On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> Here's an attempt to extend CFS (v13) to be fair at a group level, rather than
> just at task level. The patch is in a very premature state (passes
> simple tests, smp load balance not supported yet) at this point. I am sending 
> it out early to know if this is a good direction to proceed.

Well, SMP load balancing is what makes all this hard. sched_yield()
semantics are yet another twist.


On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> Salient points which needs discussion:
> 1. This patch reuses CFS core to achieve fairness at group level also.
>    To make this possible, CFS core has been abstracted to deal with generic 
>    schedulable "entities" (tasks, users etc).

The ability to handle deeper hierarchies would be useful for those
who want such semantics.


On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> 2. The per-cpu rb-tree has been split to be per-group per-cpu.
>    schedule() now becomes two step on every cpu : pick a group first (from
>    group rb-tree) and a task within that group next (from that group's task
>    rb-tree)

That assumes per-user scheduling groups; other configurations would
make it one step for each level of hierarchy. It may be possible to
reduce those steps to only state transitions that change weightings
and incremental updates of task weightings. By and large, one needs
the groups to determine task weightings as opposed to hierarchically
scheduling, so there are alternative ways of going about this, ones
that would even make load balancing easier.


On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> 3. Grouping mechanism - I have used 'uid' as the basis of grouping for
>    timebeing (since that grouping concept is already in mainline today).
>    The patch can be adapted to a more generic process grouping mechanism
>    (like http://lkml.org/lkml/2007/4/27/146) later.

I'd like to see how desirable the semantics achieved by reflecting
more of the process hierarchy structure in the scheduler groupings are.
Users, sessions, pgrps, and thread_groups would be the levels of
hierarchy there, where some handling of orphan pgrps is needed.


On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> Some results below, obtained on a 4way (with HT) Intel Xeon box. All 
> number are reflective of single CPU performance (tests were forced to 
> run on single cpu since load balance is not yet supported).
> 			         uid "vatsa"	           uid "guest"
> 		             (make -s -j4 bzImage)    (make -s -j20 bzImage)
> 
> 2.6.22-rc1		          772.02 sec		497.42 sec (real)
> 2.6.22-rc1+cfs-v13 	          780.62 sec		478.35 sec (real)
> 2.6.22-rc1+cfs-v13+this patch     776.36 sec		776.68 sec (real)
> [ An exclusive cpuset containing only one CPU was created and the
> compilation jobs of both users were run simultaneously in this cpuset ]
> I also disabled CONFIG_FAIR_USER_SCHED and compared the results with
> cfs-v13:
> 					uid "vatsa"
> 					make -s -j4 bzImage
> 
> 2.6.22-rc1+cfs-v13			395.57 sec (real)
> 2.6.22-rc1+cfs-v13+this_patch		388.54 sec (real)
> There is no regression I can see (rather some improvement, which I can't
> understand atm). I will run more tests later to check this regression aspect.
> Request your comments on the future direction to proceed!

Kernel compiles are markedly poor benchmarks. Try lat_ctx from lmbench,
VolanoMark, AIM7, OAST, SDET, and so on.


-- wli
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* William Lee Irwin III <wli@holomorphy.com> wrote:

> [...] sched_yield() semantics are yet another twist.

that's nonsense, sched_yield() semantics are totally uninteresting. It 
is a fundamentally broken interface.

	Ingo
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On Wed, May 23, 2007 at 11:03:16AM -0700, William Lee Irwin III wrote:
> Well, SMP load balancing is what makes all this hard.

Agreed. I am optimistic that we can achieve good degree of SMP
fairness using similar mechanism as smpnice ..

> On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> > Salient points which needs discussion:
> > 1. This patch reuses CFS core to achieve fairness at group level also.
> >    To make this possible, CFS core has been abstracted to deal with generic 
> >    schedulable "entities" (tasks, users etc).
> 
> The ability to handle deeper hierarchies would be useful for those
> who want such semantics.

sure, although the more levels of hierarchy scheduler recoginizes, more
the (accounting/scheduling) cost is!

> On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> > 2. The per-cpu rb-tree has been split to be per-group per-cpu.
> >    schedule() now becomes two step on every cpu : pick a group first (from
> >    group rb-tree) and a task within that group next (from that group's task
> >    rb-tree)
> 
> That assumes per-user scheduling groups; other configurations would
> make it one step for each level of hierarchy. It may be possible to
> reduce those steps to only state transitions that change weightings
> and incremental updates of task weightings. By and large, one needs
> the groups to determine task weightings as opposed to hierarchically
> scheduling, so there are alternative ways of going about this, ones
> that would even make load balancing easier.

Yeah I agree that providing hierarchical group-fairness at the cost of single 
(or fewer) scheduling levels would be a nice thing to target for,
although I don't know of any good way to do it. Do you have any ideas
here? Doing group fairness in a single level, using a common rb-tree for tasks 
from all groups is very difficult IMHO. We need atleast two levels.

One possibility is that we recognize deeper hierarchies only in user-space,
but flatten this view from kernel perspective i.e some user space tool
will have to distributed the weights accordingly in this flattened view
to the kernel.

> On Wed, May 23, 2007 at 10:18:59PM +0530, Srivatsa Vaddagiri wrote:
> > 3. Grouping mechanism - I have used 'uid' as the basis of grouping for
> >    timebeing (since that grouping concept is already in mainline today).
> >    The patch can be adapted to a more generic process grouping mechanism
> >    (like http://lkml.org/lkml/2007/4/27/146) later.
> 
> I'd like to see how desirable the semantics achieved by reflecting
> more of the process hierarchy structure in the scheduler groupings are.
> Users, sessions, pgrps, and thread_groups would be the levels of
> hierarchy there, where some handling of orphan pgrps is needed.

Good point. Essentially all users should get fair cpu first, then all
sessions/pgrps under a user should get fair share, followed by
process-groups under a session, followed by processes in a
process-group, followed by threads in a process (phew) .. ?

The container patches by Paul Menage at http://lkml.org/lkml/2007/4/27/146
provide a generic enough mechanism to group tasks in a hierarchical
manner for each resource controller. For ex: for the cpu controller, if the 
desired fairness is as per the above scheme (user/session/pgrp/threads etc), 
then it is possible to write a script which creates such a tree under cpu
controller filesystem:

	# mkdir /dev/cpuctl
	# mount -t container -o cpuctl none /dev/cpuctl

/dev/cpuctl is the cpu controller filesystem which can look like this:

	/dev/cpuctl
		|----uid root
		|       |-- sid 10
		|	|      |------ pgrp 20
		| 	|      | 	  |-- process 100
		|	|      |          |-- process 101
		|	|      |          |
		|       |
		|	|-- sid 11
		|
		|--- uid guest

(If the cpu controller really supports those many levels that is!)

user scripts can be written to modify this filesystem tree upon every 
login/session/user creation (if that is possible to trap on). Essentially it 
lets this semantics (what you ask) be dynamic/tunable by user.

> Kernel compiles are markedly poor benchmarks. Try lat_ctx from lmbench,
> VolanoMark, AIM7, OAST, SDET, and so on.

Thanks for this list of tests. I intend to run all of them if possible
for my next version.

-- 
Regards,
vatsa
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