Integrated: 8275056: Virtualize G1CardSet containers over heap region

Thomas Schatzl tschatzl at
Mon Nov 15 18:12:49 UTC 2021

On Thu, 21 Oct 2021 07:49:56 GMT, Thomas Schatzl <tschatzl at> wrote:

> 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

This pull request has now been integrated.

Changeset: 1830b8da
Author:    Thomas Schatzl <tschatzl at>
Stats:     242 lines in 12 files changed: 148 ins; 68 del; 26 mod

8275056: Virtualize G1CardSet containers over heap region

Reviewed-by: sjohanss, ayang



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