RFR: JDK-8297796 GenShen: instrument the remembered set scan [v2]

[Y. Srinivas Ramakrishna]

On Thu, 8 Dec 2022 14:42:53 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> Thanks for sharing this code. A few overview comments:
> 
> 1. Yes, I think it would be useful to see the data collected for each mark scan and each update-reference scan independently.  Sometimes, abnormal behavior of the application causes spikes in performance, and it would be nice to understand the degree to which remembered set scanning is part of this spike.
> 2. It is also useful to have a cumulative summary of all costs at the end of a run, probably still separating out the mark scans from the update-refs scans.

I'll make those changes.

> 3. Is it possible to eliminate the overhead entirely of this instrumentation by compiling it out for release builds?

It essentially is compiled out of product/release builds, but compiled only into optimized/release and *debug builds. I'll gather numbers to support that, as well as include the `.s` listing for process_clusters that should be unaffected by the presence of the stat calls which would be inlined and constant folded out.

I'll make the changes for 1. and 2., and add the supporting data for 3.

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PR: https://git.openjdk.org/shenandoah/pull/176