Re: [RFC PATCH] blk-iocost: introduce 'linear-max' cost model for cloud disk

[Jialin Wang <wjl.linux@gmail.com>]

> This formula correctly models the dual-bucket behavior of cloud disks.
> It ensures that for any block size, the calculated cost aligns with the
> actual bottleneck (IOPS or BPS). This allows the system to reach close
> to the provisioned BPS/IOPS limits without premature throttling, while
> still maintaining the latency protection benefits of iocost.

This model still has some limitations. Under workloads with mixed IO sizes and
vrate max at 100%, it fail to fully saturate the hardware performance.
However, it still demonstrates a clear improvement over the linear model.

The following fio benchmarks were conducted with two cgroups assigned equal weights:

Cgroup A: fio --bs=32k ...
Cgroup B: fio --bs=1M  ...

Results:

Model       | Cgroup A (32k)         | Cgroup B (1M)        | Total
------------+------------------------+----------------------|----------------------
linear      | 1137 IOPS (35.5 MiB/s) | 79 IOPS (79.5 MiB/s) | 1216 IOPS 115.0 MiB/s
linear-max  | 1781 IOPS (55.7 MiB/s) | 83 IOPS (83.9 MiB/s) | 1864 IOPS 139.6 MiB/s