Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-8261492

Shenandoah: reconsider forwardee accesses memory ordering

    XMLWordPrintable

    Details

    • Type: Enhancement
    • Status: New
    • Priority: P4
    • Resolution: Unresolved
    • Affects Version/s: 8-shenandoah, 11.0.9, 16, 17
    • Fix Version/s: None
    • Component/s: hotspot
    • Subcomponent:
      gc

      Description

      Shenandoah carriers forwardee information in object's mark word. Installing the new mark word is effectively "releasing" the object copy, and reading from the new mark word is "acquiring` that object copy.

      For the forwardee update side, Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64. This seems to be excessive for Shenandoah forwardee updates, and "release" is enough.

      For the forwardee load side, we need to guarantee "acquire". We do not do it now, reading the markword without memory semantics. It does not seem to pose a practical problem today, because GC does not access the object contents in the new copy, and mutators get this from the JRT-called stub that has enough barriers. It still should be cleaner to "acquire" the mark on load to avoid surprises.

      It would not help AArch64 much until JDK-8261579 actually implements acq/rel.

        Attachments

          Issue Links

            Activity

              People

              Assignee:
              shade Aleksey Shipilev
              Reporter:
              shade Aleksey Shipilev
              Votes:
              0 Vote for this issue
              Watchers:
              1 Start watching this issue

                Dates

                Created:
                Updated: