Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer

[Nauman Rafique <nauman@google.com>]

On Fri, May 29, 2009 at 9:57 AM, Fabio Checconi <fchecconi@gmail.com> wrote:
>> From: Vivek Goyal <vgoyal@redhat.com>
>> Date: Fri, May 29, 2009 12:06:10PM -0400
>>
>> On Thu, May 28, 2009 at 12:41:27PM -0700, Nauman Rafique wrote:
> ...
>> > I have some concerns about the new preemption logic.
>>
>> Actually we need a more proper definition of in-class preemption. Across
>> class preemption means that RT class always gets to run first.
>>
>> What does in-class preemption mean? If I look at the current CFQ code,
>> it does look like that preempting process will gain share. It is always
>> added to the front of the tree with "rb_key=0" and that means, this new
>> queue will get fresh time slice (even if it got time slice very recently).
>>
>> Currently I have just tried to make the behavior same as CFQ to reduce
>> the possiblility of regressions. That's a different thing that we can
>> discuss what should be the exact behavior in case of in-class preemption
>> and first it needs to be fixed in CFQ, if current behavior is an issue.
>>
>> On the other hand, I am not sure if previous bfq preemption logic was
>> working. We were checking if the new request belonged to the queue which
>> will be served next, then preempt the existing queue. While looking
>> for the next queue, I think we did not consider the current active
>> entity (as it was not on the tree). So after expiry of the current
>> queue, it might get selected next if it has not got its share. So there
>> was no point in preempting the queue. If queue already got its share, then
>> anyway the next queue will be selected next and there is no point in
>> preempting the current queue.
>>
>
> BFQ had no preemption logic, as far as I know; it simply was not
> preemptive, and the guarantees it provided took that into account.
>
> I don't know what is the best way to introduce a CFQ-like preemption logic
> into the wf2q+ code; for sure anything that does not schedule according
> to the algorithm's timestamps is a good candidate to break the guarantees
> the scheduler can provide, making it an extremely complex way to get
> the same worst-case delays of a (much simpler) round-robin scheduler.
>

What you guys think of my suggestion of handling preemption?
Basically, we don't modify the start/finish tags, so overall the
fairness properties should not be broken. But in short term, we still
allow preemption and let one queue jump another.