RFR: 8322043: HeapDumper should use parallel dump by default

[Thomas Stuefe]

On Fri, 12 Apr 2024 02:17:34 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

> The fix makes VM heap dumping parallel by default.
> `jcmd GC.heap_dump` and `jmap -dump` had parallel dumping by default, the fix affects `HotSpotDiagnosticMXBean.dumpHeap()`, `-XX:+HeapDumpBeforeFullGC`, `-XX:+HeapDumpAfterFullGC` and `-XX:+HeapDumpOnOutOfMemoryError`.
> 
> Testing:
>   - manually tested different heap dump scenarios with `-Xlog:heapdump`;
>   - tier1,tier2,hs-tier5-svc;
>   - all reg.tests that use heap dump.

I am curious: what is the memory overhead for parallel mode, and (I am not familiar with the logic) how many threads are involved? Is the number of thread bounded?

I ask because, especially for the OnOOM handling, we may already be at a limit memory-wise. Starting to swap will probably be worse than running single-threaded.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/18748#issuecomment-2051046673