CFV: New Project: ZGC

Andrew Hughes gnu.andrew at
Mon Oct 30 18:30:39 UTC 2017

On 25 October 2017 at 20:45, Per Liden <per.liden at> wrote:
> I hereby propose the creation of the ZGC Project with myself (Per Liden) as
> the Lead and the HotSpot Group as the sponsoring Group.
> In accordance with the OpenJDK guidelines [1], this project will provide a
> home for the continued development of the Z Garbage Collector, also known as
> ZGC. ZGC is a new garbage collector optimized for low latency and very large
> heaps. We've developed ZGC internally at Oracle so far, and we're now
> open-sourcing it so as to broaden the base of both contributors and users.
> ZGC has been designed with the following goals in mind:
> * Handle multi-terabyte heaps
> * GC pause times not exceeding 10ms
> * No more than 15% application throughput reduction compared to using G1
> We have strong ambitions to meet these goals for a large set of relevant
> workloads. At the same time we want to acknowledge that we don't see these
> goals as hard requirements for every conceivable workload. We are however
> currently able to meet or exceed these goals on some well-known industry
> standard benchmarks.
> At a glance, ZGC is a concurrent, currently single-generation, region-based,
> incrementally compacting collector. Stop-The-World phases are limited to
> root scanning, meaning GC pause times do not increase with the heap- or
> live-set size.
> While there is still work to do, the design and implementation is reasonably
> mature and stable. ZGC today executes the following GC tasks/phases
> concurrently:
> * Marking
> * Reference processing (java.lang.ref.*)
> * Relocation set selection
> * Relocation/Compaction
> And we're actively working on making the remaining GC tasks/phases
> concurrent. These are:
> * Weak root processing (StringTable, JNIWeakGlobalRefs)
> * Class unloading
> A core design principle/choice in ZGC is the use of load barriers in
> combination with colored object pointers (i.e. colored oops). This is what
> enables ZGC to do concurrent operations, such as object relocation, while
> Java application threads are running. From a Java thread's perspective, the
> act of loading a reference field in a Java object is subject to a load
> barrier. In addition to an object address, a colored object pointer contains
> information used by the load barrier to determine if some action needs to be
> taken before allowing a Java thread to use the pointer. For example, the
> object might have been relocated, in which case the load barrier will detect
> the situation and take appropriate action.
> Compared to alternative techniques, we believe the colored pointers scheme
> offers some very attractive properties. To name a few:
> * It allows us to reclaim and reuse memory during the relocation/compaction
> phase, before pointers pointing into the reclaimed/reused regions have been
> fixed. This helps keep the general heap overhead down. It also means that
> there is no need to implement a separate mark-compact algorithm to handle
> "Full GC".
> * It allows us to have relatively few and simple GC barriers. This helps
> keep the runtime overhead down. It also means that it's easier to implement,
> optimize and maintain the GC barrier code in our interpreter and JIT
> compilers.
> * We currently store marking and relocation related information in the
> colored pointers. However, the versatile nature of this scheme allows us to
> store any type of information (as long as we can fit it into the pointer)
> and let the load barrier take any action it wants to based on that
> information. We believe this will lay the foundation for many future
> features. To pick one example, in a heterogeneous memory environment, this
> could be used to track heap access patterns to guide GC relocation decisions
> to move rarely used objects to "cold storage".
> Much of the remaining work involves addressing latency issues in non-GC
> subsystems in HotSpot, such as being able to concurrently unlink stale
> entries in the StringTable. We hope and expect to see a fair bit of
> collaboration with people working on other garbage collectors in areas where
> we have a common interest.
> Some of the work coming out of the ZGC project has already been seen, either
> in the form of general improvements, or because a feature has found use
> cases outside of ZGC, such as:
> * Atomics re-write
> * GC Barrier API
> * Thread local handshakes
> I (Per Liden) am a member of the HotSpot GC team at Oracle, and have been
> working on JRockit and HotSpot projects for the past 8 years. I'm the
> initial author of ZGC, but many people have made significant contributions
> since then.
> Special thanks to Stefan Karlsson, who has been working with me on ZGC since
> the very early phases of this project.
> The initial Reviewers and Committers will be (based on people who have
> contributed to ZGC development within Oracle so far):
> * Stefan Karlsson (Reviewer)
> * Erik Österlund (Committer)
> * Mikael Gerdin (Committer)
> * Kim Barret (Committer)
> * Nils Eliasson (Committer)
> * Rickard Bäckman (Committer)
> * Roland Westrelin (Committer)
> * Coleen Philimore (Committer)
> * Robin Ehn (Committer)
> * Gerard Ziemski (Committer)
> The initial source of this project will be based on a clone of a JDK 10
> repository, plus the latest ZGC patch set. Changes from the JDK 10 parent
> will be synced into ZGC periodically. Change review policy will be
> determined by the Lead and a consensus of Reviewers. Review is expected to
> be relaxed initially, but made more strict as we get closer to integration.
> The project will host at least the following mailing list:
> * zgc-dev for developers
> Votes are due by 23:59 CET on Wednesday, November 8, 2017.
> Only current OpenJDK Members [1] are eligible to vote on this motion. Votes
> must be cast in the open on the discuss list. Replying to this message is
> sufficient if your mail program honors the Reply-To header.
> For Lazy Consensus voting instructions, see [2].
> Regards,
> Per Liden
> [1]
> [2]

Vote: Yes
Andrew :)

Senior Free Java Software Engineer
Red Hat, Inc. (

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