RFR: 8306836: Remove pinned tag for G1 heap regions [v6]

[Serguei Spitsyn]

On Tue, 2 May 2023 16:47:06 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that removes the pinned tag from `HeapRegion`.
>> 
>> So that "pinned" tag for G1 heap regions indicates that the region should not move during (young) gc. This applies to now removed archive regions and humongous objects/regions.
>> 
>> With "real" g1 region pinning to deal with gclocker in g1 once and for all upcoming we need a refcount, a single bit is not sufficient anymore. Further there will be a naming conflict as this kind of "pinning" is different to g1 region pinning "pinning". The former indicates "contents can not be moved, but can be reclaimed", while the latter means "contents can not be moved and not reclaimed".
>> 
>> The (current) pinned flag is surprisingly little used, only for policy decisions.
>> 
>> The suggestion this change implements is to remove the "pinned" tag as it is, and reserve it for future g1 region pinning (that needs to store the pinning attribute differently as a refcount anyway).
>> 
>> Testing: tier1-3, gha
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove is_young_gc_movable

This looks good in general.
I can't judge on the GC side decision about this removal
and all updated comments but it looks consistent.
Thanks,
Serguei

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Marked as reviewed by sspitsyn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13643#pullrequestreview-1409712756