linux-kernel - Re: bfq-mq performance comparison to cfq

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Date:   Wed, 26 Apr 2017 22:12:36 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "paolo.valente@...aro.org" <paolo.valente@...aro.org>,
        "juri.lelli@....com" <juri.lelli@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "patrick.bellasi@....com" <patrick.bellasi@....com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "joelaf@...gle.com" <joelaf@...gle.com>,
        "andresoportus@...gle.com" <andresoportus@...gle.com>,
        "morten.rasmussen@....com" <morten.rasmussen@....com>,
        "aherrmann@...e.com" <aherrmann@...e.com>
Subject: Re: bfq-mq performance comparison to cfq

On Wed, 2017-04-26 at 10:18 +0200, Paolo Valente wrote:
> I guess that both the above issues may not be dramatic. In contrast,
> the following last issue seems harder to address: BFQ uses two
> different privileging schemes, one suitable for interactive
> applications, and one suitable for soft real-time applications. So,
> what scheme should BFQ enable for processes in the RT I/O class?
>
> Because of these concerns, also for I/O I would find much clearer and
> flexible an ad-hoc, complete and explicit solution like the one(s)
> Juri reports (I've already nagged some of the recipients here to get
> support and collaboration on such sort of extensions of the basic
> benefits of a good I/O scheduler).

The numerical values of I/O priorities are part of the API between kernel
and user space API and hence the numerical value associated with a class
must not change. But we would associate different priority values with
interactive and soft real-time applications, e.g. IOPRIO_CLASS_RT(0) for
soft real-time applications and IOPRIO_CLASS_RT(7) for interactive
applications. See also http://man7.org/linux/man-pages/man2/ioprio_set.2.html.

In my opinion the above proposal does not contradict with what has been
proposed for informed run-times. We could e.g. add support for configuring
the I/O priority to the block I/O controller cgroup.

No matter how informed run-times communicate application constraints to the
kernel, the configured I/O scheduler and the block layer will have to realize
these constraints. If anyone thinks that there is a mechanism that is better
suited to communicate these constraints to the kernel than I/O priorities I'm
interested to hear about that alternative.

Bart.