The results of the "request for comments" batch to enhance the md/bitmap code is mighty impressive. Here's straight from the patch message:
"Optimize scattered address space. Achieves significant improvements in both throughput and latency.
Maximize thread-level parallelism and reduce CPU suspension time caused by lock contention. Achieve increased overall storage throughput by 89.4% on a system with four PCIe 4.0 SSDs. (Set the iodepth to 32 and employ libaio. Configure the I/O size as 4 KB with sequential write and 16 threads. In RAID5 consist of 2+1 1TB Samsung 980Pro SSDs, throughput went from 5218MB/s to 9884MB/s.)"
This ~200 line optimization patch comes from a Chinese research paper over optimizing Linux software RAID for next-generation storage devices. With the work from the Chinese university researchers they improve throughput by nearly 90% while also decreasing the 99.99th percentile latency by 85% compared to the existing MD RAID code.
Those interested can find the patch and more details via the Linux kernel mailing list while this MD optimization patch undergoes review.