Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-8254315

Shenandoah: Concurrent weak reference processing

    XMLWordPrintable

Details

    • gc
    • b23
    • generic
    • generic

    Description

      Until now, references (as in java.lang.ref.Reference and its subclasses WeakReference, SoftReference, PhantomReference and the non-public FinalReference - I'll collectively call them weak references for the purpose of clarity). Workloads that make heavvy use of such weak references will therefore potentially cause significant GC pauses.

      There are 3 main items that contribute to pause time linear to number of references, or worse:
      - We need to scan and consider each reference on the various 'discovered' lists.
      - We need to mark through subgraph of objects that are reachable only through FinalReference. Notice that this is theoretically only bounded by the live data set size.
      - Finally, all no-longer-reachable references need to be enqueued in the 'pending list'

      The problem is somewhat mitigated by pre-cleaning the discovered list: Any weak reference that we find to be strongly reachable will be removed before we go into the final-mark-pause. However, that is only a band-aid.

      The solution to this is two-fold:
      1. Extend concurrent marking to also mark the 'finalizable' subgraph of the heap. This requires to extend the marking bitmap to allow for two kinds of reachability: each object can now be strongly and finalizably reachable. Whenever marking encounters a FinalReference, it will mark through the referent and switch to 'finalizably' reachability for all objects starting from the referent. When marking encounters finalizably reachable objects while marking strongly, it will 'upgrade' reachability of such objects to strongly reachable. All of this can be done concurrently. Any encounter of a Reference (or subclass) object will enqueue that object into a thread-local 'discovered' list. Except for FinalReference, marking stops there, and does not mark through the referent.
      2. Concurrent processing is performed after the final-mark pause. GC workers scan all discovered lists that have been collected by concurrent marking, and depending on reachability of the referent, either drop the Reference, or enqueue it into the global 'pending' list (from where it will be processed by Java reference handler thread). In addition to that, we must ensure that no referents become resurrected by accessing Reference.get() on it. In order to achieve this, we employ special barriers in Reference.get() intrinsics that return NULL when the referent is not reachable.

      Attachments

        Issue Links

          blocks

          Enhancement - null JDK-8261493 Shenandoah: reconsider bitmap access memory ordering

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved
          is blocked by

          Enhancement - null JDK-8255457 Shenandoah: cleanup ShenandoahMarkTask

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved
          relates to

          Bug - A problem which impairs or prevents the functions of the product. JDK-8255852 Shenandoah: Fix incorrect copyright header in new files added by 8254315

          • P2 - Crashes, loss of data, severe memory leak.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8256011 Shenandoah: Don't resurrect finalizably reachable objects

          • P2 - Crashes, loss of data, severe memory leak.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8257793 Shenandoah: SATB barrier should only filter out already strongly marked oops

          • P2 - Crashes, loss of data, severe memory leak.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8257817 Shenandoah: Don't race with conc-weak-in-progress flag in weak-LRB

          • P2 - Crashes, loss of data, severe memory leak.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8258714 Shenandoah: Process references before evacuation during degen

          • P2 - Crashes, loss of data, severe memory leak.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8255991 Shenandoah: Apply 'weak' LRB on cmpxchg and xchg

          • P3 - Major loss of function.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8256040 Shenandoah: Allow NULL referent in ShenandoahReferenceProcessor::should_discover()

          • P3 - Major loss of function.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8260998 Shenandoah: Restore reference processing statistics reporting

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8263030 Remove Shenandoah leftovers from ReferenceProcessor

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8255999 Shenandoah: Invoke native-LRB only on non-strong refs, again

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Closed

          Enhancement - null JDK-8259377 Shenandoah: Enhance weak reference processing time tracking

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Enhancement - null JDK-8260586 Shenandoah: simplify "Concurrent Weak References" logging

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Enhancement - null JDK-8261984 Shenandoah: Remove unused ShenandoahPushWorkerQueuesScope class

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Enhancement - null JDK-8264374 Shenandoah: Remove leftover parallel reference processing argument

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved

          Bug - A problem which impairs or prevents the functions of the product. JDK-8258706 Shenandoah: Prevent recycle-assist during conc-roots

          • P2 - Crashes, loss of data, severe memory leak.
          • Closed

          Enhancement - null JDK-8269924 Shenandoah: Introduce weak/strong marking asserts

          • P4 - Minor loss of function, or other problem where easy workaround is present.
          • Resolved
          links to

          Commit Commit openjdk/jdk/f64a15d6

          Review Review openjdk/jdk/505

          There are no Sub-Tasks for this issue.

          Activity

            People

              rkennke Roman Kennke
              rkennke Roman Kennke
              Votes:
              0 Vote for this issue
              Watchers:
              4 Start watching this issue

              Dates

                Created:
                Updated:
                Resolved: