Re: [PATCH 7/7] blk-mq: add documentation for new queue attribute async_dpeth

[Nilay Shroff]

On 9/30/25 12:41 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> 
> Explain the attribute and the default value in different case.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  Documentation/ABI/stable/sysfs-block | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
> index 0ed10aeff86b..09b9b3db9a1f 100644
> --- a/Documentation/ABI/stable/sysfs-block
> +++ b/Documentation/ABI/stable/sysfs-block
> @@ -609,6 +609,16 @@ Description:
>  		enabled, and whether tags are shared.
>  
>  
> +What:		/sys/block/<disk>/queue/async_depth
> +Date:		August 2025
> +Contact:	linux-block@xxxxxxxxxxxxxxx
> +Description:
> +		[RW] This controls how many async requests may be allocated in the
> +		block layer. If elevator is none, then this value is nr_requests.
> +		By default, this value is 75% of nr_requests for bfq and kyber,
> +		abd nr_requests for mq-deadline.
> +
Hmm, it seems we need to further elaborate above documentation, seeing the
way this new sysfs interface is playing out now for different I/O schedulers. 
I'd suggest rewriting this as follow (you may further modify/simplify it based
on your taste, if needed):

Description:
[RW] Controls how many asynchronous requests may be allocated in the
block layer. The value is always capped at nr_requests.

  When no elevator is active (none):
  - async_depth is always equal to nr_requests.

  For bfq scheduler:
  - By default, async_depth is set to 75% of nr_requests. 
    Internal limits are then derived from this value:
    * Sync writes: limited to async_depth (≈75% of nr_requests).
    * Async I/O: limited to ~2/3 of async_depth (≈50% of nr_requests).

    If a bfq_queue is weight-raised:
    * Sync writes: limited to ~1/2 of async_depth (≈37% of nr_requests).
    * Async I/O: limited to ~1/4 of async_depth (≈18% of nr_requests).

  - If the user writes a custom value to async_depth, BFQ will recompute
    these limits proportionally based on the new value.

  For Kyber:
  - By default async_depth is set to 75% of nr_requests.
  - If the user writes a custom value to async_depth, then it override the
    default and directly control the limit for writes and async I/O.

  For mq-deadline:
  - By default async_depth is set to nr_requests.
  - If the user writes a custom value to async_depth, then it override the
    default and directly control the limit for writes and async I/O.

Thanks,
--Nilay