Virtual Threads

• JDK 21
• From Project Loom
• Is also a Thread: use as Platform Thread, have ThreadLocal

Virtual Threads vs Platform Thread

-	Virtual Thread	Platform Thread
Allocation	On Platform Thread	On OS Thread
Size	200 ~ 300 bytes	2~4 Mb

• We can allocate much more virtual threads than platform threads
• Virtual threads DON'T make programs run faster, but it makes scaling much better

Context Switching

TODO: more details

Virtual Threads mechanism

• JVM maintains a pool of platform threads, created and maintained by a dedicated ForkJoinPool
• size = min (256, CPU cores)
• Each virtual thread is created, then the JVM schedules its execution on a platform thread.
  • The Platform thread is called the Carrier Thread
• One platform thread can only execute a virtual thread at a time
• When the virtual thread blocks on a blocking operation, the carrier thread select the next virtual thread in queue
• When virtual thread finish its blocking task, it is scheduled again for next execution

Utilizing Virtual Threads for highly concurrency apps

What Platform Threads stuck

• Size
• Resources Limitation: Number of cores
• Context Switching

For Highly concurrency applications, using Threads is not possible solution

Future?

• Also uses a Thread Pool, which uses Platform Thread => Same issues

Virtual Threads proposal

• For any time-consuming tasks, run it using a Virtual Thread
  • E.g: API calls, database queries, files I/O...