Explaining how virtual threads work 2 – Concurrency – Virtual Threads, Structured Concurrency

Capturing virtual threads

So far we learned that a virtual thread is mounted by JVM to a platform thread which becomes its carrier thread. Moreover, the carrier thread runs the virtual thread until it hit a blocking (I/O) operation. At that point, the virtual thread is unmounted from the carrier thread and it will be rescheduled after the blocking (I/O) operation is done.While this scenario is true for most of the blocking operations resulting in unmounting the virtual threads and freeing the platform thread (and the underlying OS thread), there are a few exceptional cases when the virtual threads are not unmounted. There are two main causes for this behavior:

Limitations on OS (for instance, a significant number of filesystem operations)

Limitations on JDK (for instance, Object#wait())

When the virtual thread cannot be unmounted from its carrier thread it means that the carrier thread and the underlying OS thread are blocked. This may affect the scalability of the application, so, if the platform threads pool allows it, JVM can take the decision of adding one more platform thread. So, for a period of time, the number of platform threads may exceed the number of available cores.

Pinning virtual threads

There are also two other use cases when a virtual thread cannot be unmounted:

When the virtual thread runs code inside a synchronized method/block

When the virtual thread invokes a foreign function or native method (topic covered in Chapter 7)

In this scenario, we say that the virtual thread is pinned to the carrier thread. This may affect the scalability of the application, but JVM will not increase the number of platform threads. Instead of this, we should take action and refactor the synchronized blocks to ensure that the locking code is simple, clear, and short. Whenever possible prefer java.util.concurrent locks instead of synchronized blocks. If we manage to avoid long and frequent locking periods then we will not face any significant scalability issues. In future releases, the JDK team aims to eliminate the pinning inside synchronized blocks.