RFR: 8277137: Set OnSpinWaitInst/OnSpinWaitInstCount defaults to "isb"/1 for Arm Neoverse N1 [v3]

[Andrew Haley]

On Wed, 17 Nov 2021 12:31:10 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> One `ISB` implementation of `Thread.OnSpinWait` shows performance improvements on Graviton2 (Arm Neoverse N1 implementation), e.g. https://github.com/openjdk/jdk/pull/5562#issuecomment-966153163. 
>> 
>> Testing:
>> - `make test TEST=gtest`: Passed
>> - `make run-test TEST=tier1`: Passed
>> - `make run-test TEST=tier2`: Passed
>> - `make run-test TEST=hotspot/jtreg/compiler/onSpinWait`: Passed
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Set defaults for OnSpinWaitInst/OnSpinWaitInstCount independently

Did we establish that this is the right default for Neoverse N1? I know that we've found a benchmark where it's a win, bit I'm not sure that's the same thing. On the other hand, do we know of possible cases where ISB makes things worse?

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PR: https://git.openjdk.java.net/jdk/pull/6415