RFR: 8322043: HeapDumper should use parallel dump by default

[Alex Menkov]

On Fri, 12 Apr 2024 06:07:31 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> 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.

Good question.
It think it's several MB per each additional thread (1MB output buffer, DumperClassCacheTable - 1031 elements max, element size depends on class field numbers, if HeapDumpGzipLevel is set, some buffers for gzip compressors)
Number of threads by default is min of `os::initial_active_processor_count() * 3 / 8` and number of GC workers.

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PR Comment: https://git.openjdk.org/jdk/pull/18748#issuecomment-2052312786