Future of CMS

Jon Masamitsu jon.masamitsu at oracle.com
Sat Jun 11 22:06:29 UTC 2016

On 6/11/2016 6:37 AM, Volker Simonis wrote:
> Hi Jon,
> I'm a little bit disappointed that you didn't think about SAP when you 
> sent your initial mail ;)

Yup, I should have just addressed it to the alias so that everyone's 
mail filters
would have pushed it off to the side for later reading :-)

> But seriously: SAP is supporting CMS and will probably do so for quite 
> a long time (simply because we do support old Java releases for a very 
> long time).
> Completely removing CMS from the HotSpot code base may increase these 
> support costs considerably for us.
> Do you plan to really delete the sources from the repos or do you only 
> plan to disable it at build time?

I just don't know how this is going to play out.  I expect that the 
OpenJDK community
will want the sources to stay.  In such a case Oracle would want them 
segregated sufficiently
that we don't build the the sources.

> I think only disable it at build time would make it easier for us and 
> others to still support it in the future. But in that case we really 
> have to come up with a better development model which would allow 
> external developers to directly push CMS changes (much like ppc64 or 
> aarch64 changes). Everything else would be a real PITA.

I think some model like ppc64/aarch64 would be a good way to go. That 
would be a
nice side effect of this change.  You know better than I on how to get 


> Thank you and best regards,
> Volker
> On Saturday, June 11, 2016, Jon Masamitsu <jon.masamitsu at oracle.com 
> <mailto:jon.masamitsu at oracle.com>> wrote:
>     Added hotspot-gc-dev.
>     On 6/10/2016 6:17 PM, Jon Masamitsu wrote:
>         Jungwoo, Ramki, Tony (and other friends of CMS),
>         I wanted to be sure you guys saw this JEP.
>         Deprecate the CMS garbage collector (UseConcMarkSweepGC).
>         http://openjdk.java.net/jeps/8142518
>         I know you do development on CMS and wanted to know if
>         had thoughts about the future of CMS.
>         At some point Oracle is going to stop supporting CMS.
>         The burden of testing and maintaining the CMS code doesn't
>         make much sense given that we think G1 is the better
>         collector.    If we deprecate CMS in jdk9, support would be
>         dropped
>         no earlier than jdk10.
>         Have you guys talked about what you would like to do with regard
>         to continuing CMS development without Oracle's help?
>         I've been told that if CMS exists in the Oracle binaries, then
>         customers will expect it to work.  So not supporting CMS
>         means it is not in the build.
>         Jon

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