c.f: [https://bugs.openjdk.org/browse/JDK-8339420](https://bugs.openjdk.org/browse/JDK-8339420)
Summary ------- Add `jcmd <pid> Thread.vthread_summary` to print summary information that is useful when trying to diagnose issues with virtual threads. Problem ------- The JDK is lacking tooling to diagnose issues with virtual threads. Solution -------- Add a new command that the `jcmd` command line tool can use to print information about virtual threads. The output includes the virtual thread scheduler, the schedulers used to support timeouts, and the I/O pollers used to support virtual threads doing socket I/O, and a summary of the thread groupings. Here is sample output. The output is intended for experts and is not intended for automated parsing. Virtual thread scheduler: java.util.concurrent.ForkJoinPool@4a624db0[Running, parallelism = 16, size = 2, active = 0, running = 0, steals = 2, tasks = 0, submissions = 0] Timeout schedulers: [0] java.util.concurrent.ScheduledThreadPoolExecutor@1f17ae12[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0] [1] java.util.concurrent.ScheduledThreadPoolExecutor@6193b845[Running, pool size = 1, active threads = 0, queued tasks = 1, completed tasks = 0] [2] java.util.concurrent.ScheduledThreadPoolExecutor@c4437c4[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0] [3] java.util.concurrent.ScheduledThreadPoolExecutor@3f91beef[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0] Read I/O pollers: [0] sun.nio.ch.KQueuePoller@524bf25 [registered = 1] Write I/O pollers: [0] sun.nio.ch.KQueuePoller@25c41da2 [registered = 0] Thread groupings: <root> [platform threads = 11, virtual threads = 0] java.util.concurrent.ScheduledThreadPoolExecutor@c4437c4 [platform threads = 0, virtual threads = 0] java.util.concurrent.ScheduledThreadPoolExecutor@3f91beef [platform threads = 0, virtual threads = 0] ForkJoinPool.commonPool/jdk.internal.vm.SharedThreadContainer@4fa374ea [platform threads = 0, virtual threads = 0] java.util.concurrent.ThreadPoolExecutor@506e1b77 [platform threads = 1, virtual threads = 0] java.util.concurrent.ScheduledThreadPoolExecutor@1f17ae12 [platform threads = 0, virtual threads = 0] java.util.concurrent.ThreadPerTaskExecutor@24155ffc [platform threads = 0, virtual threads = 2] ForkJoinPool-1/jdk.internal.vm.SharedThreadContainer@48a03463 [platform threads = 2, virtual threads = 0] java.util.concurrent.ScheduledThreadPoolExecutor@6193b845 [platform threads = 1, virtual threads = 0] The "Virtual thread scheduler" section show the target parallelism, the number of threads in the scheduler's pool, and other useful counters. The "Timeout schedulers" section provides information on the usage of the ScheduledThreadPoolExecutor used to support virtual thread doing timed operations. The "I/O pollers" section will vary by OS. The sample output here is macOS which, by default, has a one read and one write poller. This is useful to see how many blocking network I/O operations on virtual threads are in progress. The "Thread groupings" appears a support people to get a sense of how the virtual threads are grouped. Virtual threads created directly with the Thread API are in the "root" grouping. Some servers may have one or more ThreadPerTaskExecutor instances. For now, this section doesn't print the tree of thread groupings that arises when using the Structured Concurrency API. Specification ------------- The output from `jcmd <pid> Thread.vthread_summary -help` is: Thread.vthread_summary` Print the virtual thread scheduler, timeout schedulers, I/O pollers, and thread groupings. Impact: Low ------------- Commit messages: - JDK-8337199: removed thread groupings iimpl from dcmd as per Alan B, and disabled asssociated tests - JDK-8337199: merge master - Sync with loom repo - Sync up from loom repo - Update from loom repo - Initial commit Changes: https://git.openjdk.org/jdk/pull/22121/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22121&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8337199 Stats: 466 lines in 11 files changed: 456 ins; 6 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/22121.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/22121/head:pull/22121 PR: https://git.openjdk.org/jdk/pull/22121
