On Fri, 27 Oct 2023 20:53:29 GMT, Albert Mingkun Yang <ay...@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 a... > > src/hotspot/share/gc/g1/g1EvacFailureRegions.hpp line 36: > >> 34: class HeapRegionClaimer; >> 35: >> 36: // This class records for every region on the heap whether it has to be >> retained > > I feel the term "retain" has two diff meanings in this PR: > > 1. retain == pinned or evac-fail > 2. should_retain_evac_failed_region > > 1 is during scavenging while 2 is after scavenging. Maybe rename `should_retain_evac_failed_region` to `should_keep_retained_region[_in_old]` or something? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1376219339
