Remove the GC combinations that were previously deprecated in JDK 8
via [JEP 173][jep173].
It is not a goal of this work to implement replacements for the removed
GC combinations. For the most part the remaining collectors should
perform as well or better than the removed GC combinations. It is not a
goal of this work to enhance any of the garbage collectors to perform as
well as the garbage collectors being removed. Only tuning of the garbage
collectors will be considered as part of satisfying the performance goal.
* No deprecation messages are printed for the concerned JVM
options. Instead the JVM considers them to be unknown flags.
* The performance impact of changing from ParNew + SerialOld to
ParallelScavenge + SerialOld is well understood for a reasonable set
As described in [JEP 173][jep173] there is a large maintenance cost
associated with keeping all of the existing GC combinations around.
Removing the deprecated GC combinations will allow for many
simplifications in the HotSpot GC code, which in turn will reduce the
number of bugs and allow for more rapid development of the remaining GC
The flags to control the GC combinations listed as deprecated in
[JEP 173][jep173], as well as the flags to enable the CMS foreground collector
(deprecated as part of [JDK-8027876](https://bugs.openjdk.java.net/browse/JDK-8027876))
will be removed from the code base. This means that no warning messages
will be printed for them any longer; the JVM will, instead, not start if
these flags are used.
Once the flags have been removed then any code that is now dead will be
removed for the GC code base. There may be simplifications in the code
base that are possible to do because of this work but large in
scope. Such simplifications may be separated out as separate changes.
Here is a detailed summary of the flags and flag combinations that will
DefNew + CMS : -XX:-UseParNewGC -XX:+UseConcMarkSweepGC
ParNew + SerialOld : -XX:+UseParNewGC
ParNew + iCMS : -Xincgc
ParNew + iCMS : -XX:+CMSIncrementalMode -XX:+UseConcMarkSweepGC
DefNew + iCMS : -XX:+CMSIncrementalMode -XX:+UseConcMarkSweepGC -XX:-UseParNewGC
CMS foreground : -XX:+UseCMSCompactAtFullCollection
CMS foreground : -XX:+CMSFullGCsBeforeCompaction
CMS foreground : -XX:+UseCMSCollectionPassing
For the ParNew + SerialOld combination the work for this JEP will also
include performance testing comparing ParNew + SerialOld with
ParallelScavenge + SerialOld. This should result in tuning suggestions
for migrating from ParNew + SerialOld to ParallelScavenge + SerialOld.
Keep all or some of the deprecated options in the code. This would prevent
significant cleanup work in the GC code and slow down new development in
the code base.
There are current tests that verify that the listed options print warning
messages. These tests need to be updated to expect messages that the
options are unknown.
Risks and Assumptions
Users who are using any of the flags that are being removed will have to
update their JVM-startup command lines. If they are moving from JDK 8 to
JDK 9 then they will already have seen warning messages and thus should
not be surprised. The assumption is that most users will benefit from
changing from the flags that are being removed to more modern GC tuning.
This work depends on [JEP 173: Retire Some Rarely-Used GC Combinations][jep173].
That JEP was implemented in JDK 8, so there are no outstanding
dependencies left at this time.