Hi all, now that it's Christmas and JDK12 ramping down I would like to give an overview of JDK12 changes to GC from the Oracle Hotspot GC team's view that may impact end-users.
>From a statistics POV there have been 197 changes for this release in GC so far [0], 70 bugs fixed, 124 enhancements, and 3 JEPs implemented. As part of that there has been a significant increase in external contributions in the last six months as you will see - that kept the reviewers quite busy :) Let's walk over a selection of improvements categorized by garbage collector: G1 - Abortable mixed collections (JEP 344 [1]) made it into JDK12 [2]: this feature is intended to improve G1 meeting pause time goals when incrementally collecting the old generation by incrementally collecting old generation regions within a particular GC. More information about this feature in last year's FOSDEM presentation [3] - the changes for Promptly return unused committed memory from G1 (JEP 346 [4][5]) let G1 perform regular garbage collection cycles to give back Java heap memory to the operating system if the system is idle. This change has been initially suggested and contributed by Rodrigo B. and Ruslan S. from JElastic. - Kishor K. from Intel contributed changes to allow only the old generation on alternate memory devices like NVDIMMs [6]. (Note that this feature is technically still not in the release yet, but requesting late commit, so it *might* still be moved off to next release) - G1 will now try to give back Java heap memory at every concurrent marking cycle if normal heap sizing heuristics indicate so, e.g. -Xms is smaller than -Xmx, there is a lot of excess free heap and others, as opposed to require a full gc to do that [7]. - _all_ collectors use a new, more latency optimized way of terminating a set of worker threads [8] contributed by RedHat, and generic optimizations to the work stealing mechanism have been implemented [9]. This reduces pause times for all collectors, particularly when using many threads. - optimizations to G1 pause times: the usual bunch of micro- optimizations for shorter pauses [10][11][12][13]. :) - G1 adds a few more JFR events to make its contents closer to what you can get from the logs [14]. ZGC - ZGC added class unloading, and doing that concurrently to the application at the same time [15]. :) - some remaining parts of the ZGC pauses were parallelized or moved into the concurrent phase to further decrease pause times [16][17] - ZGC is now built and available, still as experimental, by default [18] Oracle JDK12 EA builds can already be downloaded right now from http://jdk.java.net/12/ - please report issues here or at https://bugreport.java.com/ to make the VM even better. Thanks again to the many contributors who made this list of changes possible, and stay tuned for JDK13+ GC changes :) Have fun digging references, and enjoy! Thanks, Thomas [0] https://bugs.openjdk.java.net/browse/JDK-8215548?jql=project%20%3D%20JDK%20AND%20issuetype%20in%20(Bug%2C%20Enhancement)%20AND%20fixVersion%20in%20(%2212%22%2C%2012.0.1%2C%2012.0.2)%20AND%20component%20%3D%20hotspot%20AND%20Subcomponent%20%3D%20gc [1] http://openjdk.java.net/jeps/344 [2] https://bugs.openjdk.java.net/browse/JDK-8213890 [3] https://archive.fosdem.org/2018/schedule/event/g1/ [4] http://openjdk.java.net/jeps/346 [5] https://bugs.openjdk.java.net/browse/JDK-8212657 [6] https://bugs.openjdk.java.net/browse/JDK-8202286 [7] https://bugs.openjdk.java.net/browse/JDK-6490394 [8] https://bugs.openjdk.java.net/browse/JDK-8204947 [9] https://bugs.openjdk.java.net/browse/JDK-8205921 [10] https://bugs.openjdk.java.net/browse/JDK-8212911 [11] https://bugs.openjdk.java.net/browse/JDK-8212753 [12] https://bugs.openjdk.java.net/browse/JDK-8211853 [13] https://bugs.openjdk.java.net/browse/JDK-8209843 [14] https://bugs.openjdk.java.net/browse/JDK-8196341 [15] https://bugs.openjdk.java.net/browse/JDK-8214897 [16] https://bugs.openjdk.java.net/browse/JDK-8210883 [17] https://bugs.openjdk.java.net/browse/JDK-8210064 [18] https://bugs.openjdk.java.net/browse/JDK-8214476 _______________________________________________ hotspot-gc-use mailing list hotspot-gc-use@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
