RFR: 8327452: G1: Improve scalability of Merge Log Buffers [v3]

[Thomas Schatzl]

On Fri, 8 Mar 2024 11:49:05 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Hi all,
>> 
>> Please review this change to reduce contention on the DCQS. The parallel work done by the worker threads is very small, therefore contention on the DCQS dominates the Merge Log Buffers phase. In this change, we add a sequential phase to distribute the Log Buffers to the worker threads. This removes the DCQS bottleneck in the highly contended case, at a small cost to the cases with low contention.
>> 
>> Testing Tier 1-3
>> 
>> The graphs below are using Bigramtester at 20gb, "Distribute and Avg Log Buffers" combines the duration of the sequential distribute phase and the avg duration of the parallel `Merge Log Buffers` phase.
>> | Time (ms)  | # Cards |
>> | ------------- | ------------- |
>> | ![128_concurrent_start](https://github.com/openjdk/jdk/assets/69453999/c6663ed9-6b81-4e32-bb1e-17627591406f) | ![128_concurrent_start_cards](https://github.com/openjdk/jdk/assets/69453999/1909564b-2482-4767-a334-9f5612d5cb8c)|
>> | ![128_mixed](https://github.com/openjdk/jdk/assets/69453999/d8dd8933-3aae-4795-b426-b4022bcf1ff2)| ![128_mixed_cards](https://github.com/openjdk/jdk/assets/69453999/ed5b8279-8d70-4dc6-b53d-59bf04b6e12e)|
>> | ![64_concurrent_start](https://github.com/openjdk/jdk/assets/69453999/5fb03cc5-2b86-4b9d-83ff-caa0eb482859) | ![64_concurrent_start_cards](https://github.com/openjdk/jdk/assets/69453999/c62fb05a-dca1-4b3f-895c-62837f7af2f8)|
>> | ![64_mixed](https://github.com/openjdk/jdk/assets/69453999/896c4138-a26b-457d-a5b6-d4b4285c4b8f) |  ![64_mixed_cards](https://github.com/openjdk/jdk/assets/69453999/2a6a234e-f180-4f02-9ad6-3867c5fd1881) |
>> | ![32_mixed](https://github.com/openjdk/jdk/assets/69453999/25a45fa6-f1dd-48e9-85af-702be67f7b9b) | ![32_mixed_cards](https://github.com/openjdk/jdk/assets/69453999/f5d80538-efde-47e4-860f-9b15409085a7) |
>> |  ![8_mixed](https://github.com/openjdk/jdk/assets/69453999/112b23c5-f048-4678-8d04-2c682696b357) | ![8_mixed_cards](https://github.com/openjdk/jdk/assets/69453999/0fdb9b6b-242c-424c-83ef-3d05703f5c42) |
>
> Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Thomas Review

Changes requested by tschatzl (Reviewer).

src/hotspot/share/gc/g1/g1RemSet.cpp line 1336:

> 1334:         _dirty_card_buffers[i].~Stack();
> 1335:       }
> 1336: #endif

I do not think this should be put under `#ifdef` as the destructor itself is always there afair. The compiler should automatically compile the whole loop away in product mode anyway.

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PR Review: https://git.openjdk.org/jdk/pull/18134#pullrequestreview-1924816897
PR Review Comment: https://git.openjdk.org/jdk/pull/18134#discussion_r1517678208