RFC: JEP: Remove the Concurrent Mark Sweep Garbage Collector

Kirk Pepperdine kirk at kodewerk.com
Thu Aug 8 16:37:16 UTC 2019

Hi Thomas,

"n the meantime the Oracle garbage collection team introduced a new garbage collector, ZGC, and Red Hat contributed the Shenandoah collector. Oracle further improved G1, which has been its designated successor since initial introduction in JDK6u14, to a point where we believe there is little reason to use the CMS collector in deployments.”

I fear my experience in tuning GC 1000s of JVMs leaves me at odds with the premise that there is little reason to use CMS. In my experience CMS overheads are no where near the level of those seen with G1. This is not just my experience but there are other organizations that have reached the same conclusion. Further more, with the removal of CMS we are now recommending that customers consider Parallel GC as it offers a far better experience than G1. Again, I’m not alone is seeing this as a growing trend.

Although I do have high hopes for both ZGC and Shenandoah, they are not an option for most sites at this point in time. I would suggested that depreciation of CMS was premature as there was no viable alternative. I would further suggest that removal is also premature as there is still no viable alternative for the majority of workloads that work exceptionally well with CMS.

Kind regards,
Kirk Pepperdine

> On Aug 3, 2019, at 1:37 PM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> Hi all,
>  as already discussed during the OCW last week the Oracle garbage collection team is set to remove the CMS collector from OpenJDK for the reasons stated there and in the JEP in JDK 14.
> I wrote up a first draft available at
> https://bugs.openjdk.java.net/browse/JDK-8229049
> Comments and reviewers to move it along appreciated ;)
> Thanks,
>  Thomas

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