Does Java's implementation of virtual threads perform any kind of work stealing when a particular physical thread has no virtual threads to run (e.g. they are all blocked on I/O)?

It does. They get scheduled onto the ForkJoinPool which is a work stealing pool.

"they run virtual threads as needed" - so when one virtual thread is no longer needed and another one is needed, they switch context, yes?

This is called mounting/un-mounting and is much cheaper than a context switch.

This is a type of context switch. You are saying dollars are cheaper than money.

It's been a really long time since I dealt with anything this low level, but in my very limited and ancient experience when people talk about context switching they're talking specifically about the userland process yielding execution back to the kernel so that the processor can be reassigned to a different process/thread. Naively, if the JVM isn't actually yielding control back to the kernel, it has the freedom to do things in a much more lightweight manner than the kernel would have to.

So I think it's meaningful to define what we mean by context switch here.

When a real thread is allocated from a virtual thread to another, the JVM needs to save into the heap the stack of the first virtual thread and restore from the heap the stack of the second virtual thread, see slide 13 of [1]. This is in fact called mounting/unmounting as already pointed out, and occurs via Java Continuation, but from the JVM perspective this is a context switch. It's called JVM and the V stands for Virtual, so yes, it's not the kernel doing it, but it's happening, and it's more frequent the more virtual threads you have in your application.

[1] https://www.eclipsecon.org/sites/default/files/slides/JavaLo...

swapcontext(3) is a userland context switch, and is named so.

The JVM will need to do context switching when reallocating the real thread that is running a blocked virtual thread to the next available virtual thread. It won't be CPU context switching, but context switching happens at the JVM level and represents an effort.

Ok. This JVM-level switching is called mounting/un-mounting of the virtual thread and is supposed to be several orders of magnitude cheaper compared to normal context switch. You should be fine with millions of virtual threads.