, they can use fewer context switches, and possibly even fewer cache misses etc.
I think it’s definitely worth a read.
You can create a thread in your code and not worry whether that thing will then be some day run in a huge loop or receive thousands of requests and therefore spend all your memory on thread overhead. Go and other languages (in Java's ecosystem there's Kotlin for example) employ similar mechanisms to avoid native thread overhead, but you have to think about them. Like, there's tutorial code where everything is nice & simple, and then there's real world code where a lot of it must run in these special constructs that may have little to do with what you saw in those first "Hello, world" samples.
Java's approach tries to erase the difference between virtual and real threads. The programmer should have to employ no special techniques when using virtual threads and should be able to use everything the language has to offer (this isn't true in many languages' virtual/green threads implementations). Old libraries should continue working and perhaps not even be aware they're being run on virtual threads (although, caveats do apply for low level/high performance stuff, see above posts). And libraries that you interact with don't have to care what "model" of green threading you're using or specifically expose "red" and "blue" functions.
This gives the chance to the JVM to use real threads more efficiently, avoiding that threads remain unused while waiting on I/O (e.g. a response from a stream). As soon as the JVM detects that a physical thread is blocked on I/O, a semaphore, a lock or anything, it will reallocate that physical thread to running a new virtual thread. This will reduce latency, context switch time (the switching is done by the JVM that already globally manages the memory of the Java process in its heap) and will avoid or at least largely reduce the chance that a real thread remains allocated but idle as it's blocked on I/O or something else.
My understanding is that virtual threads mostly eliminate context switching - for N CPUs JVM creates N platform threads and they run virtual threads as needed. There is no real context switching apart from GC and other JVM internal threads.
A platform thread picking another virtual thread to run after its current virtual thread is blocked on IO is not a context switch, that is an expensive OS-level operation.
So I think it's meaningful to define what we mean by context switch here.
[1] https://www.eclipsecon.org/sites/default/files/slides/JavaLo...
Virtual threads are less general than kernel threads. If you use a virtual thread to call out of the JVM you lose their benefits, because the JVM becomes like the kernel and can't make any assumptions about the stack.
But if you are running code controlled by the JVM, then it becomes possible to do optimizations (mostly stack related) that otherwise can't be done, because the GC and the compiler and the threads runtime are all developed together and work together.
Specifically, what HotSpot can do moving stack frames to and from the heap very fast, which interacts better with the GC. For instance if a virtual thread resumes, iterates in a loop and suspends again, then the stack frames are never copied out of the heap onto the kernel stack at all. Hotspot can incrementally "pages" stack frames out of the heap. Additionally, the storage space used for a suspended virtual thread stack is a lot smaller than a suspended kernel stack because a lot of administrative goop doesn't need to be saved at all.
Virtual Threads are very fast to create and allocate only the memory needed by the actual call stack, which can be much less than for OS Threads.
Also, blocking code is very simple compared to the equivalent async code. So using blocking code makes your code much easier to follow. Check out examples of reactive frameworks for Java and you will quickly understand why.
I'm not familiar with windows, but this certainly isn't the case on Linux. It only costs 2mb-8mb of virtual address space, not actual physical memory. And there's no particular reason to believe the JVM can have a list of threads and their states more efficiently than the kernel can.
All you really save is the syscall to create it and some context switching costs as the JVM doesn't need to deal with saving/restoring registers as there's no preemption.
The downside though is you don't have any preemption, which depending on your usage is a really fucking massive downside.
Nobody is taking OS threads away, so you can choose to use them when they better fit your use case.
Of course there is. The JVM is able to store the current stack for the Thread efficiently in the pre-allocated heap. Switching execution between Virtual Threads is very cheap. Experiments show you can have millions of VTs, but only a few thousand OS Threads.
I don't know why you think preemption is a big downside?! The JVM only suspends a Thread at safe points and those are points where it knows exactly when to resume. I don't believe there's any downsides at all.
The goal is to maximize the number of tasks you can run concurrently, while imposing on the developers a low cognitive load to write and maintain the code.
> Why did you need a thread pool before but not any more?
You still need a thread pool. Except with virtual threads you are no longer bound to run a single task per thread. This is specially desirable when workloads are IO-bound and will expectedly idle while waiting for external events. If you have a never-ending queue of tasks waiting to run, why should you block a thread consuming that task queue by running a task that stays idle while waiting for something to happen? You're better off starting the task and setting it aside the moment it awaits for something to happen.
> What resource was exhausted to prevent you from putting every request on a thread?
if creating gazillion threads on modern hardware is super cheap why not? I have transparency and debuggability what threads are running, can check stacktrace of each and what are they blocked on.
virtual threads adds lots of magic under the hood, and if there will be some bug or lib in your infra with no vthreads support it is absolutely not clear how to debug it.
Some usage types don’t care, some do.
From what I gather virtual threads are an alternative to “callback-hell” (js) or async coloring (python).
I suspect if you care about threads overhead, you won't pick Java, because there will be overhead in other areas too
> From what I gather virtual threads are an alternative to “callback-hell” (js) or async coloring (python).
there is also existing ExecutorService and futures in Java
Yes, virtual threads are an alternative also to those. (Kind of)
Virtual threads are a performance improvement over threads, no matter how cheap to create threads are. Virtual threads run on threads. If threads become cheaper to create, so do virtual threads. They are not mutually exclusive.
Virtual threads are on top of that a developer experience improvement. Code is easier to write and maintain.
Virtual threads improve throughput because the moment a task is waiting for anything like IO, the thread is able to service any other task in the queue.
except now you need to prove somehow that all 100 libs in your project support virtual threads.
> Virtual threads improve throughput because the moment a task is waiting for anything like IO, the thread is able to service any other task in the queue.
from reading similar discussions, linux for example doesn't have true IO async API, you just push lock of Java thread to lock of thread in the kernel
io_uring has been around for a few years at this point, with vulnerabilities having been fixed to the point that it's fit for broadband usage.
Unfortunately kafka, for example, has not: https://github.com/spring-projects/spring-kafka/commit/ae775...
Also, all the database vendors provided their drivers implementing the JDBC API - good luck getting Oracle or IBM contribute to R2DBC.. (Actually, I stand corrected: there is an Oracle R2DBC driver now - it was released fairly recently though.)
EDIT: "failed miserably" is maybe too strong - but R2DBC certainly doesn't have the support and acceptance of JDBC.
JDBC knows nothing about the amount of connections a server can handle, but to try so many connections until it won’t connect any more.
| In any case, for the developer it would be a more streamlined experience, with less code needed, and better defaults.
I agree it would be best not to bother the dev with what is going on under the hood.
It might be different for some exotic databases, but I don't see any reason why ordinary JDBC driver couldn't reuse single TCP connection for multiple logical JDBC connections in this case.
My question is though: Why even do alleged “non-blocking” _at all_? What are people trying to optimize against?
A modern thread-per-core shared-nothing architecture takes this even further and tries to eliminate blocking at the hardware level for the same basic reason.
Nope. You can go async all the way down, right to the electrical signals if you want. We usually impose some amount of synchronous clocking/polling for sanity, at various levels, but you don't have to; the world is not synchronised, the fastest way to respond to a stimulus will always be to respond when it happens.
> Does it shake out to any real advantage?
Of course it does - did you miss the whole C10K discussions 20+ years ago? Whether it matters for your business is another question, but you can absolutely get a lot more throughput by being nonblocking, and if you're doing request-response across the Internet you generally can't afford not to.
When you spawn an OS thread you are paying at worst the full cost of it, and at best the max depth seen so far in the program, and stack overflows can happen even if the program is written correctly. Whereas a virtual thread can grow the stack to be exactly the size it needs at any point, and when GC runs it can rewrite pointers to any data on the stack safely.
Virtual/green/user space threads aka stackful coroutines have proven to be an excellent tool for scaling concurrency in real programs, while threads and processes have always played catchup.
> “something” will block eventually no matter what…
The point is to allow everything else to make progress while that resource is busy.
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At a broader scale, as a programming model it lets you architect programs that are designed to scale horizontally. With the commodization of compute in the cloud that means it's very easy to write a program that can be distributed as i/o demand increases. In principle, a "virtual" thread could be spawned on a different machine entirely.
Throughput.
Some workloads are not CPU-bound or memory-bound, and spend the bulk of their time waiting for external processes to make data available.
If your workloads are expected to stay idle while waiting for external events, you can switch to other tasks while you wait for those external events to trigger.
This is particularly convenient if the other tasks you're hoping to run are also tasks that are bound to stay idle while waiting for external events.
One of the textbook scenarios that suits this pattern well is making HTTP requests. Another one is request handlers, such as the controller pattern used so often in HTTP servers.
Perhaps the poster child of this pattern is Node.js. It might not be the performance king and might be single-threaded, but it features in the top spots in performance benchmarks such as TechEmpower's. Node.js is also highly favoured in function-as-a-service applications, as it's event-driven architecture is well suited for applications involving a hefty dose of network calls running on memory- and CPU-constrained systems.
You are right that everything blocks, even when going to L1 cache you have to wait 1 nanoseconds. But blocking in this context means waiting for “real” IO like a network request or spinning disk access. Virtual threads take away the problem that the thread sits there doing nothing for a while as it is waiting for data, before it is context switched.
Virtual threads won’t improve CPU-bound blocking. There the thread is actually occupying the CPU, so there is no problem of the thread doing nothing as with IO-bound blocking.
In one project I had to basically turn a reactive framework into a one thread per request framework, because passing around the MDC (a kv map of extra logging information) was a horrible pain. Getting it to actually jump ship from thread to thread AND deleting it at the correct time was basically impossible.
Has that improved yet?
Does the same apply to virtual threads?
Edit: I think I answered my own question. Java virtual threads have the same memory model as regular java threads so yes, I need to use the same semantics. That rules replacing the eventloop model for us.
See https://openjdk.org/jeps/429
If you keep ThreadLocal variables, they get inherited by child Threads. If you make many thousands of them, the memory footprint becomes completely unacceptable. If the memory used by ThreadLocal variables is large, it also makes it more expensive to create new Threads (virtual or not), so you lose most advantages of Virtual Threads by doing that.
You used to be able to depend on OS threads getting reused because you were pooling them. You can do the same with virtual threads if you wish and you will get the same behavior. The difference is we ought to spawn new threads per task now.
Side note, you have to specifically use InheritableThreadLocal to get the inheritance behavior you speak of.
class MyExecutor implements Executor {
private final Executor delegate;
public MyExecutor(Executor delegate) {
this.delegate = delegate;
}
@Override
public void execute(@NotNull Runnable command) {
var mdc = MDC.getCopyOfContextMap();
delegate.execute(() -> {
MDC.setContextMap(mdc);
try {
command.run();
} finally {
MDC.clear();
}
});
}
}[1] https://davidvlijmincx.com/posts/virtual-thread-performance-...
It’s a shame this article paints a neutral (or even negative) experience with virtual threads.
We rewrote a boring CRUD app that spent 99% of its time waiting the database to respond to be async/await from top-to-bottom. CPU and memory usage went way down on the web server because so many requests could be handled by far fewer threads.
Well somewhat but also not really. They are green threads like async/await, but it's use is more transparent, unlike async/await.
So there are no special "async methods". You just instantiate a "VirtualThread" where you normally instantiate a (kernel) "Thread" and then use it like any other (kernel) thread. This works because for example all blocking IO API will be automatically converted to non-blocking IO underwater.
Not really. What C# does is sort of similar but it has the disadvantages of splitting your code ecosystem into non-blocking/blocking code. This means you can “accidentally” start your non-blocking code. Something which may cause your relatively simple API to consume a ridiculous amount of resources. It also makes it much more complicated to update and maintain your code as it grows over the years. What is perhaps worse is that C# lacks an interruption model.
Java’s approach is much more modern but then it kind of had to be because the JVM already supported structured concurrency from Kotlin. Which means that Java’s “async/await” had to work in a way which wouldn’t break what was already there. Because Java is like that.
I think you can sort of view it as another example of how Java has overtaken C# (for now), but I imagine C# will get an improved async/await model in the next couple of years. Neither approach is something you would actually chose if concurrency is important to what you build and you don’t have a legacy reason to continue to build on Java/C# . This is because Go or Erlang would be the obvious choice, but it’s nice that you at least have the option if your organisation is married to a specific language.
I seem to remember that is was some pretty basic operations (like maybe read or something) that caused the thread not to unmount, and therefore just block the underlying os thread. At that point you've just invented the world's most complicated thread pool.
Some operations that don't cause a task switch to another virtual thread are:
- If you've called into a native library and back into Java that then blocks. In practice this never happens because Java code doesn't rely on native libraries or frameworks that much and when it does happen it's nearly always in-and-out quickly without callbacks. This can't be fixed by the JVM, however.
- File IO. No fundamental problem here, it can be fixed, it's just that not so many programs need tens of thousands of threads doing async file IO.
- If you're holding a lock using 'synchronized'. No fundamental problem here, it's just annoying because of how HotSpot is implemented. They're fixing this at the moment.
In practice it's mostly the last one that causes issues in real apps. It's not hard to work around, and eventually those workarounds won't be needed anymore.
Java has the power that they make relatively more decisions about the language and the libs that they don’t have to fix later. That’s a great value if you’re not building throw-away software but SaaS or something that has to live long.
Kotlin's design had no bearing on Java's or the JVM's implementation.
C# has an interruption model through CancellationToken as far as I'm aware.
With virtual threads, it seems like if you don't hit gotchas, you can spawn a thead, and run straight through blocking code and not worry about too many threads, etc. So you could do thread per connection/user chat servers and http servers and what not.
Yes, it's still shared memory, so you can miss out on the simplifying effect of explicit communication instead of shared memory communication and how that makes it easy to work with remote and local communication partners. But you can build a mailbox system if you want (it's not going to be as nice as built in one, of course). I'm not sure if Java virtual threads can kill each other effectively, either.
It's (with caveats, of course):
- a thread crashing will not bring the system down
- a thread cannot hog all processing time as the system ensures all threads get to run. The entire system is re-entrant and execution of each thread can be suspended to let other threads continue
- all CPU cores can and will be utilized transparently to the user
- you can monitor a thread and if it crashes you're guaranteed to receive info on why and how it crashed
- immutable data structures play a huge part of it, of course, but the above is probably more important
That's why Go's concurrency is not that good, actually. Goroutines are not even half-way there: an error in a goroutine can panic-kill your entire program, there are no good ways to monitor them etc.
The bigger problem with Go in this regard is how easy it is to cause a panic thanks to nil.
Go made half a step in the right direction with goroutines, but never committed fully
The Go authors are adamant that goroutines not be addressable (from without) or identifiable (from within). This is diametrically opposed to Erlang, where processes are meant to be addressed/identified. I can't say I've ever found a case where a problem couldn't be solved due to this constraint in Go, but it does complicate some things.
For example, you can actually share a thread with another runtime.
Cooperative threading allows for implicit critical sections that can be cumbersome in preemptive threading.
Async/await and virtual threads are solving different problems.
> What is perhaps worse is that C# lacks an interruption model
Btw, You'd just use OS threads if you really needed pre-emptively scheduled threads. Async tasks run on top of OS threads so you get both co-opertive scheduling within threads and pre-emptive scheduling of threads onto cores.
I’m not sure I said outdated, but I can see what you mean by how I called Javas approach “more modern”. What I should have called Javas approach was “correctly designed”.
C#’s async/await isn’t all terrible as you point out, but it’s designed wrong from the bottom up because computation should always be blocking by default. The fact that you can accidentally start running your code asynchronous is just… Aside from trapping developers with simple mistakes, it’s also part of what has lead to the ecosystem irrecoverably being split into two.
I was actually a little surprised to see Microsoft make their whole .Net to .Net core without addressing some of the glaring issues with it, when that massive disruption process uprooted everything anyway.
Why go? It has a quite anemic standard library for concurrent data structures, compared to java and is a less expressive , and arguably worse language on any count, verbosity included.
No, the I/O is still blocking with respect to the application code.
The biggest difference is that C# async/await code is rewritten by the compiler to be able to be async. This means that you see artifacts in the stack that weren’t there when you wrote the code.
There are no rewrites with virtual threads and the code is presented on the stack just as you write it.
They solve the same problem but in very different ways.
Yes. Async/await is stackless, which leads to the “coloured functions” problem (because it can only suspend function calls one-by-one). Threads are stackful (the whole stack can be suspended at once), which avoids the issue.
It does not enable easy concurrency and task/future composition the way C#/JS/Rust do, which offer strictly better and more comprehensive model.
In Java they're working on a structured concurrency library to bridge this gap, but IMO, it'll end up looking like async/await with all its ups and downs but with less sugar.
Async/await is able to achieve good UX around explicitly defining what goes on your Main thread and what goes elsewhere. Its trivial to mix UI thread and background thread code by bouncing between synchronization contexts as needed.
When the threading model is implicit its impossible to have this control.
Links:
https://github.com/dotnet/runtimelab/issues/2398
https://github.com/dotnet/runtimelab/blob/feature/green-thre...
> Green threads introduce a completely new async programming model. The interaction between green threads and the existing async model is quite complex for .NET developers. For example, invoking async methods from green thread code requires a sync-over-async code pattern that is a very poor choice if the code is executed on a regular thread.
Also to note that even the current model is complex enough to warrant a FAQ,
https://devblogs.microsoft.com/dotnet/configureawait-faq
https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/b...
JVM Green Threads here serve predominantly back-end scenarios, where most of the items on the list are not of concern. This list also exists to address bad habits that carried over from before the tasks were introduced, many years ago.
In general, the perceived want of green threads is in part caused by misunderstanding of that one bad article about function coloring. And that one bad article about function coloring also does not talk about the way you do async in C#.
Async/await in C# in back-end is a very easy to work with model with explicit understanding where a method returns an operation that promises to complete in the future or not, and composing tasks[0] for easy (massive) concurrency is significantly more idiomatic than doing so with green threads or completable futures that existed in Java before these. And as evidenced by adoption of green threads by large scale Java projects, turns out the failure modes share similarities except green threads end up violating way more expectations and the code author may not have any indication or explicit mechanism to address this, like using AsyncLocal.
Also one change to look for is "Runtime Handled Tasks" project in .NET that will replace Roslyn-generated state machine code with runtime-provided suspension mechanism which will only ever suspend at true suspension points where task's execution actually yields asynchronously. So far numbers show at least 5x decrease in overhead, which is massive and will bring performance of computation heavy async paths in line with sync ones:
https://github.com/dotnet/runtimelab/blob/feature/async2-exp...
Note that you were trivially able to have millions of scheduled tasks even before that as they are very lightweight.
[0]: e.g. sending requests in parallel is just this
using var http = new HttpClient() {
BaseAddress = new("https://news.ycombinator.com/news")
};
var requests = Enumerable
.Range(1, 4)
.Select(n => $"?p={n}")
.Select(http.GetStringAsync);
var pages = await Task.WhenAll(requests);You'd jsut have to define a ThreadPool with n Threads before, where each request would've blocked one pending thread. Now it just keeps going.
So your equivalent Java example should've been something like this, but again: the completeable futures api is pretty old at this point.
@HttpExchange(value = "https://news.ycombinator.com")
interface HnClient {
@GetExchange("news?p={page}")
CompletableFuture<String> getNews(@PathVariable("page") Integer page);
}
@RequiredArgsConstructor
@Service
class HnService {
private final HnClient hnClient;
List<String> getNews() {
var requests = IntStream.rangeClosed(1, 4)
.boxed().map(hnClient::getNews).toList();
return requests.stream().map(CompletableFuture::join).toList();
}
}Also, I wouldnt consider that the equivalent Java code. That is all Spring and Lombok magic. Just write the code and just use java.net.HttpClient.
No.
That being said, imo you can’t call something equivalent when doing a bunch of spring magic. This disregards that OPs logic isn’t equivalent at all. It waits for each future 1 by 1 instead of doing something like CompletableFuture.allOf or in JS: Promise.all.
Take for instance Go. It is well liked in part, because its so easy to do concurrency with goroutines, and they're easy to reason about, easy to call, easy to write, and for how much heavy weight they're lifting, relatively simple to understand.
The reason Java is getting alot of kudos here for their implementation of green threads is exactly the same reason people talk about Go being an easy language to use for concurrency: It doesn't gate code behind specialized idioms / syntax / features that are only specific to asynchronous work. Rather, it largely utilizes the same idioms / syntax as synchronous code, and therefore is easier to reason about, adopt, and ultimately I think history is starting to show, to use.
Java is taking an approach paved by Go, and ultimately I think its the right choice, because having worked extensively with C# and other languages that use async / await, there are simply less footguns for the average developer to hit when you reduce the surface area of having to understand async / sync boundaries.
[0]: https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...
[1]: HN discussion: https://news.ycombinator.com/item?id=8984648
I'm tired of this, that one article is bad, and incorrect, and promotes straight-up harmful intuition and probably sets the industry in terms of concurrent and asynchronous programming back by 10 years in the same way misinterpreting Donald Knuth's quote did in terms of performance.
Also, nothing prevents you from building your own, more limited but safer (the two always come together!) abstraction on top, but you couldn’t express Loom on async as the primitive.
(I guess scaling to ridiculous levels you could be approaching trouble if you have O(100k) outstanding DB queries per application server, hope you have a DB that can handle millions of oustanding DB queries then!)
But the bottom line with virtual threads, go-routines, or kotlin's co-routines is that it indeed allows for imperative code style code that is easy to read and understand. Of course you still need to understand all the pitfalls of concurrency bugs and all the weird and wonderful way things can fail to work as you expect. And while Java's virtual threads are designed to work like magic pixie dust, it does have some nasty failure modes where a single virtual thread can end up blocking all your virtual threads. Having a lot of synchronized blocks in legacy code could cause that.
However, the use of JAVA for me is for admin backend or heavy weight services for enterprises or startups I coded for, so for my taste I can't use it without spring or jboss, etc.. , and in that way I think simplicity went out the window a long long time ago :) It took me years to learn all the quirks of these frameworks... and the worse thing about it is that they keep changing every few months...
For example ktor works on the JVM but you can also build native applications with it. And I use ktor client in the browser. When running in the browser it uses the browser fetch API. When running on the jvm you can configure it to use any of a wide range of Java http clients. On native it uses curl.
Go is a next-gen trumpian language that rejects sum types, pattern matching, non-nil pointers, and for years, generics; it's unhinged.
> pretending Erlang does not exist
For better or worse it doesn't to most programmers. The syntax is not nearly as approachable as GoLang. Luckily Elixir exists.