[master] RFR: Implement sliding forwarding scheme that preserves upper header bits [v12]

Roman Kennke rkennke at openjdk.java.net
Mon Jul 5 15:09:11 UTC 2021

> The current way of storing forwarding information - by storing the forwardee address into the header - poses a problem for sliding collectors: it overrides the upper bits that keep the (compressed) Klass*. Sliding collectors have no way to recover the Klass*, and therefore need a different forwarding scheme that preserves the upper bits. I propose a scheme that compresses the forwarding pointer and by taking advantage of the fact that each region only ever forwards to at most two other regions (when dividing the heap into equal-sized logical regions), it can address all of the heap, regardless of its size. This obviouly works well with regionalized collectors, but it also works with contiguous-heap collectors like serial or parallel GC. The latter would divide the heap into logical regions sized by 4G (or single region if heap is smaller than that). Notice that evacuating and scavenging collectors don't have this problem: they can safely stomp over the from-space copy of objects, t
 he Klass* information is preserved in the to-space copy.
> G1 is special: the fallback from parallel to serial full GC means that we can have maximum of N target regions with N == ParallelGCThreads. For this reason, I use an encoding with 5 bits for target region index, that is enough to address 32 target regions from each region, assuming a maximum heap region size of 32M. In the future we will be able to steal bits from the Klass* in the upper half of the header, allowing us to address even more regions. Alternatively, we can change the full-GC fallback to re-forward all objects serially, instead of trying to re-use the already-forwarded state from the parallel full GC.
> Serial GC and Shenandoah GC use only 1 bit to address target regions, because they really can only ever have 2 target regions out of a single region.
> Testing:
> - [x] manual testing with +passive and -degen-gc
> - [x] hotspot_gc_shenandoah
> - [x] tier1
> - [x] tier2
> - [x] tier1 (x86_32)

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Disable G1 serial full-GC, simplify sliding forwarding


  - all: https://git.openjdk.java.net/lilliput/pull/8/files
  - new: https://git.openjdk.java.net/lilliput/pull/8/files/92c2f1ae..83a607f2

 - full: https://webrevs.openjdk.java.net/?repo=lilliput&pr=8&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=lilliput&pr=8&range=10-11

  Stats: 222 lines in 27 files changed: 84 ins; 78 del; 60 mod
  Patch: https://git.openjdk.java.net/lilliput/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/lilliput pull/8/head:pull/8

PR: https://git.openjdk.java.net/lilliput/pull/8

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