RFR: 8255984: Shenandoah: "adaptive" heuristic is prone to missing load spikes [v3]

[earthling-amzn]

On Mon, 16 Nov 2020 08:39:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> earthling-amzn has updated the pull request incrementally with six additional commits since the last revision:
>> 
>>  - Inline calls to gc decision methods (vestige of an earlier design)
>>  - Use os::elapsedTime to avoid type issues and to be consistent with other heuristics code
>>  - Reuse instantaneous_rate method instead of duplicating code
>>  - Rename variables to improve readability
>>  - Make logging messages more consistent
>>  - Restore call to reset allocation counter at cycle start
>
> I did some performance tests on the variant of this patch, and while it regresses throughput in some scenarios, that's the price we pay for additional spike safety. I think we can drop current alloc-spike and penalty tracking (even from `shenandoahHeuristics`?).
> 
> Another round of tune-ups follows.

@shipilev , what workload are you running? I'd like to run it as well.

I'm reluctant to remove the headroom and penalty adjustments. They're an additional layer of safety and let the user provide information about the application that we wouldn't otherwise know. Perhaps we could externalize the penalty adjustments for headroom as command line arguments? If somebody wants to run closer to the edge, they could set the headroom and penalties to zero.

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PR: https://git.openjdk.java.net/jdk/pull/1099