CFV: New Project: ZGC

Andrew Dinn adinn at
Fri Oct 27 08:11:44 UTC 2017

Vote: Yes

> On 2017-10-25 21:45, Per Liden 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]

Andrew Dinn
Senior Principal Software Engineer
Red Hat UK Ltd
Registered in England and Wales under Company Registration No. 03798903
Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander

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