RFR: 8277736: G1: Allow forced evacuation failure of first N regions in collection set

[Hamlin Li]

On Tue, 30 Nov 2021 09:58:54 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Currently, debug options of evacuation failure, like G1EvacuationFailureALotInterval and G1EvacuationFailureALotCount only config the ratio of objects failing evacuation, basically the distribution of objects failing evacuation is even among regions.
>> 
>> JDK-8256265 is trying to improve parallelism in regions that failed evacuation, it'll be helpful if there is option to config the distribution of objects failing evacuation among regions, so implementation of JDK-8256265 could be profiled more precisely.
>> 
>> This is to add the support and necessary vm options, and the implementation will be to select regions from start, i.e. not randomly, the motivation is to configure a more stable selection of evacuation failure regions, which will help verify the effect of JDK-8256265 implementation.
>
> src/hotspot/share/gc/g1/g1_globals.hpp line 78:
> 
>> 76:           "evacuation pauses")                                              \
>> 77:                                                                             \
>> 78:   product(uintx, G1EvacuationFailureALotCSetPercent, 100,                   \
> 
> Why `uintx`? Does `uint` work?

I will refine the explanation specifically.
As Thomas suggested, an random selection will be added later, I plan to add another bool option to indicate if it's random selection or not, so will keep `G1EvacuationFailureALotCSetPercent`.

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PR: https://git.openjdk.java.net/jdk/pull/6561