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Message-ID: <cbdb4a5a-a8ca-436b-a2ed-59f650a3dce7@linux.ibm.com>
Date: Thu, 2 Oct 2025 20:42:48 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, bvanassche@....org,
ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com,
johnny.chenyi@...wei.com
Subject: Re: [PATCH 7/7] blk-mq: add documentation for new queue attribute
async_dpeth
On 9/30/25 12:41 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
>
> Explain the attribute and the default value in different case.
>
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
> 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@...r.kernel.org
> +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
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