Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP_IOSCHED is disabled

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 27 set 2022, alle ore 03:02, Yu Kuai <yukuai1@huaweicloud.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.

Thanks,
Paolo

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