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

[Ivan Walulya]

> 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:
  - all: https://git.openjdk.org/jdk/pull/18134/files
  - new: https://git.openjdk.org/jdk/pull/18134/files/9582ed1b..8a782c89

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18134&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18134&range=01-02

  Stats: 16 lines in 2 files changed: 10 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18134/head:pull/18134

PR: https://git.openjdk.org/jdk/pull/18134