[master] Integrated: 8347711: [Lilliput] Parallel GC support for compact identity hashcode

[Roman Kennke]

On Fri, 14 Mar 2025 11:25:23 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> The Parallel GC does not yet support Lilliput 2 until now. The big problem has been that the Parallel Full GC is too rigid with respect to object sizes, and we could not make it work with compact identity hashcode, which requires that objects can grow during GC.
> 
> The PR implements an alternative full GC for Parallel GC, which is more flexible. The algorithm mostly follows G1 and Shenandoah, with the difference that it creates temporary 'regions' (because Parallel GC does not use heap regions), with boundaries such that no object crosses region boundaries, and then after GC fill any gaps at end of regions with dummy objects.
> 
> The implementation has a special 'serial' mode, which sets up only 4 regions which exactly match the 4 heap spaces (old, eden, from, to), and performs the forwarding and compaction phases serially to achieve perfect compaction at the expense of performance. (The marking and adjust-refs phases will still be done with parallel workers).
> 
> I've run the micro benchmarks for systemgc, there seem to be only minor differences, and looks mostly like a few milliseconds offset in the new implementation:
> 
> Baseline Full GC:
> 
> AllDead.gc                        ss   25   31.120 ±  0.447  ms/op
> AllLive.gc                        ss   25   83.655 ±  2.238  ms/op
> DifferentObjectSizesArray.gc      ss   25  179.725 ±  1.171  ms/op
> DifferentObjectSizesHashMap.gc    ss   25  186.011 ±  1.409  ms/op
> DifferentObjectSizesTreeMap.gc    ss   25   65.668 ±  3.333  ms/op
> HalfDeadFirstPart.gc              ss   25   64.862 ±  0.696  ms/op
> HalfDeadInterleaved.gc            ss   25   67.764 ±  3.139  ms/op
> HalfDeadInterleavedChunks.gc      ss   25   59.160 ±  1.667  ms/op
> HalfDeadSecondPart.gc             ss   25   66.210 ±  1.167  ms/op
> HalfHashedHalfDead.gc             ss   25   69.584 ±  2.276  ms/op
> NoObjects.gc                      ss   25   18.462 ±  0.270  ms/op
> OneBigObject.gc                   ss   25  587.425 ± 27.493  ms/op
> 
> 
> New Parallel Full GC:
> 
> 
> AllDead.gc                        ss   25   39.891 ±  0.461  ms/op
> AllLive.gc                        ss   25   87.898 ±  1.940  ms/op
> DifferentObjectSizesArray.gc      ss   25  184.109 ±  0.795  ms/op
> DifferentObjectSizesHashMap.gc    ss   25  189.620 ±  2.236  ms/op
> DifferentObjectSizesTreeMap.gc    ss   25   69.915 ±  3.308  ms/op
> HalfDeadFirstPart.gc              ss   25   70.664 ±  0.804  ms/op
> HalfDeadInterleaved.gc            ss   25   71.318 ±  1.583  ms/op
> HalfDeadInterleavedChunks.gc      ss   25   65.050...

This pull request has now been integrated.

Changeset: 4b6094a0
Author:    Roman Kennke <rkennke at openjdk.org>
URL:       https://git.openjdk.org/lilliput/commit/4b6094a023e6d9f3c74a0afc65192d7e81c7e3a0
Stats:     2081 lines in 12 files changed: 2065 ins; 4 del; 12 mod

8347711: [Lilliput] Parallel GC support for compact identity hashcode

Reviewed-by: stuefe

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PR: https://git.openjdk.org/lilliput/pull/195