On Tue, 24 Oct 2023 09:56:57 GMT, Thomas Schatzl <tscha...@openjdk.org> wrote:
> The JEP covers the idea very well, so I'm only covering some implementation > details here: > > * regions get a "pin count" (reference count). As long as it is non-zero, we > conservatively never reclaim that region even if there is no reference in > there. JNI code might have references to it. > > * the JNI spec only requires us to provide pinning support for typeArrays, > nothing else. This implementation uses this in various ways: > > * when evacuating from a pinned region, we evacuate everything live but the > typeArrays to get more empty regions to clean up later. > > * when formatting dead space within pinned regions we use filler objects. > Pinned regions may be referenced by JNI code only, so we can't overwrite > contents of any dead typeArray either. These dead but referenced typeArrays > luckily have the same header size of our filler objects, so we can use their > headers for our fillers. The problem is that previously there has been that > restriction that filler objects are half a region size at most, so we can end > up with the need for placing a filler object header inside a typeArray. The > code could be clever and handle this situation by splitting the to be filled > area so that this can't happen, but the solution taken here is allowing > filler arrays to cover a whole region. They are not referenced by Java code > anyway, so there is no harm in doing so (i.e. gc code never touches them > anyway). > > * G1 currently only ever actually evacuates young pinned regions. Old pinned > regions of any kind are never put into the collection set and automatically > skipped. However assuming that the pinning is of short length, we put them > into the candidates when we can. > > * there is the problem that if an applications pins a region for a long > time g1 will skip evacuating that region over and over. that may lead to > issues with the current policy in marking regions (only exit mixed phase when > there are no marking candidates) and just waste of processing time (when the > candidate stays in the retained candidates) > > The cop-out chosen here is to "age out" the regions from the candidates > and wait until the next marking happens. > > I.e. pinned marking candidates are immediately moved to retained > candidates, and if in total the region has been pinned for > `G1NumCollectionsKeepUnreclaimable` collections it is dropped from the > candidates. Its current value is fairly random. > > * G1 pauses got a new tag if there were pinned regions in the collection set. > I.e. in addition to something like: > > `GC(6) P... This pull request has now been integrated. Changeset: 38cfb220 Author: Thomas Schatzl <tscha...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/38cfb220ddadbb401cc15f313aadb8234f626210 Stats: 1823 lines in 59 files changed: 1150 ins; 435 del; 238 mod 8318706: Implement JEP 423: Region Pinning for G1 Reviewed-by: ayang, iwalulya, sjohanss ------------- PR: https://git.openjdk.org/jdk/pull/16342
