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.
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.