RFR: 8352181: Shenandoah: Evacuate thread roots after early cleanup

[Aleksey Shipilev]

On Mon, 17 Mar 2025 21:37:14 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Moving the evacuation of thread roots after early cleanup allows Shenandoah to recycle immediate garbage a bit sooner in the cycle.

I believe the reason we do thread roots earlier is to do the bulk of the stack processing before mutator sees it. If mutator does it by itself, it will go through armed nmethod barriers, which might be introducing extra latency. 

So we need to think if the benefit of doing the immediate cleanup earlier is worth accepting more active nmethod barriers in mutator.

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PR Review: https://git.openjdk.org/jdk/pull/24090#pullrequestreview-2693567054