Parallelizing the full compaction of CMS

Jon Masamitsu jon.masamitsu at
Wed Jun 3 18:57:50 UTC 2015

On 6/3/2015 2:32 AM, Volker Simonis wrote:
> Hi Jeremy,
> thanks for your answer. Maybe you misunderstood Ivan but it is 
> definitely his and our (i.e. SAP's) intention to make this part of the 
> OpenJDK.
> I can also fully understand the pains you have with integrating 
> changes into the HotSpot. We've gone through this painful process with 
> our ppc64 port a year ago but we think it will finally pay off. After 
> all you (i.e. Google) used it as the base for your ppc64le port 
> (Alexander Smundak contributed most of the LE-specific changes).
> We're facing the exactly same problems regarding the support of 
> proprietary changes in a downstream version of the OpenJDK. That was 
> actually the main reason why we started contributing to the OpenJDK. 
> What we want to kindly ask you is to make your changes and extensions 
> more transparently visible to the community if you intend to integrate 
> them into the OpenJDK. If Hiroshi had posted his changes directly to 
> the hotspot mailing lists instead of sending them privately to Jon I'm 
> pretty sure we would have picked them up. And maybe together we would 
> have managed to push them through (we have some reviewers as well :). 
> After all, there are already quite a lot of features in the OpenJDK 
> which are supported and maintained by the community (i.e. RedHat, IBM, 
> SAP, ...) and not by Oracle.


With regards to integrating such a change as discussed here,  how do you 
see "supported and maintained by the community" working?   I really 
don't know what to expect.


PS.  I actually never  saw the patch from Hiroshi.
> I don't have much experience with other Open Source projects, but with 
> OpenJDK and especially HotSpot it's pretty pointless to try to 
> contribute something by writing mails like "..I've implemented this 
> cool feature XXX. If somebody is interested I can post more 
> details..". You'll almost never get any feedback to such kind of 
> requests. You'll at least have to post a complete buildabel and 
> runnable patch against the head revision. I know that's a lot of 
> overhead and we're having this very same discussion if it is worth 
> doing it nearly every day for most of the features/additions we are 
> implementing. Nevertheless, I think it is worth doing it although it 
> sometimes may take years until you get your changes back in the 
> version you are using productively.
> So coming back to Ivan's initial request, we would kindly ask you to 
> just publish your changes in their current form against your current 
> version (8u45 or whatsoever). We'll take a look at them an may invest 
> some time and effort (also jointly with you if desired) to forward 
> port them to 9 and propose them for integration into the head revision 
> if that makes sense. Thus that sound reasonable to you?
> Thank you and best regards,
> Volker
> On Tue, Jun 2, 2015 at 5:00 PM, Jeremy Manson <jeremymanson at 
> <mailto:jeremymanson at>> wrote:
>     We'd much rather it be part of OpenJDK, because it is large and
>     fairly complicated, so that would save us time and energy forward
>     porting it every year or so.  It's pretty large (for 8u45, it is a
>     2600 line change and touches 20 files), so it tends to be a
>     lengthy and complicated forward port.  We would much rather spend
>     that time doing something more interesting.
>     Just posting an updated patch whenever we need to update it would
>     be a little annoying (although not impossible).
>     Hiroshi Yamauchi tried to offer it to (I think it was) Jon
>     Masamitsu a few years ago, and Jon didn't know anyone with the
>     bandwidth to review it.  (It was around the same time we
>     contributed the parallel initial mark).  I believe that was when
>     CMS was unowned for a while.  Perhaps there's a different story
>     now?  Oracle volunteers?
>     (Procedural: One problem is that Hotspot reviews have to go
>     through someone at Oracle. We had it written by an OpenJDK author
>     and reviewed by a reviewer.  We've been using it in production for
>     years without too many headaches (although we have yet to
>     inflict^Wlaunch 8u45, which has the latest updates, widely).  I
>     totally understand that Oracle folks are the ones who have to pay
>     the price for a bad check-in, but it's a pretty big hurdle for us.)
>     Jeremy
>     On Tue, Jun 2, 2015 at 6:12 AM, Galkin, Ivan <ivan.galkin at
>     <mailto:ivan.galkin at>> wrote:
>         Hi,
>         in the thread „JEP 248: Make G1 the Default Garbage Collector”
>         [1] there is an email from Jeremy Manson, who mentions the
>         enhancements made by Google to improve CMS.
>         Especially the parallelizing of full compaction is a great
>         improvement and we at SAP see the strong demand of it.
>         @Jeremy: Are these changes published somewhere? May we have
>         insight into the diffs you've made? We could collaborate in
>         order to try to bring your changes into OpenJDK.
>         Kind regards and thank you in advance,
>         Ivan
>         [1]

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the hotspot-gc-dev mailing list