[Devel] Re: [ckrm-tech] [RFC] [PATCH 0/3] Add group fairness to CFS

[Kirill Korotaev]

Srivatsa Vaddagiri wrote:
> On Fri, May 25, 2007 at 05:05:16PM +0400, Kirill Korotaev wrote:
> 
>>>That way the scheduler would first pick a "virtual CPU" to schedule, and 
>>>then pick a user from that virtual CPU, and then a task from the user. 
>>
>>don't you mean the vice versa:
>>first use to scheduler, then VCPU (which is essentially a runqueue or rbtree),
>>then a task from VCPU?
>>
>>this is the approach we use in OpenVZ [...]
> 
> 
> So is this how it looks in OpenVZ?
> 
> 	CONTAINER1 => VCPU0 + VCPU1 
> 	CONTAINER2 => VCPU2 + VCPU3
>
> PCPU0 picks a container first, a vcpu next and then a task in it
> PCPU1 also picks a container first, a vcpu next and then a task in it.

correct.

> Few questions:
> 
> 1. Are VCPU runqueues (on which tasks are present) global queues?
>   
>    That is, let's say that both PCPU0 and PCPU1 pick CONTAINER1 to schedule
>    (first level) at the same time and next (let's say) they pick same vcpu
>    VCPU0 to schedule (second level). Will the two pcpu's now have to be 
>    serialized for scanning task to schedule next (third level) within VCPU0 
>    using a spinlock?  Won't that shootup scheduling costs (esp on large
>    systems), compared to (local scheduling + balance across cpus once in a 
>    while, the way its done today)?
>    Or do you required that two pcpus don't schedule the same vcpu at the
>    same time (the way hypervisors normally work)? Even then I would
>    imagine a fair level of contention to be present in second step (pick
>    a virtual cpu from a container's list of vcpus).

2 physical CPUs can't select the same VCPU at the same time.
i.e. VCPU can be running on 1 PCPU only at the moment.
and vice versa: PCPU can run only 1 VCPU at the given moment.

So serialization is done when we need to assign VCPU to PCPU moment only,
not when we select a particular task from the runqueue.

About the contention: you can control how often VCPUs should be rescheduled,
so the contention can be quite small. This contention is unavoidable in any fair
scheduler since fairness implies across CPUs accounting and decision making at least
with some period of time.

Well it is possible to avoid contention at all - if we do fair scheduling
separately on each CPU. But in this case we still do user-based balancing
(which requires serialization) and precision can be nasty.

> 2. How would this load balance at virtual cpu level and sched domain based
>    load balancing interact?
> 
>    The current sched domain based balancing code has many HT/MC/SMT related 
>    optimizations, which ensure that tasks are spread across physical 
>    threads/cores/packages in a most efficient manner - so as to utilize 
>    hardware bandwidth to the maximum. You would now need to introduce
>    those optimizations essentially at schedule() time ..? Don't know
>    if that is a wise thing to do.

load balancing is done taking into account *current* VCPUs assignments to PCPUs.
i.e. sched domains are taken into account.
nothing is introduces at schedule() time - not sure what you meant actually by this.

> 3. How do you determine the number of VCPUs per container? Is there any
>    relation for number of virtual cpus exposed per user/container and
>    the number of available cpus? For ex: in case of user-driven
>    scheduling, we would want all users to see the same number of cpus
>    (which is the number available in the system).

by default every user is given num_online_cpus() VCPUs, i.e. it can
run on all physical CPUs at the same time. If needed a user can be limited.


> 4. VCPU ids (namespace) - is it different for different containers?

yes.

>    For ex: can id's of vcpus belonging to different containers (say VCPU0 and 
>    VCPU2), as seen by users thr' vgetcpu/smp_processor_id() that is, be same?

yes.

>    If so, then potentially two threads belonging to different users may find
>    that they are running -truly simultaneously- on /same/ cpu 0 (one on
>    VCPU0/PCPU0 and another on VCPU2/PCPU1) which normally isn't possible!

yes. but for user space this has no any implications. You see, there is no way for user space
to determine whether it is "-truly simultaneously- running on /same/ cpu 0".

>    This may be ok for containers, with non-overlapping cpu id namespace,
>    but when applied to group scheduling for, say, users, which require a
>    global cpu id namespace, wondering how that would be addressed ..

very simple imho.
the only way from user space to get some task CPU id is /proc.
All you need is to return *some* value there.
For example, one can report PCPU id to which VCPU is assigned.

>>and if you don't mind I would propose to go this way for fair-scheduling in 
>>mainstream.
>>It has it's own advantages and disatvantages.
>>
>>This is not the easy way to go and I can outline the problems/disadvantages
>>which appear on this way:
>>- tasks which bind to CPU mask will bind to virtual CPUs.
>>  no problem with user tasks, [...]
> 
> 
> Why is this not a problem for user tasks? Tasks which bind to different
> CPUs for performance reason now can find that they are running on same
> (physical) CPU unknowingly.

if there is no high load - tasks will be running on different PCPUs
as the author was planning, since VCPUs will get different PCPUs for sure.
Otherwise - it is not performance critical, and moreover *controversial* to *fairness*.

Let me provide an example why binding is controversial to fairness.
Imagine that we have 2 USERs - USER1 and USER2 and 2 CPUs in the system.
USER1 has 50 tasks binded to CPU0 and 50 tasks binded to CPU1.
USER2 has 1 task.

Let USER2 to be as important as USER1 is, so these USERs should
share summary CPU time as 1:1.
How will it work with your approach?

>>but some kernel threads
>>  use this to do CPU-related management (like cpufreq).
>>  This can be fixed using SMP IPI actually.
>>- VCPUs should no change PCPUs very frequently,
>>  otherwise there is some overhead. Solvable.
>>
>>Advantages:
>>- High precision and fairness.
> 
> 
> I just don't know if this benefit of high degree of fairness is worth the 
> complexity it introduces. Besides having some data which shows how much better 
> is is with respect to fairness/overhead when compared with other approaches 
> (like smpnice) would help I guess. I will however let experts like Ingo make 
> the final call here  :)

sure. The "perfect" solution doesn't exist :( So I would be happy to know Ingo
opinion as well.

Thanks,
Kirill

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