linux-kernel - Re: [PATCH] blk-throttle: support io merge over iops

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Message-ID: <c1e467a9-7499-e42b-88ed-b8e34b831515@huaweicloud.com>
Date: Wed, 12 Mar 2025 09:51:30 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Tejun Heo <tj@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc: Ming Lei <ming.lei@...hat.com>, axboe@...nel.dk, josef@...icpanda.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-throttle: support io merge over iops_limit

Hi,

在 2025/03/12 3:34, Tejun Heo 写道:
> Hello,
> 
> On Tue, Mar 11, 2025 at 11:08:00AM +0800, Yu Kuai wrote:
> ...
>>> That said, I'm not sure about changing the behavior now. blk-throtl has
>>> mostly used the number of bios as long as it has existed and given that
>>> there can be a signficant difference between the two metrics, I'm not sure
>>> the change is justified at this point.
>>
>> If we really concern about the behavior change, can we consider a new
>> flag that can switch to the old behavior? We'll see if any user will
>> complain.
> 
> Yeah, that may be the right way to go about this, but let me turn this
> around and ask you why adding a new behavior would be a good idea. What
> problems are you trying to solve?

In the case of dirty pages writeback, BIO is 4k, while RQ can be up to
hw_sectors_kb. Our user are limiting iops based on real disk capacity
and they found BIO merge will be broken.

The idea way really is rq-qos based iops limit, which is after BIO merge
and BIO merge is ensured not borken. In this case, I have to suggest
them set a high iops limit or just remove the iops limit.

Thanks,
Kuai

> 
> Thanks.
>