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

[Bin Liao]

On Mon, 23 Aug 2021 10:50:45 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> 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`.

> 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)

Thank you for your advice, it seems that (CollectedHeap::safepoint_workers()->active_workers()) looks better.

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