RFR: 8262398: Shenandoah: Disable nmethod barrier and stack watermark when running with passive mode

[Zhengyu Gu]

On Thu, 25 Feb 2021 20:20:55 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> If it is possible at all, it would be most useful if we could have flags like ShenandoahNMethodBarrier and ShenandoahStackWatermark, and let them drive enabling and disabling of the corresponding barriers/features. It would be even better, if we could configure concurrent modes such that they do STW class-unloading and stack-processing (if possible with reasonable amount of work). That would help ports where those features are not yet present (current ports under development with the problem are PPC and Graal).

Disabling nmethod barrier and/or stack watermark for concurrent GC means crashes, we no longer have backup (e.g. processing thread roots and code roots at pauses) so I don't think it will help ports.

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PR: https://git.openjdk.java.net/jdk/pull/2727