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  1. JDK
  2. JDK-8273122

ZGC: Load forwarding entries without acquire semantics

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    Details

    • Type: Enhancement
    • Status: Open
    • Priority: P4
    • Resolution: Unresolved
    • Affects Version/s: 18
    • Fix Version/s: tbd
    • Component/s: hotspot
    • Subcomponent:
      gc
    • CPU:
      aarch64
    • OS:
      generic

      Description

      JDK-8272138 introduced an unbound membar release() between the object copy and the forwarding table insertion.

      Thread A (Relocation):
      copy();
      release();
      cas_forwarding_table();
      cas_self_heal();

      The membar guarantees that the contents of the forwarded object are ready after a forwarding entry is loaded. Since load_object_content(ref) depends on the result of load_forwarding_table(), load_acquire can be safely changed to a simple load.

      Thread B (Remapping/Relocation):
      ref = load_forwarding_table(); // acquire (current version) -> relaxed (our proposal)
      load_object_content(ref);

      Our experiment on heapothesys demonstrates >5% time reduction spent on concurrent mark/relocation on AArch64.

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              Assignee:
              Unassigned Unassigned
              Reporter:
              haotang Hao Tang
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                Dates

                Created:
                Updated: