Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Fabio Checconi <fchecconi@gmail.com>]

> From: Nauman Rafique <nauman@google.com>
> Date: Tue, Nov 18, 2008 02:33:19PM -0800
>
> On Tue, Nov 18, 2008 at 4:05 AM, Fabio Checconi <fchecconi@gmail.com> wrote:
...
> >  it should be possible without altering the code.  The slices can be
> > assigned in the time domain using big values for max_budget.  The logic
> > is: each process is assigned a budget (in the range [max_budget/2, max_budget],
> > choosen from the feedback mechanism, driven in __bfq_bfqq_recalc_budget()),
> > and if it does not complete it in timeout_sync milliseconds, it is
> > charged a fixed amount of sectors of service.
> >
> > Using big values for max_budget (where big means greater than two
> > times the number of sectors the hard drive can transfer in timeout_sync
> > milliseconds) makes the budgets always to time out, so the disk time
> > is scheduled in slices of timeout_sync.
> >
> > However this is just a temporary workaround to do some basic testing.
> >
> > Modifying the scheduler to support time slices instead of sector
> > budgets would indeed simplify the code; I think that the drawback
> > would be being too unfair in the service domain.  Of course we
> > have to consider how much is important to be fair in the service
> > domain, and how much added complexity/new code can we accept for it.
> >
> > [ Better service domain fairness is one of the main reasons why
> >  we started working on bfq, so, talking for me and Paolo it _is_
> >  important :) ]
> >
> > I have to think a little bit on how it would be possible to support
> > an option for time-only budgets, coexisting with the current behavior,
> > but I think it can be done.
> 
> I think "time only budget" vs "sector budget" is dependent on the
> definition of fairness: do you want to be fair in the time that is
> given to each cgroup or fair in total number of sectors transferred.
> And the appropriate definition of fairness depends on how/where the IO
> scheduler is used.  Do you think the work-around that you mentioned
> would have a significant performance difference compared to direct
> built-in support?
> 

In terms of throughput, it should not have any influence, since tasks
would always receive a full timeslice.  In terms of latency it would
bypass completely the feedback mechanism, and that would have a
negative impact (basically the scheduler would not be able to
differentiate between tasks with the same weight but with different
interactivity needs).

In terms of service fairness it is a little bit hard to say, but I
would not expect anything near to what can be done with a service domain
approach, independently from the scheduler used.