> OpenJDK Colleagues: > > Please review this proposed integration of Generational mode for Shenandoah > GC under https://bugs.openjdk.org/browse/JDK-8307314. > > Generational mode of Shenandoah is enabled by adding > `-XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational` to a > command line that already specifies ` -XX:+UseShenandoahGC`. The > implementation automatically adjusts the sizes of old generation and young > generation to efficiently utilize the entire heap capacity. Generational > mode of Shenandoah resembles G1 in the following regards: > > 1. Old-generation marking runs concurrently during the time that multiple > young generation collections run to completion. > 2. After old-generation marking completes, we perform a sequence of mixed > collections. Each mixed collection combines collection of young generation > with evacuation of a portion of the old-generation regions identified for > collection based on old-generation marking information. > 3. Unlike G1, young-generation collections and evacuations are entirely > concurrent, as with single-generation Shenandoah. > 4. As with single-generation Shenandoah, there is no explicit notion of eden > and survivor space within the young generation. In practice, regions that > were most recently allocated tend to have large amounts of garbage and these > regions tend to be collected with very little effort. Young-generation > objects that survive garbage collection tend to accumulate in regions that > hold survivor objects. These regions tend to have smaller amounts of > garbage, and are less likely to be collected. If they survive a sufficient > number of young-generation collections, the “survivor” regions are promoted > into the old generation. > > We expect to refine heuristics as we gain experience with more production > workloads. In the future, we plan to remove the “experimental” qualifier > from generational mode, at which time we expect that generational mode will > become the default mode for Shenandoah. > > **Testing**: We continuously run jtreg tiers 1-4 + hotspot_gc_shenandoah, > gcstress, jck compiler, jck runtime, Dacapo, SpecJBB, SpecVM, Extremem, > HyperAlloc, and multiple AWS production workload simulators. We test on Linux > x64 and aarch64, Alpine x64 and aarch64, macOS x64 and aarch64, and Windows > x64.
Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision: JDK-8309322: [GenShen] TestAllocOutOfMemory#large failed When generational Shenandoah is used, there may be an additional alignment related heap size adjustment that the test should be cognizant of. Such alignment might also happen in the non-generational case, but in this case the specific size used in the test was affected on machines with larger than usual os page size settings. The alignment related adjustment would have affected all generational collectors (except perhaps Gen Z). In the future, we might try and relax this alignment constraint.alignment. ------------- Changes: - all: https://git.openjdk.org/jdk/pull/14185/files - new: https://git.openjdk.org/jdk/pull/14185/files/221c88ff..88958669 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=14185&range=10 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14185&range=09-10 Stats: 27 lines in 1 file changed: 16 ins; 0 del; 11 mod Patch: https://git.openjdk.org/jdk/pull/14185.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/14185/head:pull/14185 PR: https://git.openjdk.org/jdk/pull/14185
