[giamma]

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.

[frant-hartm]

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.