RFR: 8318706: Implementation of JDK-8276094: JEP 423: Region Pinning for G1 [v15]

Thomas Schatzl tschatzl at openjdk.org
Tue Nov 7 18:11:52 UTC 2023


> 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...

Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:

 - Merge branch 'master' into 8318706-implementation-of-region-pinning-in-g1
 - "GCLocker Initiated GC" is not a valid GC cause for G1 any more
 - Fix tests after merge
 - Merge tag 'jdk-22+22' into 8318706-implementation-of-region-pinning-in-g1
   
   Added tag jdk-22+22 for changeset d354141a
 - Merge tag 'jdk-22+21' into 8318706-implementation-of-region-pinning-in-g1
   
   Added tag jdk-22+21 for changeset d96f38b8
 - iwalulya review
 - typos
 - ayang review - renamings + documentation
 - Add documentation about why and how we handle pinned regions in the young/old generation.
 - Renamings to (almost) consistently use the following nomenclature for evacuation failure and types of it:
   
   * evacuation failure is the general concept. It includes
     * pinned regions
     * allocation failure
   
   One region can both be pinned and experience an allocation failure.
   
   G1 GC messages use tags "(Pinned)" and "(Allocation Failure)" now instead of "(Evacuation Failure)"
   
   Did not rename the G1EvacFailureInjector since this adds a lot of noise.
 - ... and 9 more: https://git.openjdk.org/jdk/compare/45e68ae2...83eff9fe

-------------

Changes: https://git.openjdk.org/jdk/pull/16342/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16342&range=14
  Stats: 1820 lines in 59 files changed: 1147 ins; 430 del; 243 mod
  Patch: https://git.openjdk.org/jdk/pull/16342.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16342/head:pull/16342

PR: https://git.openjdk.org/jdk/pull/16342


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