linux-kernel - Re: Why do we use cpu nice priority for ioprio?

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Message-ID: <BANLkTinKyWsB-Oj4B-tDWEV7SPxvi--RaQ@mail.gmail.com>
Date:	Wed, 30 Mar 2011 20:59:34 -0700
From:	Justin TerAvest <teravest@...gle.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Chad Talbott <ctalbott@...gle.com>
Subject: Re: Why do we use cpu nice priority for ioprio?

On Wed, Mar 30, 2011 at 11:34 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Wed, Mar 30, 2011 at 11:23:45AM -0700, Justin TerAvest wrote:
>> On Fri, Mar 25, 2011 at 12:48 AM, Jens Axboe <jaxboe@...ionio.com> wrote:
>> > On 2011-03-25 03:38, Justin TerAvest wrote:
>> >> It's not clear why the cpu nice value should be mapped to the ioprio
>> >> for a task when none is picked.
>> >>
>> >> Internally, at cfq_init_prio_data(), we just set:
>> >>               ioprio = IOPRIO_NORM;
>> >>               ioprio_class = IOPRIO_CLASS_BE;
>> >> if ioprio_class is IOPRIO_CLASS_NONE.
>> >>
>> >>
>> >> The problem is that  today, SCHED_RR and SCHED_FIFO threads
>> >> automatically get bumped up to RT class. This all happens behind the
>> >> curtains and the io_class of the thread is still shown as NONE with
>> >> sys_ioprio_get(). What's the motivation behind this promotion of
>> >> ioprio class?
>> >
>> > It was decided back in the day when io priorities were introduced. I
>> > still think it's a good idea.
>>
>> This is the part I'm especially curious about. If we're managing the
>> resources separately,
>> why should be be adjusting io_class based on the nice value?
>
> What's wrong with taking a hint about ioclass and ioprio from cpu
> scheduling class and nice value?

I guess it's fine. I was just curious how strongly everyone felt about
the way it works today. It sounds like we expect nice values and io
scheduling classes to be closely matched.

Thanks,
Justin

>
> I think in general if some task is important from cpu perspective then
> most likely it is important from IO perspective too. And if user thinks
> otherwise, then one can explicitly set ioclass and ioprio of the task.
>
> So this makes sense to me.
>
> Thanks
> Vivek
>
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