G1 as a CMS replacement

Jon Masamitsu jon.masamitsu at oracle.com
Fri Jun 5 17:46:42 UTC 2015


 From the mail below (from another thread) it sounds like you might
have some suggestions about what is needed to make G1 a replacement
for CMS.   Perhaps some features that CMS has that G1 doesn't.  Or
some characterizations of applications (or even small benchmarks)
where CMS is doing better.

I'm aware of the applications which almost all of the heap
is place in the young gen and promotions to the old/cms
gen is very low and can be handled by the CMS concurrent

I also know that the static initiating occupancy of G1 can be
a hindrance and that the larger G1 process footprint is a
disadvantage.  Are either of those blocker for transition to
G1 for some applications.

I'd appreciate anything you can share.

Same applies to anyone else  that would like to share blockers to
migration from CMS to G1.



On 6/5/2015 10:11 AM, Ben Evans wrote:
> Hi,
> I like the plan, but this statement is really quite bullish,
> especially (IMO) on the available evidence.
> G1 was always marketed as the replacement for CMS. In field experience
> so far, that hasn't happened, and I feel that we are glossing over the
> fact that we are now thinking of G1 as a replacement for Parallel.
> Certainly none of the testing I've done has been about Parallel -> G1
> - it's all been CMS vs G1. I will be happy to start working with
> clients to try to cover the Parallel vs G1 space, but that hasn't
> happened up until now.
> Neither do I think we should be under any illusions that a very large
> number of installs are going to be affected.
> It may not be all that representative, but here are the results of a
> quick community straw poll I ran over the last couple of days:
> A single question: "Which Garbage Collector Does Your Application Use?
> " yielded these results:
> Concurrent Mark & Sweep (CMS)
> 23.94%
> 85
>> Garbage First (G1GC)
> 10.70%
> 38
>> Parallel (because I explicitly set it)
> 5.07%
> 18
>> Default (this actually gives you Parallel)
> 39.15%
> 139
>> I Don't Know
> 21.13%
> 75
> Total: 355
> The clear, stand-out winner is Default, with ~40% of installs using
> this, and therefore being exposed to the proposed change. That makes
> me very nervous.
> So, whilst I'm not saying we shouldn't do this, and I know that
> community members, including Kirk, myself & the other jClarity folks
> will help to get some better data, I'd argue that we're still a long
> way from being certain that we're ready for this change.
> Thanks,
> Ben

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