RFR: 8363932: G1: Better distribute KlassCleaningTask [v2]

[Albert Mingkun Yang]

On Mon, 22 Sep 2025 10:19:02 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change to parallel klass cleaning to improve performance.
>> 
>> The current implementation only parallelizes cleaning of weak class links, while the main work, cleaning the object tree is single-threaded. Hence in practice, the current mechanism does not scale beyond 2-3 threads.
>> 
>> Cleaning an object graph in an application that loads some jars and instantiates central classes within them, with around 180k classes the current `G1 Complete Cleaning` task (which executes this code) can take 80ms (with 25 threads).
>> 
>> The suggested change is to walk the object graph by (live) `ClassLoaderData` klass by klass, fixing only the links of that particular klass.
>> 
>> E.g.
>> 
>> CLD1 has klasses A, B, C, CLD2 has klasses a, b, c and CLD3 has klasses 0, 1, 2, 4;
>> vertical links are subklass references, while horizontal links are sibling references.
>> 
>> j.l.O
>>   |
>>   A - B - c - 3
>>   |
>>   0 - 2 - C - 1
>> 
>> 
>> CLD 3 is dead. Thread 1 claims CLD 1, Thread 2 claims CLD 2 (and nobody claims CLD3 because it's dead).
>> 
>> So thread 1, when reaching `A` fixes its subklass link to `C`, and otherwise does nothing with `A`. When looking at `C`, it will remove the link to `1`.
>> Thread 2 will only remove the link to `3` of `c`.
>> 
>> The result is
>> 
>> j.l.O
>>   |
>>   A - B - c
>>   |
>>   C
>> 
>> 
>> There should be no unnecessary object graph walking.
>> 
>> There is a slight change in printing during unlinking: previously the code, when cleaning subklasses it printed `unlinking class (subclass)`for every class that has been removed on the way to the next live one. In above case, it would print
>> 
>> 
>> unlinking class (subclass): 0
>> unlinking class (subclass): 2
>> 
>> 
>> With the change, to avoid following the subklasses of the graph twice, it prints
>> 
>> 
>> unlinking class (subclass): 0
>> unlinking class (sibling): 0
>> 
>> 
>> because the string in brackets is the actual link that is followed. I can revert that change.
>> 
>> With the change "Complete Cleaning" time for 200k classes takes 7.6ms (The test is a bit random on when it does the class unloading).
>> 
>> Testing: tier1-5
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   * ayang review

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27316#pullrequestreview-3281701843