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

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Message-ID: <BANLkTimzt7QhBdayBpB3dRSOSY3G3VCfcw@mail.gmail.com>
Date:	Wed, 30 Mar 2011 11:23:45 -0700
From:	Justin TerAvest <teravest@...gle.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Chad Talbott <ctalbott@...gle.com>
Subject: Re: Why do we use cpu nice priority for ioprio?

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?

> You could argue that auto-bumping up to
> IOPRIO_CLASS_RT is a bit heavy handed. But is it really that different
> from the CPU scheduling?

But why do auto bumping at all? Is this for convenience, so users only
have to nice their tasks?

>
> So, from memory, the difference between RR and FIFO on the CPU
> scheduling side is that one of them will relingiush the CPU for a brief
> period once in a while. Correct? That does not happen for
> IOPRIO_CLASS_RT. So at least from that perspective, it's not a complete
> match and might be a cause for concern.
>
> --
> Jens Axboe
>
>
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