CFV: New Project: ZGC

Per Liden per.liden at
Fri Oct 27 17:12:45 UTC 2017

Hi Volker,

On 2017-10-26 17:42, Volker Simonis wrote:
> 1. What are the benefits / advantages of ZGC over Shenandoah?

In my proposal I outlined some of the benefits we expect to achieve 
through the current ZGC design.

With respect to an evaluation of specifics of the ZGC implementation 
with other approaches, I believe that's something that makes most sense 
to discuss once the ZGC code is actually open source, and starts to move 
ahead as part of the OpenJDK Community in a Project of its own, rather 
than in a vacuum.

> 2. Does it make sense to have both, ZGC and Shenandoah in the main code line?
> 2.1 If yes, why?
> 2.2 If no, why you propose ZGC at all and not contribute to
> Shenandoah? It's there since more than two years! That you already
> worked on ZGC for some time as well doesn't count here because we (the
> OpenJDK community) couldn't see that and had no chance to contribute
> to it.

As Roman and Andrew have already mentioned, there are multiple possible 
outcomes here (pick one, have both, merge them, etc), and only time and 
experience with both code bases will help us here. This is just a 
proposal for a new, independent ZGC project. It's not a proposal to 
merge ZGC into the JDK itself.

An important part here is that we make sure that adding GCs doesn't come 
with an explosion in maintenance cost. The ongoing collaboration between 
Oracle and Red Hat on the GC and Barrier APIs is key to making sure that 
doesn't happen.

> 3. Oracle has just deprecated CMS and other collectors because
> according to Oracle it was just too expensive to maintain that many
> collectors in parallel. Now I wonder how yet another new collector
> fits into the picture?

The CMS code base is complex to maintain. It's also not a collector we 
at Oracle believe is a good foundation for any future work. By 
deprecating CMS we've freed up resources at Oracle that can be invested 
in collectors we believe will serve everyone better going forward.


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