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SubjectRe: [RFC] CFQ group scheduling structure organization
On Wed, Dec 16, 2009 at 2:52 PM, Vivek Goyal <> wrote:
> Hi All,
> With some basic group scheduling support in CFQ, there are few questions
> regarding how group structure should look like in CFQ.
> Currently, grouping looks as follows. A, and B are two cgroups created by
> user.
> Proposal 1:
> =========
>                        grp-service-tree
>                        /      |     \
>                    root       A     B
> One issue with this structure is that RT tasks are not system wide. So an
> RT tasks inside root group has RT priority only with-in root group. So a
> BE task inside A will get it fair share despite the fact that root has got
> RT tasks.
> Proposal 2:
> ==========
> One proposal to solve this issue is that make RT and IDLE tasks system
> wide and provide weight based service differentiation only for BE class
> tasks. So RT or IDLE tasks running in any of the groups will automatically
> move to one global RT group maintained by CFQ internally. Same is true for
> IDLE tasks. But BE class tasks will honor the cgroup limitations and will
> get differentiated service according to weight.
> Internal structure will look as follows.
>     grp-RT-service-tree  grp-BE-service-tree   grp-IDLE-service-tree
>             |                /  \                      |
>        all_RT_task_group    A   B               all_idle_tasks_grp
> Here A and B are two cgroups and some BE tasks might be running inside
> those groups. systemwide RT tasks will move under all_RT_task_group and
> all idle tasks will move under all_idle_tasks_grp.
> So one will notice service differentiation only for BE tasks.

What is the use case for this kind of setup? Do we need RT tasks
system wide that could basically starve out all the cgroups present in
the system?

> Proposal 3:
> ===========
> One can argue that we need group service differentiation for RT class
> tasks also and don't move tasks automatically across groups. That means
> we need to support "group class" type also. Probably we can support
> three classes of cgroups RT, BE and IDLE and CFQ will use that data to
> put cgroups in respective tree.
> Things should look as follows.
>     grp-RT-service-tree  grp-BE-service-tree   grp-IDLE-service-tree
>             / \                      /  \             /   \
>            C  D                     A   B            E    F
> Here A and B are BE type groups created by user.
> C and D are RT type cgroups created by user.
> E and F are IDLE type cgroups created by user.
> Now in this scheme of things, by default root will be of type BE. Any task
> RT task under "root" group will not be system wide RT task. It will be RT
> only with-in root group. To make it system wide idle, admin shall have to
> create a new cgroup, say C, of type RT and move task in that cgroup.
> Because RT group C is system wide, now that task becomes system wide RT.
> So this scheme might throw some surprise to existing users. They might
> create a new group and not realize that their RT tasks are no more system
> wide RT tasks and they need to specifically create one RT cgroup and move
> all RT tasks in that cgroup.
> Practically I am not sure how many people are looking for group service
> differentiation for RT and IDLE class tasks also.

IMHO, this is the setup that offers the most flexibility. With this,
we could set up a cgroup which has RT tasks internal to it, but it
still obeys the proportions imposed by system. And if one wanted a
real RT behavior, a cgroup with RT class can be used.

> Proposal 4:
> ==========
> Treat task and group at same level. Currently groups are at top level and
> at second level are tasks. View the whole hierarchy as follows.
>                        service-tree
>                        /   |  \  \
>                       T1   T2  G1 G2
> Here T1 and T2 are two tasks in root group and G1 and G2 are two cgroups
> created under root.
> In this kind of scheme, any RT task in root group will still be system
> wide RT even if we create groups G1 and G2.
> So what are the issues?
> - I talked to few folks and everybody found this scheme not so intutive.
>  Their argument was that once I create a cgroup, say A,  under root, then
>  bandwidth should be divided between "root" and "A" proportionate to
>  the weight.
>  It is not very intutive that group is competing with all the tasks
>  running in root group. And disk share of newly created group will change
>  if more tasks fork in root group. So it is highly dynamic and not
>  static hence un-intutive.
>  To emulate the behavior of previous proposals, root shall have to create
>  a new group and move all root tasks there. But admin shall have to still
>  keep RT tasks in root group so that they still remain system-wide.
>                        service-tree
>                        /   |    \  \
>                       T1  root  G1 G2
>                            |
>                            T2
>  Now admin has specifically created a group "root" along side G1 and G2
>  and moved T2 under root. T1 is still left in top level group as it might
>  be an RT task and we want it to remain RT task systemwide.
>  So to some people this scheme is un-intutive and requires more work in
>  user space to achive desired behavior. I am kind of 50:50 between two
>  kind of arrangements.
> I am looking for some feedback on what makes most sense.
> For the time being, I am little inclined towards proposal 2 and I have
> implemented a proof of concept version on top of for-2.6.33 branch in block
> tree.  These patches are compile and boot tested only and I have yet to do
> testing.
> Thanks
> Vivek
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