On Fri, 27 Oct 2023 20:38:19 GMT, Albert Mingkun Yang <[email protected]> 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/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.
Apart from having an early return in the `should_compact`-if, one option would
be making `has_pinned_objects()` more clever by stating something like:
bool has_pinned_objects() const {
return pinned_count() > 0 || (is_continues_humongous() &&
humongous_start_region()->pinned_count() > 0);
}
Then this predicate would get shorter. Or add a local helper for that (as
suggested in the next commit). Either is fine with me.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1376208039
