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

[Vivek Goyal]

On Fri, May 29, 2009 at 12:06:03PM -0700, Nauman Rafique wrote:
> On Fri, May 29, 2009 at 9:57 AM, Fabio Checconi <fchecconi at gmail.com> wrote:
> >> From: Vivek Goyal <vgoyal at 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.

It sounded complicated from the description of it. I would prefer either
we get rid of in-class preemtion thing completely or do in-class preemtption
at the cost of gaining share, like cfq does.

In fact, to begin with, I prefer to be as close as possible to CFQ and then
change things selectively one piece at a time so that we can analyze the
impact well.

Thanks
Vivek
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