Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-8239779

Documentation for Phaser (& related) does not define task re-schedule strategy

    Details

      Description

      ADDITIONAL SYSTEM INFORMATION :
      Relates to directly to openjdk-15-loom+3-20_windows-x64_bin but indirectly to all previous releases.

      A DESCRIPTION OF THE PROBLEM :
      Was looking at https://stackoverflow.com/a/12787137 : a proposed use of Phaser for the good old "Ping Pong" example.

      Although this example 'works' it ONLY does so because it is relying on an implicit implementation feature: that tasks that have come together at awaitAdvance() are re-scheduled in a fixed, known, repeatable fashion (ie FIFO I believe [as a result of the current implementation being based on ArrayBlockingQueue]).

      This is surely an incorrect assumption for the 'ideal' Phaser (& other Synchronizers such as CountDownLatch, etc.).
      In any case, it is not stated in any doco.

      IMHO, the order that tasks become eligible for re-scheduling should NOT be assumed...one should assume randomness unless one is told otherwise.

      The documentation should make the intended behavior clear, and state either:
      + tasks are released to execution in FIFO order

      Or:
      + tasks may be resumed with no guarantee as to ordering (i.e.: NOT FIFO or anything else)

      In the former case, FIFO behavior is formalized and becomes a known part of the API for these classes. In the latter case randomness is formalized.

      *I* believe it is better to assume randomness.

      In any case, the definition and doco becomes a bit better.

      Note that this is not covered by the statements regarding "Memory Consistency Properties/happens-before"...this is about a re-scheduling decision after memory consistency is taken care of.

      REGRESSION : Last worked in version 8

      STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
      See

      https://download.java.net/java/early_access/jdk15/docs/api/java.base/java/util/concurrent/Phaser.html

      Also "Synchronizers" at:
      https://download.java.net/java/early_access/jdk15/docs/api/java.base/java/util/concurrent/package-summary.html

      for example.


      FREQUENCY : always


        Attachments

          Activity

            People

            • Assignee:
              martin Martin Buchholz
              Reporter:
              webbuggrp Webbug Group
            • Votes:
              0 Vote for this issue
              Watchers:
              5 Start watching this issue

              Dates

              • Created:
                Updated:
                Resolved: