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Subject: Re: [PATCH v3 3/5] block, bfq: don't disable wbt if
 CONFIG_BFQ_GROUP_IOSCHED is disabled
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <84f52be5-0fc4-ad22-d1ce-27e3dbc28fe7@huaweicloud.com>
Date:   Tue, 27 Sep 2022 18:14:39 +0200
Cc:     Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
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Message-Id: <E29E2557-9D79-40A3-B0EA-5EBAD6DD5A1F@linaro.org>
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 <988a86f2-e960-ba59-4d41-f4c8a6345ee9@huaweicloud.com>
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 <84f52be5-0fc4-ad22-d1ce-27e3dbc28fe7@huaweicloud.com>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Precedence: bulk


> Il giorno 27 set 2022, alle ore 03:02, Yu Kuai =
<yukuai1@huaweicloud.com> ha scritto:
>=20
> Hi, Jan
>=20
> =E5=9C=A8 2022/09/26 22:22, Jan Kara =E5=86=99=E9=81=93:
>> Hi Kuai!
>> On Mon 26-09-22 21:00:48, Yu Kuai wrote:
>>> =E5=9C=A8 2022/09/23 19:03, Jan Kara =E5=86=99=E9=81=93:
>>>> Hi Kuai!
>>>>=20
>>>> On Fri 23-09-22 18:23:03, Yu Kuai wrote:
>>>>> =E5=9C=A8 2022/09/23 18:06, Jan Kara =E5=86=99=E9=81=93:
>>>>>> On Fri 23-09-22 17:50:49, Yu Kuai wrote:
>>>>>>> Hi, Christoph
>>>>>>>=20
>>>>>>> =E5=9C=A8 2022/09/23 16:56, Christoph Hellwig =E5=86=99=E9=81=93:
>>>>>>>> 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.
>>>>>>>>=20
>>>>>>>> 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?
>>>>>>>> .
>>>>>>>>=20
>>>>>>>=20
>>>>>>> That's a good point,
>>>>>>>=20
>>>>>>> Before this patch wbt is simply disabled if elevator is bfq.
>>>>>>>=20
>>>>>>> With this patch, if elevator is bfq while bfq doesn't throttle
>>>>>>> any IO yet, wbt still is disabled unnecessarily.
>>>>>>=20
>>>>>> 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.
>>>>>>=20
>>>>>=20
>>>>> Thanks for your explanation, I understand that bfq and wbt should =
not
>>>>> work together.
>>>>>=20
>>>>> 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.
>>>>=20
>>>> 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.
>>>>=20
>>> Hi, Jan
>>>=20
>>> Just to be curious, have you ever think about or tested wbt with
>>> io-cost? And even more, how bfq work with io-cost?
>>>=20
>>> 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 :)
>=20
> Just realize I made a clerical error, I actually want to saied that
> *can't* work well together.
>=20

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.
>=20
> Thanks,
> Kuai
>> 								Honza