linux-kernel - Re: [PATCH V4 00/15] blk-throttle: add .high limit

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Date:   Mon, 14 Nov 2016 16:41:33 -0800
From:   Bart Van Assche <bart.vanassche@...disk.com>
To:     Shaohua Li <shli@...com>
CC:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Kernel-team@...com" <Kernel-team@...com>,
        "axboe@...com" <axboe@...com>, "tj@...nel.org" <tj@...nel.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>
Subject: Re: [PATCH V4 00/15] blk-throttle: add .high limit

On 11/14/2016 04:05 PM, Shaohua Li wrote:
> On Mon, Nov 14, 2016 at 02:46:22PM -0800, Bart Van Assche wrote:
>> On 11/14/2016 02:22 PM, Shaohua Li wrote:
>>> The background is we don't have an ioscheduler for blk-mq yet, so we can't
>>> prioritize processes/cgroups. This patch set tries to add basic arbitration
>>> between cgroups with blk-throttle. It adds a new limit io.high for
>>> blk-throttle. It's only for cgroup2.
>>
>> My understanding of this work is that a significant part of it will have to
>> be reverted once blk-mq supports I/O scheduling, e.g. the code for detecting
>> whether the I/O submitter is idle. Shouldn't this kind of infrastructure be
>> added after support has been added in blk-mq for I/O scheduling?
>
> Sure, if we have a CFQ-like io scheduler for blk-mq, this is largly not
> required. But we don't have one yet and nothing is floating around either. The
> conservative throttling is relatively easy to implement and achive similar
> goal. The throttling could be still useful even with ioscheduler as throttling
> is faster if we are talking about CFQ-like scheduler. I don't think this should
> be blocked to wait for I/O scheduling. There was a long discussion in last
> post, and we agreed the throttling and io scheduler aren't mutually exclusive.
> http://marc.info/?l=linux-kernel&m=147552964708965&w=2

Hello Shaohua,

Thank you for pointing me to the discussion thread about v3 of this 
patch series. Did I see correctly that one of the conclusions was that 
for users this mechanism is hard to configure? Are we providing a good 
service to Linux users by providing a mechanism that is hard to configure?

Thanks,

Bart.