Parallelizing the full compaction of CMS

Jeremy Manson jeremymanson at
Tue Jun 2 15:00:07 UTC 2015

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

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.)


On Tue, Jun 2, 2015 at 6:12 AM, Galkin, Ivan <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]
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