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

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Message-ID: <60a26451-22cd-409b-920d-eb9d8490e246@yukuai.org.cn>
Date: Mon, 6 Oct 2025 10:00:21 +0800
From: Yu Kuai <hailan@...uai.org.cn>
To: Nilay Shroff <nilay@...ux.ibm.com>, 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

Hi,

在 2025/10/2 23:12, Nilay Shroff 写道:
>
> 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.

This is great! I will use this in the next version.

Thanks
Kuai

> Thanks,
> --Nilay
>
>