On Fri, 3 Nov 2023 13:46:51 GMT, Thomas Schatzl <[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...
>
> Thomas Schatzl has updated the pull request incrementally with one additional
> commit since the last revision:
>
> typos
LGTM!
Nits:
src/hotspot/share/gc/g1/g1FullCollector.cpp line 465:
> 463: continue;
> 464: } else if (is_compaction_target(region_index)) {
> 465: assert(!hr->has_pinned_objects(), "pinned objects should not be
> compaction targets");
Suggestion:
assert(!hr->has_pinned_objects(), "pinned regions should not be
compaction targets");
src/hotspot/share/gc/g1/g1YoungCollector.cpp line 430:
> 428: _claimer(_g1h->workers()->active_workers()),
> 429: _humongous_total(0),
> 430: _humongous_candidates(0) { }
Suggestion:
_humongous_candidates(0) { }
-------------
Marked as reviewed by iwalulya (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/16342#pullrequestreview-1707910606
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1382164253
PR Review Comment: https://git.openjdk.org/jdk/pull/16342#discussion_r1382135577
