RFR: 8351167: ZGC: Lazily initialize livemap [v2]

[Thomas Schatzl]

On Tue, 4 Mar 2025 20:17:28 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> Memory for the bitmap inside the livemap of a ZPage is currently allocated upon calling its constructor, which adds a latency overhead when allocating pages. As preparation for the Mapped Cache ([JDK-8350441](https://bugs.openjdk.org/browse/JDK-8350441)), but also as a standalone improvement, we want to instead lazily initialize the livemap's bitmap.
>> 
>> This patch holds off with allocating memory for the bitmap that the livemap uses until the livemap is written to the first time (i.e. by calling ZLiveMap::set). The effect of this is that the latency impact of allocating the bitmap will only be taken by GC threads and not by mutator threads, since only GC threads mark objects before pushing them onto the mark stack. This improvement will reduce page allocation latencies somewhat.
>> 
>> In addition to lazily allocating the bitmap, I've converted the static C-style cast to a checked cast for `ZPage::object_max_count()`, which is passed as the size to the bitmaps. This is because a value not contained in 32 bits will overflow with the C-style cast and give a too small bitmap when passed to the livemap. This is not an observed issue, just more of a sanity check.
>> 
>> Testing:
>> * Tiers 1-5
>> * GHA
>
> Joel Sikström has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Copyright years

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23907#pullrequestreview-2678106656