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

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Date:   Wed, 28 Sep 2022 11:30:05 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Paolo Valente <paolo.valente@...aro.org>,
        Yu Kuai <yukuai1@...weicloud.com>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <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,

在 2022/09/28 0:14, Paolo Valente 写道:
> 
> 
>> Il giorno 27 set 2022, alle ore 03:02, Yu Kuai <yukuai1@...weicloud.com> ha scritto:
>>
>> Hi, Jan
>>
>> 在 2022/09/26 22:22, Jan Kara 写道:
>>> Hi Kuai!
>>> On Mon 26-09-22 21:00:48, Yu Kuai wrote:
>>>> 在 2022/09/23 19:03, Jan Kara 写道:
>>>>> Hi Kuai!
>>>>>
>>>>> On Fri 23-09-22 18:23:03, Yu Kuai wrote:
>>>>>> 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.
>>>>>
>>>>> Well, BFQ implements for example idling on sync IO queues which is one of
>>>>> the features that upsets blk-wbt. That does not depend on
>>>>> CONFIG_BFQ_GROUP_IOSCHED in any way. Also generally the idea that BFQ
>>>>> assigns storage *time slots* to different processes and IO from other
>>>>> processes is just queued at those times increases IO completion
>>>>> latency (for IOs of processes that are not currently scheduled) and this
>>>>> tends to confuse blk-wbt.
>>>>>
>>>> Hi, Jan
>>>>
>>>> Just to be curious, have you ever think about or tested wbt with
>>>> io-cost? And even more, how bfq work with io-cost?
>>>>
>>>> I haven't tested yet, but it seems to me some of them can work well
>>>> together.
>>> No, I didn't test these combinations. I actually expect there would be
>>> troubles in both cases under high IO load but you can try :)
>>
>> Just realize I made a clerical error, I actually want to saied that
>> *can't* work well together.
>>
> 
> You are right, they can't work together, conceptually. Their logics would simply keep conflicting, and none of the two would make ti to control IO as desired.

Yes, I just run some simple tests, test result is very bad...

Perhaps we can do something like bfq does to disable wbt.

Thanks,
Kuai
> 
> Thanks,
> Paolo
> 
>> I'll try to have a test the combinations.
>>
>> Thanks,
>> Kuai
>>> 								Honza
> 
> .
>