CFV: New Project: ZGC
per.liden at oracle.com
Wed Oct 25 19:45:23 UTC 2017
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 , 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
* Reference processing (java.lang.ref.*)
* Relocation set selection
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
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
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  are eligible to vote on this motion.
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