linux-kernel - Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP

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Message-ID: <95998ae6-8bbf-b438-801b-7033ceaf9c36@huaweicloud.com>
Date:   Fri, 23 Sep 2022 18:23:03 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Jan Kara <jack@...e.cz>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     Christoph Hellwig <hch@...radead.org>, paolo.valente@...aro.org,
        axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3 3/5] block, bfq: don't disable wbt if
 CONFIG_BFQ_GROUP_IOSCHED is disabled

Hi, Jan

在 2022/09/23 18:06, Jan Kara 写道:
> On Fri 23-09-22 17:50:49, Yu Kuai wrote:
>> Hi, Christoph
>>
>> 在 2022/09/23 16:56, Christoph Hellwig 写道:
>>> On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote:
>>>> wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled.
>>>
>>> Umm, wouldn't this be something decided at runtime, that is not
>>> if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build
>>> if the hierarchical cgroup based scheduling is actually used for a
>>> given device?
>>> .
>>>
>>
>> That's a good point,
>>
>> Before this patch wbt is simply disabled if elevator is bfq.
>>
>> With this patch, if elevator is bfq while bfq doesn't throttle
>> any IO yet, wbt still is disabled unnecessarily.
> 
> It is not really disabled unnecessarily. Have you actually tested the
> performance of the combination? I did once and the results were just
> horrible (which is I made BFQ just disable wbt by default). The problem is
> that blk-wbt assumes certain model of underlying storage stack and hardware
> behavior and BFQ just does not fit in that model. For example BFQ wants to
> see as many requests as possible so that it can heavily reorder them,
> estimate think times of applications, etc. On the other hand blk-wbt
> assumes that if request latency gets higher, it means there is too much IO
> going on and we need to allow less of "lower priority" IO types to be
> submitted. These two go directly against one another and I was easily
> observing blk-wbt spiraling down to allowing only very small number of
> requests submitted while BFQ was idling waiting for more IO from the
> process that was currently scheduled.
> 

Thanks for your explanation, I understand that bfq and wbt should not
work together.

However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service
guarantee is not needed, does the above phenomenon still exist? I find
it hard to understand... Perhaps I need to do some test.

Thanks,
Kuai

> So I'm kind of wondering why you'd like to use blk-wbt and BFQ together...
> 
> 								Honza
>