Re: [PATCH 0/3] cfq-iosched: Fair cross-group preemption

[Chad Talbott <ctalbott@google.com>]

Would it be a better approach to avoid calling the feature real-time?
Or perhaps to use another service tree to allow strict preemption (as
is done at the task level today)?

I don't like the approach of tuning tiny slices and taking a
throughput hit all the time - even if there is no latency sensitive
group on the system.

Chad

On Tue, Mar 22, 2011 at 11:12 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Tue, Mar 22, 2011 at 10:39:36AM -0700, Chad Talbott wrote:
>> On Tue, Mar 22, 2011 at 8:09 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
>> > Why not just implement simply RT class groups and always allow an RT
>> > group to preempt an BE class. Same thing we do for cfq queues. I will
>> > not worry too much about a run away application consuming all the
>> > bandwidth. If that's a concern we could use blkio controller to limit
>> > the IO rate of a latency sensitive applicaiton to make sure it does
>> > not starve BE applications.
>>
>> That is not quite the same semantics.  This limited preemption patch
>> is still work-conserving.  If the RT task in the only task on the
>> system with IO, it will be able to use all available disk time.
>>
>
> It is not same semantics but it feels like too much of special casing
> for a single use case.
>
> You are using the generic notion of a RT thread (which in general means
> that it gets all the cpu or all the disk ahead of BE task). But you have
> changed the definition of RT for this special use case. And also now
> group RT is different from queue RT definition.
>
> Why not have similar mechanism for cpu scheduler also then. This
> application first should be able to get cpu bandwidth in same predictable
> manner before it gets the disk bandwidth.
>
> And I think your generation number patch should address this issue up
> to great extent. Isn't it? If a latency sensitive task is not using
> its fair quota, it will get a lower vdisktime and get to dispatch soon?
>
> If that soon is not enough, then we could operate with reduce base slice
> length so that we allocate smaller slices to groups and get better IO
> latencies at the cost of total throughput.
>
>> > If RT starving BE is an issue, then it is an issue with plain cfq queue
>> > also. First we shall have to fix it there.
>> >
>> > This definition that a latency sensitive task get prioritized only
>> > till it is consuming its fair share and if task starts using more than
>> > fair share then CFQ automatically stops prioritizing it sounds little
>> > odd to me. If you are looking for predictability, then we lost it. We
>> > shall have to very well know that task is not eating more than its
>> > fair share before we can gurantee any kind of latencies to that task. And
>> > if we know that task is not hogging the disk, there is anyway no risk
>> > of it starving other groups/tasks completely.
>>
>> In a shared environment, we have to be a little bit defensive.  We
>> hope that a latency sensitive task is well characterized and won't
>> exceed its share of the disk, and that we haven't over-committed the
>> disk.  If the app does do more IO than expected, then we'd like them
>> to bear the burden.  We have a choice of two outcomes.  A single job
>> sometimes failing to achieve low disk latency when it's very busy.  Or
>> all jobs on a disk sometimes being very slow when another (unrelated)
>> job is very busy.  The first is easier to understand and debug.
>
> To me you are trying to come up with a new scheduling class which is
> not RT and you are trying to overload the meaning of RT for your use
> case and that's the issue I have.
>
> Coming up with a new scheduling class is also not desirable as that
> will demand another service tree and we already have too many. Also
> it should probably be also done for task and not just group otherwise
> extending this concept to hierarchical setup will get complicated. Queues
> and groups will just not gel well.
>
> Frankly speaking, the problem you are having should be solved by your
> generation number patch and by having smaller base slices.
>
> Or You could put latency sensitive applications in an RT class and
> then throttle them using blkio controller. That way you get good
> latencies as well as you don't starve other tasks.
>
> But I don't think overloading the meaning for RT or this specific use
> case is a good idea.
>
> Thanks
> Vivek
>