RFR: 8256390: ZGC: Relocate in-place instead of having a heap reserve [v3]

[Albert Mingkun Yang]

On Mon, 23 Nov 2020 08:07:26 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Per Liden has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
>> 
>>  - Merge master
>>  - Review Stefank+Erik
>>  - Review Stefank
>>  - 8256390: ZGC: Relocate in-place instead of having a heap reserve
>
> Marked as reviewed by stefank (Reviewer).

Not a review, just a comment in passing

I believe by merging `relocate_object` and `relocate_object_inner`, one can defer `retain_page()` after the forwarding table returns nothing. This may reduce the blocking window in `detach_page` before freeing a page, since some pages may never be retained.

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PR: https://git.openjdk.java.net/jdk/pull/1228