8388931: Shenandoah: Optimize ShenandoahHeapRegion footprint

[shipilev]

ShenandoahHeapRegion is known to be a performance critical instance in current GC code. Its storage is carefully managed to avoid (false) sharing across threads where possible, while keeping its footprint low.

We have organically regressed some of the ShenandoahHeapRegion footprint. It now takes a whooping 416 bytes, or 7x64-byte cache lines! We need to optimize this.

Additional testing:

• Linux x86_64 server fastdebug, hotspot_gc_shenandoah
• Regular testing pipelines

---

• I confirm that I make this contribution in accordance with the OpenJDK Interim AI Policy.

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed (2 reviews required, with at least 1 Reviewer, 1 Author)

Issue

• JDK-8388931: Shenandoah: Optimize ShenandoahHeapRegion footprint (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/32040/head:pull/32040
$ git checkout pull/32040

Update a local copy of the PR:
$ git checkout pull/32040
$ git pull https://git.openjdk.org/jdk.git pull/32040/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 32040

View PR using the GUI difftool:
$ git pr show -t 32040

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/32040.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back shade! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@shipilev The following labels will be automatically applied to this pull request:

• hotspot-gc
• shenandoah

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

[openjdk]

The total number of required reviews for this PR has been set to 2 based on the presence of this label: hotspot-gc. This can be overridden with the /reviewers command.

[mlbridge]

Webrevs

• 00: Full (f702440f)

[shipilev]

pahole now reports this:

        const HeapWord *           _bottom;              /*     0     8 */
        const HeapWord *           _end;                 /*     8     8 */
        const uint32_t             _index;               /*    16     4 */
        Atomic                     _state;               /*    20     4 */
        HeapWord *                 _new_top;             /*    24     8 */
        HeapWord *                 _top_before_promoted; /*    32     8 */
        HeapWord *                 _top_at_evac_start;   /*    40     8 */
        HeapWord *                 _coalesce_and_fill_boundary; /*    48     8 */
        size_t                     _mixed_candidate_garbage_words; /*    56     8 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        Atomic                     _live_data;           /*    64     8 */
        Atomic                     _critical_pins;       /*    72     8 */
        Atomic                     _update_watermark;    /*    80     8 */
        HeapWord *                 _top;                 /*    88     8 */
        uint32_t                   _tlab_allocs;         /*    96     4 */
        uint32_t                   _gclab_allocs;        /*   100     4 */
        uint32_t                   _plab_allocs;         /*   104     4 */
        float                      _empty_time;          /*   108     4 */
        Atomic                     _has_self_forwards;   /*   112     1 */
        Atomic                     _recycling;           /*   113     1 */
        uint8_t                    _age;                 /*   114     1 */
        bool                       _needs_bitmap_reset;  /*   115     1 */
        bool                       _promoted_in_place;   /*   116     1 */