Parallelizing the full compaction of CMS

Jon Masamitsu jon.masamitsu at
Tue Jun 2 18:51:02 UTC 2015

On 06/02/2015 08:00 AM, Jeremy Manson 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,
Yes, that was me.
> and Jon didn't know anyone with the bandwidth to review it.
My understanding was that the change was based on a parallelization of
the serial mark-sweep-compact (used by CMS for full collections).
If it was based on the parallel-old compaction used by ParallelGC,
then my bad.  It should have been reviewed.

> (It was around the same time we contributed the parallel initial mark).

Which we accepted and works great.

> I believe that was when CMS was unowned for a while. Perhaps there's a 
> different story now?  Oracle volunteers?

CMS is still unowned.

> (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.)

It's less the up front costs as opposed to the continuing costs.

Testing (for whatever reason there are intermittent failures
that we see on some platforms and not others),
maintenance on code for which we have a limited understanding,
and performance on other platforms that we have to care about.

And priorities (we have ours just as you have yours).


> 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]

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