Multiple threads in JVM - when are they using multiple cores?

Question

I recently had a chance to listen to another discussion about the JVM. It was stated that Java Virtual Machine is well built in terms of concurrency. I could not find any satisfying answer to what I thought to be a simple question: when JVM runs multiple-threads (and therefore uses multiple virtual-cores, right?) does it make use of multiple real cores of machine's CPU?

I heard "not always" or "sometimes" as an answer; so is there any way to ensure that when we design our code to run multiple threads the JVM will use multiple cores of the CPU as well? Or the other way, what determines whether the JVM uses mutliple CPU cores or not?

I am not really able to give an example of when this would be necessary, but I find it interesting, as I know designers who prefer everything to be deterministic in their project. And what would really be the point of having multiple threads in some big applications if for real they would never be computed parallely?

Answer 1

Java threads, like regular threads, may be scheduled to use multiple cores. The real sticky thing about concurrent programming is that it's hard to "force" a program to use X number of cores and to have this thread run on this core, etc. In other words, to be deterministic.

It's ultimately up to the OS and in some sense the hardware. But at the end of the day the programmer should be thinking about concurrent programming abstractly and trusting that the scheduler is doing its job.

In other words, yes.

Answer 2

In all mainstream, modern jvms, the java threads are "real" operating system threads and therefore will be scheduled across cores just like any other OS threads.