RFR: 8272611: Use parallel heap inspection when print classhisto info in gc log

[Thomas Schatzl]

On Mon, 23 Aug 2021 10:39:59 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> This patch helps to enhance the heap inspection with parallel threads when print classhisto info in gc log.
>> 
>> I have built it on linux x86_64 and run test-tier1 successfully
>> 
>> ---------
>> ### Progress
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>> 
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>
> src/hotspot/share/gc/shared/collectedHeap.cpp line 551:
> 
>> 549:     ResourceMark rm;
>> 550:     LogStream ls(lt);
>> 551:     uint parallel_thread_num = MAX2<uint>(1, (uint)os::initial_active_processor_count() * 3 / 8);
> 
> Where do the magic numbers for the formula come from? Please consider using the ergonomically determined number of threads (`CollectedHeap::safepoint_workers()->active_workers()`)? (Maybe change this in another CR as other reviewers think is better, looking whether there is a significant difference between these numbers)

Note: this is probably copy&pasted from `ClassHistogramDCmd::execute`; instead of the copy&paste please at least add something like a static `default_parallel_thread_num()` method to `VM_GC_HeapInspection`.

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PR: https://git.openjdk.java.net/jdk/pull/5158