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

[Chad Talbott]

On Tue, Mar 22, 2011 at 8:09 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> 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.

> 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.

Chad
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