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

Yu Kuai posted 7 patches 2 months, 2 weeks ago
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[PATCH 7/7] blk-mq: add documentation for new queue attribute async_dpeth
Posted by Yu Kuai 2 months, 2 weeks ago 
From: Yu Kuai <yukuai3@huawei.com>

Explain the attribute and the default value in different case.

Signed-off-by: Yu Kuai <yukuai3@huawei.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@vger.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.
+
+
 What:		/sys/block/<disk>/queue/nr_zones
 Date:		November 2018
 Contact:	Damien Le Moal <damien.lemoal@wdc.com>
-- 
2.39.2
Re: [PATCH 7/7] blk-mq: add documentation for new queue attribute async_dpeth
Posted by Nilay Shroff 2 months, 2 weeks ago 

On 9/30/25 12:41 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Explain the attribute and the default value in different case.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.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@vger.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
Re: [PATCH 7/7] blk-mq: add documentation for new queue attribute async_dpeth
Posted by Yu Kuai 2 months, 1 week ago 
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@huawei.com>
>>
>> Explain the attribute and the default value in different case.
>>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.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@vger.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
>
>

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