Message-ID: <20100901172114.GB22149@redhat.com>
Date:	Wed, 1 Sep 2010 13:21:14 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Nauman Rafique <nauman@...gle.com>
Cc:	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Jens Axboe <axboe@...nel.dk>, Jeff Moyer <jmoyer@...hat.com>,
	Divyesh Shah <dpshah@...gle.com>,
	Corrado Zoccolo <czoccolo@...il.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC] [PATCH] cfq-iosched: add cfq group hierarchical scheduling
 support

On Wed, Sep 01, 2010 at 10:15:31AM -0700, Nauman Rafique wrote:
> On Wed, Sep 1, 2010 at 10:10 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > On Wed, Sep 01, 2010 at 08:49:26AM -0700, Nauman Rafique wrote:
> >> On Wed, Sep 1, 2010 at 1:50 AM, Gui Jianfeng <guijianfeng@...fujitsu.com> wrote:
> >> > Vivek Goyal wrote:
> >> >> On Tue, Aug 31, 2010 at 08:40:19AM -0700, Nauman Rafique wrote:
> >> >>> On Tue, Aug 31, 2010 at 5:57 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> >> >>>> On Tue, Aug 31, 2010 at 08:29:20AM +0800, Gui Jianfeng wrote:
> >> >>>>> Vivek Goyal wrote:
> >> >>>>>> On Mon, Aug 30, 2010 at 02:50:40PM +0800, Gui Jianfeng wrote:
> >> >>>>>>> Hi All,
> >> >>>>>>>
> >> >>>>>>> This patch enables cfq group hierarchical scheduling.
> >> >>>>>>>
> >> >>>>>>> With this patch, you can create a cgroup directory deeper than level 1.
> >> >>>>>>> Now, I/O Bandwidth is distributed in a hierarchy way. For example:
> >> >>>>>>> We create cgroup directories as following(the number represents weight):
> >> >>>>>>>
> >> >>>>>>>             Root grp
> >> >>>>>>>            /       \
> >> >>>>>>>        grp_1(100) grp_2(400)
> >> >>>>>>>        /    \
> >> >>>>>>>   grp_3(200) grp_4(300)
> >> >>>>>>>
> >> >>>>>>> If grp_2 grp_3 and grp_4 are contending for I/O Bandwidth,
> >> >>>>>>> grp_2 will share 80% of total bandwidth.
> >> >>>>>>> For sub_groups, grp_3 shares 8%(20% * 40%), grp_4 shares 12%(20% * 60%)
> >> >>>>>>>
> >> >>>>>>> Design:
> >> >>>>>>>   o Each cfq group has its own group service tree.
> >> >>>>>>>   o Each cfq group contains a "group schedule entity" (gse) that
> >> >>>>>>>     schedules on parent cfq group's service tree.
> >> >>>>>>>   o Each cfq group contains a "queue schedule entity"(qse), it
> >> >>>>>>>     represents all cfqqs located on this cfq group. It schedules
> >> >>>>>>>     on this group's service tree. For the time being, root group
> >> >>>>>>>     qse's weight is 1000, and subgroup qse's weight is 500.
> >> >>>>>>>   o All gses and qse which belones to a same cfq group schedules
> >> >>>>>>>     on the same group service tree.
> >> >>>>>> Hi Gui,
> >> >>>>>>
> >> >>>>>> Thanks for the patch. I have few questions.
> >> >>>>>>
> >> >>>>>> - So how does the hierarchy look like, w.r.t root group. Something as
> >> >>>>>>   follows?
> >> >>>>>>
> >> >>>>>>
> >> >>>>>>                     root
> >> >>>>>>                    / | \
> >> >>>>>>                  q1  q2 G1
> >> >>>>>>
> >> >>>>>> Assume there are two processes doin IO in root group and q1 and q2 are
> >> >>>>>> cfqq queues for those processes and G1 is the cgroup created by user.
> >> >>>>>>
> >> >>>>>> If yes, then what algorithm do you use to do scheduling between q1, q2
> >> >>>>>> and G1? IOW, currently we have two algorithms operating in CFQ. One for
> >> >>>>>> cfqq and other for groups. Group algorithm does not use the logic of
> >> >>>>>> cfq_slice_offset().
> >> >>>>> Hi Vivek,
> >> >>>>>
> >> >>>>> This patch doesn't break the original sheduling logic. That is cfqg => st => cfqq.
> >> >>>>> If q1 and q2 in root group, I treat q1 and q2 bundle as a queue sched entity, and
> >> >>>>> it will schedule on root group service with G1, as following:
> >> >>>>>
> >> >>>>>                          root group
> >> >>>>>                         /         \
> >> >>>>>                     qse(q1,q2)    gse(G1)
> >> >>>>>
> >> >>>> Ok. That's interesting. That raises another question that how hierarchy
> >> >>>> should look like. IOW, how queue and groups should be treated in
> >> >>>> hierarchy.
> >> >>>>
> >> >>>> CFS cpu scheduler treats queues and group at the same level. That is as
> >> >>>> follows.
> >> >>>>
> >> >>>>                        root
> >> >>>>                        / | \
> >> >>>>                       q1 q2 G1
> >> >>>>
> >> >>>> In the past I had raised this question and Jens and corrado liked treating
> >> >>>> queues and group at same level.
> >> >>>>
> >> >>>> Logically, q1, q2 and G1 are all children of root, so it makes sense to
> >> >>>> treat them at same level and not group q1 and q2 in to a single entity and
> >> >>>> group.
> >> >>>>
> >> >>>> One of the possible way forward could be this.
> >> >>>>
> >> >>>> - Treat queue and group at same level (like CFS)
> >> >>>>
> >> >>>> - Get rid of cfq_slice_offset() logic. That means without idling on, there
> >> >>>>  will be no ioprio difference between cfq queues. I think anyway as of
> >> >>>>  today that logic helps in so little situations that I would not mind
> >> >>>>  getting rid of it. Just that Jens should agree to it.
> >> >>>>
> >> >>>> - With this new scheme, it will break the existing semantics of root group
> >> >>>>  being at same level as child groups. To avoid that, we can probably
> >> >>>>  implement two modes (flat and hierarchical), something similar to what
> >> >>>>  memory cgroup controller has done. May be one tunable in root cgroup of
> >> >>>>  blkio "use_hierarchy".  By default everything will be in flat mode and
> >> >>>>  if user wants hiearchical control, he needs to set user_hierarchy in
> >> >>>>  root group.
> >> >>> Vivek, may be I am reading you wrong here. But you are first
> >> >>> suggesting to add more complexity to treat queues and group at the
> >> >>> same level. Then you are suggesting add even more complexity to fix
> >> >>> the problems caused by that approach.
> >> >>>
> >> >>> Why do we need to treat queues and group at the same level? "CFS does
> >> >>> it" is not a good argument.
> >> >>
> >> >> Sure it is not a very good argument but at the same time one would need
> >> >> a very good argument that why we should do things differently.
> >> >>
> >> >> - If a user has mounted cpu and blkio controller together and both the
> >> >>   controllers are viewing the same hierarchy differently, then it is
> >> >>   odd. We need a good reason that why different arrangement makes sense.
> >> >
> >> > Hi Vivek，
> >> >
> >> > Even if we mount cpu and blkio together, to me, it's ok for cpu and blkio
> >> > having their own logic, since they are totally different cgroup subsystems.
> >> >
> >> >>
> >> >> - To me, both group and cfq queue are children of root group and it
> >> >>   makes sense to treat them independent childrens instead of putting
> >> >>   all the queues in one logical group which inherits the weight of
> >> >>   parent.
> >> >>
> >> >> - With this new scheme, I am finding it hard to visualize the hierachy.
> >> >>   How do you assign the weights to queue entities of a group. It is more
> >> >>   like a invisible group with-in group. We shall have to create new
> >> >>   tunable which can speicy the weight for this hidden group.
> >> >
> >> > For the time being, the root "qse" weight is 1000 and others is 500, they don't
> >> > inherit the weight of parent. I was thinking that maybe we can determine the qse
> >> > weight in term of the queue number and weight in this group and subgroups.
> >> >
> >> > Thanks,
> >> > Gui
> >> >
> >> >>
> >> >>
> >> >> So in summary I am liking the "queue at same level as group" scheme for
> >> >> two reasons.
> >> >>
> >> >> - It is more intutive to visualize and implement. It follows the true
> >> >>   hierarchy as seen by cgroup file system.
> >> >>
> >> >> - CFS has already implemented this scheme. So we need a strong arguemnt
> >> >>   to justify why we should not follow the same thing. Especially for
> >> >>   the case where user has co-mounted cpu and blkio controller.
> >> >>
> >> >> - It can achieve the same goal as "hidden group" proposal just by
> >> >>   creating a cgroup explicitly and moving all threads in that group.
> >> >>
> >> >> Why do you think that "hidden group" proposal is better than "treating
> >> >> queue at same level as group" ?
> >>
> >> There are multiple reasons for "hidden group" proposal being a better approach.
> >>
> >> - "Hidden group" would allow us to keep scheduling queues using the
> >> CFQ queue scheduling logic. And does not require any major changes in
> >> CFQ. Aren't we already using that approach to deal with queues at the
> >> root group?
> >
> > Currently we are operating in flat mode where all the groups are at
> > same level (irrespective their position in cgroup hiearchy).
> >
> >>
> >> - If queues and groups are treated at the same level, queues can end
> >> up in root cgroup. And we cannot put an upper bound on the number of
> >> those queues. Those queues can consume system resources in proportion
> >> to their number, causing the performance of groups to suffer. If we
> >> have "hidden group", we can configure it to a small weight, and that
> >> would limit the impact these queues in root group can have.
> >
> > To limit the impact of other queues in cgroup, one can use libcgroup to
> > automatically place new threads or tasks into a subgroup.
> >
> > I understand that kernel doing it by default should help though. It is
> > less work in terms of configuration. But I am not sure that's a good
> > argument to design kernel functionality. Kernel functionality should be
> > pretty generic.
> >
> > Anyway, how would you assign the weight to the hidden group. What's the
> > interface for that? A new cgroup file inside each cgroup? Personally
> > I think that's little odd interface. Every group has one hidden group
> > where all the queues in that group go and weight of that group can be
> > specified by a cgroup file.
> 
> I think picking a reasonable default weight at compile time is not
> that bad an option, given that threads showing up in the "hidden
> group" is an uncommon case.

I don't think that's a good idea. A user should be able to determine
what's the share of the queues in a group without looking at the kernel
config file.

Vivek
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