Summary
-------

Deprecate the Concurrent Mark Sweep (CMS) garbage collector, with the
intent to stop supporting it in a future major release.

Goals
-----

Accelerate the development of other garbage collectors in HotSpot.

Motivation
----------

Dropping support for CMS and then removing the CMS code, or at least more
thoroughly segregating it, will reduce the maintenance burden of the GC
code base and accelerate new development.  The G1 garbage collector is
intended, in the long term, to be a replacement for most uses of CMS.

Description
-----------

Deprecate CMS so that a warning message is issued when it is requested
on the command line, via the `-XX:+UseConcMarkSweepGC` option.

This JEP does not specify the major release in which support for CMS
will be dropped.  The decision of when to do that will be informed by
the extent to which the G1 collector proves to be a suitable replacement
for CMS.  In the meantime, CMS users are encouraged to migrate to the G1
collector (`-XX:+UseG1GC`).


Testing
-------

Verify that the intended deprecation message is emitted when the CMS
collector is requested.


Risk and Assumptions
--------------------

 - For some applications CMS is a very good fit and might always
   outperform G1.

 - When support of CMS is ended, CMS will not be available in new
   releases.