RFR: 8327042: G1: Parallelism used for redirty logged cards needs better control. [v4]

[Albert Mingkun Yang]

On Mon, 4 Mar 2024 11:40:00 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Please review this change that provides better scaling of the `RedirtyLoggedCardsTask` with high number of worker threads. In the current implementation, the threads  contend for access to the Log Buffers through a single BufferNode resulting in a bottleneck as we increase the threads. The cost per card is pretty low, thus the work distribution overhead dominates the task. 
>> 
>> The new approach preserves the BufferNodeList states from G1ParScanThreadState, which effectively act as fingers (short cuts) into the list of buffers within `G1RedirtyCardsQueueSet`. We use these  BufferNodeList states to distribute BufferNodes for "redirtying" to the worker threads. By creating multiple points of access to the buffers, this method significantly reduces synchronization overheads and eliminates the bottleneck.
>> 
>> I have attached results from the Big Ram Tester microbenchmark on a server that spins up 163 worker threads for the `RedirtyLoggedCardsTask`
>> 
>> ![prepare_mixed](https://github.com/openjdk/jdk/assets/69453999/7120aa59-9b84-4e32-ac58-129ff31bb668)
>> ![normal](https://github.com/openjdk/jdk/assets/69453999/7aafb72e-8f18-410d-9f1b-0c0adcd78b03)
>> ![mixed](https://github.com/openjdk/jdk/assets/69453999/8a115d9c-56d3-4654-931e-1fa84987915d)
>> ![concurrent_start](https://github.com/openjdk/jdk/assets/69453999/783d8804-b7b8-42a7-976c-4c217636157b)
>> 
>> 
>> Testing: Tier 1-3
>
> Ivan Walulya has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Albert discussion
>  - Review
>  - init

Marked as reviewed by ayang (Reviewer).

src/hotspot/share/gc/g1/g1YoungGCPostEvacuateTasks.cpp line 604:

> 602:     _evac_failure_regions(evac_failure_regions) { }
> 603: 
> 604:   virtual ~RedirtyLoggedCardsTask() = default;

I don't think this class will ever have any subclasses. This can probably be left out.

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PR Review: https://git.openjdk.org/jdk/pull/17963#pullrequestreview-1914153330
PR Review Comment: https://git.openjdk.org/jdk/pull/17963#discussion_r1511061108