Loom - Where Are We?
summary
Virtual threads become permanent in JDK 21 and the HotSpot VM now uses the limited continuation implementation plus a ForkJoin carrier scheduler; the current work adapts the VM’s lightweight locking (the lock stack) to avoid object monitors pinned scenarios by freezing lock-stack entries during continuation unmounts and adding VM-initiated preemption for contended monitor enter/exit. Early-access builds since March show the prototype works for lightweight monitors, diagnostics (JFR, jcmd, heap/thread dumps, JVMTI) improve observability, one performance wrinkle remains, and JEPs for JDK 24 and previews for Structured Concurrency and Scoped Values are planned.
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chapters (11)
- 0:00Project Loom update
- 1:16Virtual thread background
- 3:07Pinning and scheduling
- 9:39Object monitor pain points
- 13:35Fixing monitor pinning
- 18:51Monitor rehabilitation
- 26:06Other pinning challenges
- 29:25Troubleshooting and tools
- 35:15Scheduler exploration
- 40:27Structured concurrency
- 44:43Scope values and summary
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