CFV: New Project: ZGC

Mario Torre neugens.limasoftware at
Thu Oct 26 07:57:13 UTC 2017

Vote: Yes


On Wed 25. Oct 2017 at 22:23, 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]

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