RFR: 8275266: Virtualize G1CardSet containers over heap region

[Thomas Schatzl]

Hi all,

can I have reviews for this change that virtualizes `G1CardSet` "regions" over a heap region, allowing the use of multiple "`G1CardSet` card regions" across a single heap region?

I.e. `HeapRegionRemSet`, which is the interface to a region's card set, simply uses multiple indexes for the remembered set of a single source heap region if necessary. E.g. on a 128MB region, heap region 0's cards would be stored as (what I call) "card region" indexes 0..3 as appropriate in its `_card_set`.

When retrieving the values, the appropriate retransformation is done (during `HeapRegionRemSet::iterate_for_merge()`).

Assigning `HeapRegionRemSet` to handle all this multiplexing required some move of the `G1CardSet::iterate_for_merge` method to `HeapRegionRemSet`, which is why there are more changes than expected.

One change I would like to have opinions on is storing the amount of card regions per region into `G1CardSetConfiguration`, maybe it is better to put this into `HeapRegionRemSet` - but I did not want to start a `HeapRegionRemSetConfiguration` (maybe also put the cached values introduced in the `split_card` optimization there as well?).

This allows unlimited actual heap region size. Currently set to 512MB (what we would set ergonomically if on a 1 TB heap), but that's just a random number basically.
Feel free to suggest a different maximum heap region size if any. We could also keep the ergonomics use a smaller heap region size (e.g. 32M as before).

There is also a CSR to look at.

Testing: tier1-5, some perf testing on region sizes up to 512M with slight improvements in specjbb2015 with larger region sizes.

Thanks,
Thomas

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Commit messages:
 - Virtual heap regions in remembered sets

Changes: https://git.openjdk.java.net/jdk/pull/6059/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6059&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275266
  Stats: 212 lines in 10 files changed: 126 ins; 67 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6059.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6059/head:pull/6059

PR: https://git.openjdk.java.net/jdk/pull/6059