JEP 291: Deprecate the Concurrent Mark Sweep (CMS) Garbage Collector

mark.reinhold at mark.reinhold at
Fri Jul 1 21:08:16 UTC 2016

2016/7/1 1:50:41 -0700, aph at
> On 30/06/16 22:35, mark.reinhold at wrote:
>> New JEP Candidate:
> I think that there is likely to be a fair amount of push-back against
> this one.  I understand that the GC team has to be responsible for
> this decision, given that they have to support it.  But there has to
> be at least a possibility that OpenJDK support for CMS might not be
> ended, and Oracle is not necessarily the only company involved in
> this.

I think that's well understood.

There are limits to what can be expressed within the structure of a JEP
so, for clarity's sake, here's my take on this.  Jon will correct me if
I've got any of it wrong, I'm sure.

Oracle's GC team is intensely focused on improving the G1 collector, so
they're trying to reduce the amount of time they spend maintaining CMS.
At the very least, therefore, they will deprecate CMS in Oracle's JDK 9
product builds and then, most likely but depending upon end-user and
customer feedback, remove it entirely from Oracle's JDK 10 builds.

Whether and when this happens is a decision for Oracle to make, just as
whether Red Hat ships an AArch64 build of JDK 9 is a decision for Red
Hat to make.  I don't think this is controversial -- there's really no
need for anyone to spin conspiracy theories about smoke-filled rooms in
Redwood Shores (but go ahead and do that if it makes you feel better).

The fate of the CMS GC code itself in any particular OpenJDK Release
Project, or in any other OpenJDK Project for that matter, is a different
question, about which JEP 291 was intended to prompt a wider discussion,
as indeed it has.

If a set of credible developers expresses a clear desire to maintain CMS
after JDK 9 then all of us who work on this code base, regardless of
employer, will find a way for that to happen.  Maybe the CMS code stays
in JDK 9 and later Release Projects but is #ifdef'd out, or maybe it
stays but the common GC interface is refactored so that other collectors
(not just G1) can evolve more independently, or maybe the CMS code is
removed from the mainline Release Projects but kept alive in a new side
Project.  Exactly what happens depends mostly, I think, on who shows up.

To put it another way, the question that JEP 291 is trying to ask is,
"Does anybody outside of Oracle wish to take on the maintenance of CMS
after JDK 9?  If so, then let's talk."

Any takers?

- Mark

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