Re: [RFC] CFQ group scheduling structure organization

[Munehiro Ikeda <m-ikeda@ds.jp.nec.com>]

Hello,

Corrado Zoccolo wrote, on 12/17/2009 06:41 AM:
> Hi,
> On Wed, Dec 16, 2009 at 11:52 PM, Vivek Goyal<vgoyal@redhat.com>  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.
>>
>> [snip]
>>
>> 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.

I agree it might be dynamic but I don't think it's un-intuitive.
I think it's reasonable that disk share of a group is
influenced by the number of tasks running in root group,
because the root group is shared by the tasks and groups from
the viewpoint of cgroup I/F, and they really share disk bandwidth.


>>   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.
>>
> This is the one I prefer: it is the most natural one if you see that
> groups are scheduling entities like any other task.
> I think it becomes intuitive with an analogy with a qemu (e.g. kvm)
> virtual machine model. If you think a group like a virtual machine, it
> is clear that for the normal system, the whole virtual machine is a
> single scheduling entity, and that it has to compete with other
> virtual machines (as other single entities) and every process in the
> real system (those are inherently more important, since without the
> real system, the VMs cannot simply exist).
> Having a designated root group, instead, resembles the xen VM model,
> where you have a separated domain for each VM and for the real system.
>
> I think the implementation of this approach can make the code simpler
> and modular (CFQ could be abstracted to deal with scheduling entities,
> and each scheduling entity could be defined in a separate file).
> Within each group, you will now have the choice of how to schedule its
> queues. This means that you could possibly have different I/O
> schedulers within each group, and even have sub-groups within groups.

Corrado exactly says my preference.

I understand current implementation, like proposal 1, was
employed to make code simple and I believe it succeeded.
However, rather I feel it's un-intuitive because it's
inconsistent with cgroup I/F.  Behavior which is inconsistent
with the I/F can lead to misconfiguration of sys-admins.
This might be problematic, IMHO.



Thanks,
Muuhh

-- 
IKEDA, Munehiro
   NEC Corporation of America
     m-ikeda@ds.jp.nec.com