linux-kernel - Re: [PATCH 0/3] cfq-iosched: Fair cross-group preemption

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Date:	Mon, 28 Mar 2011 09:17:05 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Chad Talbott <ctalbott@...gle.com>
Cc:	Gui Jianfeng <guijianfeng@...fujitsu.com>, jaxboe@...ionio.com,
	linux-kernel@...r.kernel.org, mrubin@...gle.com,
	teravest@...gle.com
Subject: Re: [PATCH 0/3] cfq-iosched: Fair cross-group preemption

On Fri, Mar 25, 2011 at 04:53:13PM -0700, Chad Talbott wrote:
> On Fri, Mar 25, 2011 at 2:32 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> >> You seem pretty unenthusiastic about a).  How do you feel about b)?
> >
> > IMHO, Using RT group with throttling avoids introducing asymmetry between
> > task and group attributes. So I will prefer that approch. Though it means
> > more code as we will be introducing RT groups but that might be useful
> > in general for something else too. (I am assuming that somebody makes
> > use of RT class for cfqq).
> >
> > The one more down side of trying to use throttling is that one needs to
> > come up with absolute limit. So one shall have to know disk capacity
> > and if there are no BE tasks running then latency sensitive task will
> > be unnecessarily throttled (until and unless some management software
> > can monitor it and change limit dynamically).
> >
> > So if you are worried about setting the absolute limit part, then I guess
> > I am fine with option a). But if you think that setting absolute limit
> > is not a problem, then option b) is preferred.
> 
> I prefer option a) - so much so that even with the older CFQ group
> implementation we did work to merge the RT and BE service trees to
> achieve that behavior.  But I see that using blkio.class is a poor
> choice of interface name.  I will rename the interface and resubmit
> the patch series (also with Gui's suggestion to keep the "_device"
> suffix for consistency).

Do you need this feature to be global or per device or both?

Thanks
Vivek
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