RFR: 8322043: HeapDumper should use parallel dump by default [v3]

[David Holmes]

On Mon, 15 Apr 2024 23:18:54 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.
>
> Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   check free_memory for OOME

src/hotspot/share/services/heapDumper.hpp line 63:

> 61:   // additional info is written to out if not null.
> 62:   // compression >= 0 creates a gzipped file with the given compression level.
> 63:   // parallel_thread_num >= 0 indicates thread numbers of parallel object dump, -1 means "auto select".

I don't understand why you need to add `-1` to mean "auto-select" instead of just setting the default parameter to be `default_num_of_dump_threads()`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/18748#discussion_r1567017789