Integrated: 8307058: Implementation of Generational ZGC

[Stefan Karlsson]

On Wed, 3 May 2023 09:04:50 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Hi all,
> 
> Please review the implementation of Generational ZGC, which can be turned on by adding -XX:+ZGenerational in addition to using -XX:+UseZGC. Generational ZGC is a major rewrite of the non-generational ZGC version that exists in the openjdk/jdk repository. It splits the heap into two generations; the young generation where newly allocated objects are born, and the old generation where long-lived objects get promoted to. The motivation for introducing generations is to allow ZGC to reclaim memory faster by not having to walk the entire object graph every time a garbage collection is run. This should make Generational ZGC suitable for more workloads. In particular workloads that previously hit allocation stalls because of high allocation rates, large live sets, or limited spare machine resources, have the potential to work better with Generational ZGC. For an in-depth description of Generational ZGC, see https://openjdk.org/jeps/439.
> 
> The development of Generational ZGC started around the same time as the development of JDK 17. At that point we forked off the Generational ZGC development into its own branch and let non-generational live unaffected in openjdk/jdk. This safe-guarded non-generational ZGC and allowed Generational ZGC to move unhindered, without the shackles of having to fit into another GC implementation's design and quirks. Since then, almost all of the ZGC files have been changed. Moving forward to today, when it's ready for us to upstream Generational ZGC, we now need to deliver Generational ZGC without disrupting our current user-base. We have therefore opted to initially include both versions of ZGC in the code base, but with the intention to deprecate non-generational ZGC in a future release. Existing users running with only -XX:+UseZGC will get the non-generational ZGC, and users that want the new Generational ZGC need to run with -XX:+ZGenerational in addition to -XX:+UseZGC. The intention i
 s to give the users time to validate and deploy their workloads with the new GC implementation.
> 
> Including both the new evolution of a GC and its legacy predecessor poses a few challenges for us GC developers. The first reaction could be to try to mash the two implementations together and sprinkle the GC code with conditional statements or dynamic dispatches. We have done similar experiments before. When ZGC was first born, we started an experiment where we converted G1 into getting the same features as the evolving ZGC. It was quite clea...

This pull request has now been integrated.

Changeset: d20034b0
Author:    Stefan Karlsson <stefank at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/d20034b09c99026e7dc2213f7d88ebdc85e5b1e7
Stats:     67359 lines in 684 files changed: 58192 ins; 4252 del; 4915 mod

8307058: Implementation of Generational ZGC

Co-authored-by: Stefan Karlsson <stefank at openjdk.org>
Co-authored-by: Erik Österlund <eosterlund at openjdk.org>
Co-authored-by: Axel Boldt-Christmas <aboldtch at openjdk.org>
Co-authored-by: Per Liden <pliden at openjdk.org>
Co-authored-by: Stefan Johansson <sjohanss at openjdk.org>
Co-authored-by: Albert Mingkun Yang <ayang at openjdk.org>
Co-authored-by: Erik Helin <ehelin at openjdk.org>
Co-authored-by: Roberto Castañeda Lozano <rcastanedalo at openjdk.org>
Co-authored-by: Nils Eliasson <neliasso at openjdk.org>
Co-authored-by: Martin Doerr <mdoerr at openjdk.org>
Co-authored-by: Leslie Zhai <lzhai at openjdk.org>
Co-authored-by: Fei Yang <fyang at openjdk.org>
Co-authored-by: Yadong Wang <yadongwang at openjdk.org>
Reviewed-by: eosterlund, aboldtch, rcastanedalo

-------------

PR: https://git.openjdk.org/jdk/pull/13771