### Summary For Shenandoah and ZGC, safepoint workers were created at startup and only used for heap dump and heap inspection. If heap dump or heap inspection never runs, these threads consume memory without doing any work. For G1 and Parallel, GC pauses and non-GC safepoints share the GC worker pool, so heap dump and heap inspection don't need a separate worker pool. The downside is that dump and inspection work contributes to GC CPU time, which can affect how G1 sizes its heap.
### Changes Safepoint workers are replaced with a diagnostic worker pool used by heap dump and heap inspection. The diagnostic worker pool is created on the first heap dump or heap inspection that requests more than one thread, if the collector supports parallel heap iteration and `ParallelGCThreads` > 1. The pool holds at most `ParallelGCThreads` workers, and workers stay alive until the VM exits. G1 and Parallel no longer lend their GC worker pool. ZGC's `ZRuntimeWorkers` is deleted because it's no longer used. For Shenandoah, diagnostic workers have no GCLAB and fall back to shared allocation when they evacuate objects. ### Testing all passed - linux-x86_64-server-fastdebug: `hotspot:tier1_gc`, `hotspot:tier1_serviceability`, `gc/shenandoah` - GHA - Internal CI pipeline --------- - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - Fix merge conflicts - Improvements to test - Create test for parallel class histogram and make workers thread safe - Remove gclab for diagnostic workers and add test coverage - Remove stale comment - Improvements - Initial version Changes: https://git.openjdk.org/jdk/pull/33218/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=33218&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8392050 Stats: 612 lines in 19 files changed: 421 ins; 136 del; 55 mod Patch: https://git.openjdk.org/jdk/pull/33218.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/33218/head:pull/33218 PR: https://git.openjdk.org/jdk/pull/33218
