May I have Project Loom Clarified?

Question

Brian Goetz got me excited about project Loom and, in order to fully appreciate it, I'll need some clarification on the status quo.

My understanding is as follows: Currently, in order to have real parallelism, we need to have a thread per cpu/core.

1. Is there then any point in having n+1 threads on an n-core machine? Project Loom will bring us virtually limitless threads/fibres, by relying on the jvm to carry out a task on a virtual thread, inside the JVM.
2. Will that be truly parallel?
3. How, specifically, will that differ from the aforementioned scenario "n+1 threads on an n-core machine "?

Thanks for your time.

Answer 1

Virtual threads allow for concurrency (IO bound), not parallelism (CPU bound). They represent causal simultaneity, but not resource usage simultaneity.

In fact, if two virtual threads are in an IO bound* state (awaiting a return from a REST call for example), then no thread is being used at all. Whereas, the use of normal threads (if not using a reactive or completable semantic) would both be blocked and unavailable for use until the calls are complete.

*Except for certain conditions (e.g., use of synchonize vs ReentrackLock, blocking that occurs in a native method, and possibly some other minor areas).