RFR: 8367332: Replace BlockTree tree logic with an intrusive red-black tree

[Casper Norrbin]

On Wed, 8 Oct 2025 12:02:13 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi everyone,
>> 
>> Metaspace's `BlockTree` is currently implemented as a simple binary search tree, using the memory blocks it stores as the nodes. While this is straightforward and generally fine, it can become unbalanced depending on insert/remove order, leading to degraded performance.
>> 
>> This is a good fit for the utilities `IntrusiveRBTree`, instead of using internal tree logic. With it, we can replace all tree functions and instead rely on a few insert/remove calls. `BlockTree` still remains as the layer coordinating these blocks, and still handles the list of same-size nodes. The intrusive red-black tree only handles balancing and linking/unlinking nodes in the tree structure. 
>> 
>> Validation and printing are preserved by using the hooks provided by the red-black tree. We keep the same checks (and get a few more from the rb-tree), and the printed output is kept identical. The only real difference is that the red-black tree traverses the tree instead.
>> 
>> Testing:
>> - Oracle tiers 1-8
>
> Looks good to me. Thanks for the cleanup.

Thank you for reviewing @jdksjolen!

Pinging @tstuefe, is this something you could have a look at?

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PR Comment: https://git.openjdk.org/jdk/pull/27212#issuecomment-3381313648