RFR: 8318706: Implementation of JDK-8276094: JEP 423: Region Pinning for G1

[Albert Mingkun Yang]

On Tue, 24 Oct 2023 09:56:57 GMT, Thomas Schatzl <tschatzl at 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...

src/hotspot/share/gc/g1/g1CollectedHeap.cpp line 444:

> 442:     }
> 443: 
> 444:     if (succeeded) {

Can these two `if`s can be merged into one, `if (succeeded) { return result; }`?

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.

src/hotspot/share/gc/g1/g1FullGCPrepareTask.inline.hpp line 82:

> 80:   } else {
> 81:     assert(hr->containing_set() == nullptr, "already cleared by PrepareRegionsClosure");
> 82:     if (hr->has_pinned_objects() ||

This `do_heap_region` method is hard to follow; there multiple occurrences of same predicates. I wonder if one can reorganize these if-else a bit. Inlining `should_compact` should make all `if` on the same level at least.

src/hotspot/share/gc/g1/g1ParScanThreadState.cpp line 494:

> 492:     // undo_allocation() method too.
> 493:     undo_allocation(dest_attr, obj_ptr, word_sz, node_index);
> 494:     return handle_evacuation_failure_par(old, old_mark, word_sz, true /* cause_pinned */);

Why is this `cause_pinned == true`? This obj can be arbitrary, not necessarily type-array.

src/hotspot/share/gc/g1/heapRegion.cpp line 734:

> 732:   // ranges passed in here corresponding to the space between live objects, it is
> 733:   // possible that there is a pinned object that is not any more referenced by
> 734:   // Java code (only by native).

Can such obj becomes referenced by java again later on? IOW, a pointer passed from native to java.

src/hotspot/share/gc/g1/heapRegion.inline.hpp line 262:

> 260: }
> 261: 
> 262: inline bool HeapRegion::can_reclaim() const {

I'd suggest inline this method to callers, because "can reclaim" is sth caller context sensitive.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1374835226
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1375035713
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1375023324
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1375009476
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1375304500
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1375030685