This PR reduces the Thread-SMR (Safe Memory Reclamation) related cost of starting and exiting threads.
Both starting and exiting a thread create a new `ThreadsList` and retire the old one. Previously, we used to scan the hazard pointers of all threads when a `ThreadsList` was retired. This made the execution cost increasingly higher as more threads were started. This PR tackles the problem in three ways: 1. By not scanning hazard pointers every time a new thread is started. Instead, we defer the scanning until either 16 `ThreadsLists` have been retired or the retired lists occupy at least 4 MB of memory. 2. We use the scan that is performed when removing a thread to also determine if it's safe to delete the thread, and thereby we potentially avoid a costly re-scan in `wait_until_not_protected()`. 3. By checking if there is no retired `ThreadsList` that can protect the thread, `wait_until_not_protected()` can also avoid a re-scan. I've run the ThreadStart reproducer that is attached to [JDK-8307970](https://bugs.openjdk.org/browse/JDK-8307970). Depending on the number of idle threads, the changes in this PR show that threads start up to ~63% faster (or in other words, a ~39% reduction in time to start a thread). To go further than this likely requires a major rewrite of the Thread-SMR system, which would not be a walk in the park. Passes tier1-3 tests. --------- - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - Added reproducer ThreadStartWithIdleThreads.java - Merge branch 'master' into 8307970_thraedstart_is_slow - Updated comments for David and Daniel - 8307970: Thread.start is slow with large number of threads Changes: https://git.openjdk.org/jdk/pull/33166/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=33166&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8307970 Stats: 217 lines in 5 files changed: 213 ins; 0 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/33166.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/33166/head:pull/33166 PR: https://git.openjdk.org/jdk/pull/33166
