Summary

The proposed change removes the use of non-generational ZGC by obsoleting the ZGenerational option.

Problem

Maintaining two different modes of ZGC comes at a cost. As a step in reducing the future maintenance burden and technical debt the non-generational mode of ZGC will be phased out through the deprecation process.

For more details see JEP 474: ZGC: Generational Mode by Default (JDK-8326667) and JEP 490: ZGC: Remove the Non-Generational Mode (JDK-8335850).

Solution

JEP 439: https://openjdk.org/jeps/439 introduced Generational ZGC as a second mode of ZGC to allow for a smoother transition for users from the non-generational ZGC. JEP 474: https://openjdk.org/jeps/474 made the generational mode the default mode of ZGC and the flag ZGenerational deprecated. As the final step in phasing out non-generational ZGC the flag ZGenerational will be made obsolete.

This will entail obsoleting ZGenerational via the standard HotSpot Command-line Flag lifecycle process.

For more details see JEP 490: ZGC: Remove the Non-Generational Mode (JDK-8335850).

Specification

Draft PR: https://github.com/openjdk/jdk/pull/21401

After the change specifying -XX:+UseZGC will always run in the generational mode. Specifying ZGenerational will result in a obsolete VM option warning.

• All product flags (except ZGenerational and ZVerifyViews) will have similar behavior
  • The flag ZCollectionInterval can be specified with finer granularity via:
    • ZCollectionIntervalMinor - "Force Minor GC at a fixed time interval (in seconds)"
    • ZCollectionIntervalMajor - "Force GC at a fixed time interval (in seconds)"
  • The flag ConcGCThreads can be specified with generational granularity via:
    • ZYoungGCThreads - "Number of GC threads for the young generation"
    • ZOldGCThreads - "Number of GC threads for the old generation"
  • The flag ZFragmentationLimit is used for the old generation, young generations is configured via:
    • ZYoungCompactionLimit - "Maximum allowed garbage in young pages"
  • The diagnostic ZVerifyViews option will be removed.
  • The ZGenerational option will be obsolete.

Because Generational ZGC has two generations and the non-generational ZGC only has one, there will be a difference in the number entities in various serviceability layers:

Memory Pools:

• "ZGC Young Generation", "ZGC Old Generation"

Memory Managers:

• "ZGC Minor Cycles", "ZGC Minor Pauses", "ZGC Major Cycles", "ZGC Major Pauses"

The same is true for the HotSpot performance counters.