RFR: 8368251: Parallel: Refactor lgrp_id used in MutableNUMASpace

[Joel Sikström]

On Mon, 22 Sep 2025 11:42:39 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`

src/hotspot/os/windows/os_windows.cpp line 539:

> 537:   if (UseNUMA) {
> 538:     thread->update_lgrp_id();
> 539:   }

I'm a bit hesistant to remove this code from Windows since I have no way to test this. However, since we set `UseNUMA = false` early on initialization on Windows, Paralell should never start with NUMA enabled, so this code should effectively have no effect.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27424#discussion_r2368082735