RFR: 8368251: Parallel: Refactor lgrp_id used in MutableNUMASpace [v2]

[Leo Korinth]

On Tue, 23 Sep 2025 08:58:19 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> Hello,
>> 
>> There are multiple cases which handle if the locality group (lgrp) id is negative (-1) in Parallel, specifically in MutableNUMASpace. -1 was historically used to indicate that the hardware topology has changed and that the lgrp id needs to be updated. This is no longer the case, since HotSpot doesn't support CPU/NUMA hotplugging, and the last piece of code setting the lgrp id to -1 was removed in [JDK-8301149](https://bugs.openjdk.org/browse/JDK-8301149).
>> 
>> I propose we simplify the code a bit by not handling edge cases where the lgrp id is -1 and removing os::numa_has_group_homing(), which always return false
>> 
>> Testing:
>>  * Running through Oracle's tier 1-4
>>  * Local NUMA testing tier1-4 with `-XX:+UseParallelGC -XX:+UseNUMA`
>
> Joel Sikström has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Don't set lgrp_id on Windows since we don't support NUMA there

src/hotspot/share/runtime/thread.cpp line 68:

> 66:   set_stack_base(nullptr);
> 67:   set_stack_size(0);
> 68:   _lgrp_id = -1;

Here I  would prefer initialising the value in an initialiser list but I think it is okay to let it be as it is (it is following the style of the file).

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PR Review Comment: https://git.openjdk.org/jdk/pull/27424#discussion_r2372403442