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

Jeremy Manson jeremymanson at google.com
Sun Aug 28 00:42:12 UTC 2016


Thanks, Jon.

If we are going to do it, I'd (selfishly) like to see it done earlier in
the JDK 10 time frame, so that we can start contributing our patches for it
for JDK 10 earlier, rather than later.  But JDK 10 time frame is fine.

Assuming you are asking me about management issues: I just (Thursday) got
back from the vacation I mentioned, so I've only just spoken about it with
our management.  Naturally, no immediate answer, but there's a lot of
interest, and we plan to have more conversations in the near term.

For a variety of reasons (most of which I've already articulated), I
strongly believe we will have to do *something*, and it will mostly be a
question of whether this is the right path.  Since we have ~three years
before the issue becomes immediate, that gives us a bit of breathing room
to do the right thing.  Other possibilities include spending the time until
JDK 10 making G1 do what we need, or figuring out another GC entirely (we'd
want to evaluate, e.g., Shenandoah).

(Since the G1 possibility is clearly of interest to you folks: We would
have to decide a) that it is technically feasible, and b) that you folks
were likely to take the patches.  In addition to finding a path forward to
lowering the cost of the write barrier dramatically and dealing with the
memory footprint issues (not sure this is possible), we would also have to
reimplement, e.g., our parallel Full GC patch, as well as a number of other
changes we've made to CMS locally.)

I'd like what we end up doing to be something palatable to the community,
since CMS is obviously filling a very important role in the ecosystem that
won't get filled if it gets abandoned.

Jeremy

On Tue, Aug 23, 2016 at 9:23 PM, Jon Masamitsu <jon.masamitsu at oracle.com>
wrote:

> All,
>
> I've been working on the examples and we're been discussing (arguing)
> about them here.  They are failing into at least the following
> classes
>
> - Use inheritance to specialize for CMS (specialize card table is
> envisioned as an example).
> - Use a GC Interface to request something different for CMS  (different
> write
> barriers for example).
> - Compile code under macros
>     Similar to INCLUDE_ALL_GCS
>     Leaves calls to specialized CMS code visible in the shared code
>     Involves some code refactoring
> - Move more code to the cms directory (ParNew for example)
> - Custom solutions which might use one or more of the above techniques.
> - Argument processing I don't know about yet.
>
> We're still arguing about the example so they're not here.
>
> As I've said Oracle would not drop support of CMS until at least
> jdk 10 so working on separating out the code would be work
> for the jdk 10 time frame.   Do people have reasons to get that
> done earlier rather than later?
>
> Also is your management's support for spending engineering time
> on this effort a slam-dunk?   Or a qualified maybe?   I'm trying to
> get a feel for how real this CMS project is.
>
> Jon
>
>
>
> On 8/1/2016 1:49 PM, Jeremy Manson wrote:
>
> For those following along at home, the meeting happened last week.  We
> took notes, which are linked from the bug:
>
> https://bugs.openjdk.java.net/browse/JDK-8142518
>
> The current plan for followup is to set up a meeting for the end of
> August, and then a f2f during JavaOne.
>
> Jeremy
>
> On Thu, Jul 21, 2016 at 9:46 AM, Jeremy Manson <jeremymanson at google.com>
> wrote:
>
>> I was drafting an agenda.  My expectation is that the meeting will go
>> something like this:
>>
>> Introductions
>>
>> Discussion of motivations for JEP 291 (Jon M)
>>
>> Discussions of concerns about JEP 291 (roundtable)
>>
>> Discussion / Brainstorming about potential ways forward, as well as an
>> understanding of what level of commitment organizations are willing to make.
>>
>> Discussion of action items and followup.
>>
>> Jeremy
>>
>>
>> On Thu, Jul 21, 2016 at 7:39 AM, Martijn Verburg <
>> martijnverburg at gmail.com> wrote:
>>
>>> Hi Jon,
>>>
>>> For me it's to see if there are enough folks who are willing *and*
>>> capable (to be blunt, it will need the backing of large companies) of
>>> maintaining CMS as a collector going forwards (it's important to
>>> several of our customers).  If that's a yes then I'd hope to have a
>>> technical discussion around the options of how we could achieve that
>>> without causing a major headache for the Oracle GC engineers.
>>>
>>>
>>> Cheers,
>>> Martijn
>>>
>>> On 20 July 2016 at 23:50, Jon Masamitsu <jon.masamitsu at oracle.com>
>>> wrote:
>>>
>>>> What are peoples expectations for this meeting?
>>>>
>>>> Jon
>>>>
>>>> On 07/18/2016 10:16 AM, Jeremy Manson wrote:
>>>>
>>>> Presumably, everyone will be happy with meeting notes rather than an
>>>> actual recording.
>>>>
>>>> Otherwise, I'll add:
>>>>
>>>> ysr1729 at gmail.com
>>>> mark.reinhold at oracle.com
>>>>
>>>>
>>>>
>>>> On Mon, Jul 18, 2016 at 8:13 AM, kirk.pepperdine at gmail.com <
>>>> kirk.pepperdine at gmail.com> wrote:
>>>>
>>>>> ok we can record if everyone is open to that.
>>>>>
>>>>> On Jul 18, 2016, at 7:48 AM, Jeremy Manson <jeremymanson at google.com>
>>>>> wrote:
>>>>>
>>>>> Richard - I'll bring someone who can take readable notes. :)
>>>>>
>>>>> Jeremy
>>>>>
>>>>> On Sat, Jul 16, 2016 at 5:07 AM, kirk.pepperdine at gmail.com <
>>>>> kirk.pepperdine at gmail.com> wrote:
>>>>>
>>>>>> Hi Richard,
>>>>>>
>>>>>> I think it’s in everyone’s interest to keep this out in the open.
>>>>>>
>>>>>> Kind regards,
>>>>>> Kirk
>>>>>>
>>>>>> On Jul 16, 2016, at 1:34 AM, Richard Warburton <
>>>>>> richard.warburton at gmail.com> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Sounds as if the consensus is a telephone / video conference in the
>>>>>>> near term, and then a F2F during / close to JavaOne.
>>>>>>>
>>>>>>> People who want to be invited include:
>>>>>>>
>>>>>>> kirk at kodewerk.com
>>>>>>> aph at redhat.com
>>>>>>> volker.simonis at gmail.com
>>>>>>> jon.masamitsu at oracle.com
>>>>>>> jeremymanson at google.com
>>>>>>> martijnverburg at gmail.com
>>>>>>> jwha at google.com
>>>>>>>
>>>>>>> Did I miss anyone?  We can take date / time planning and logistics
>>>>>>> off-list.
>>>>>>>
>>>>>>
>>>>>> Given that this change affects a lot of organisations and people in
>>>>>> the wider Java community I think it would be really appreciated that a
>>>>>> brief summary of the discussion be published somewhere in public. Maybe
>>>>>> this mailing list?
>>>>>>
>>>>>> regards,
>>>>>>
>>>>>>   Richard Warburton
>>>>>>
>>>>>>   http://insightfullogic.com
>>>>>>   @RichardWarburto <http://twitter.com/richardwarburto>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>
>
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