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.
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.
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).
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.
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 think it’s definitely worth a read.
