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

[Fabio Checconi]

> From: Vivek Goyal <vgoyal at redhat.com>
> Date: Tue, Nov 18, 2008 09:07:51AM -0500
>
> On Tue, Nov 18, 2008 at 01:05:08PM +0100, Fabio Checconi wrote:
...
> > 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.
> > 
> 
> IIUC, bfq and cfq are different in following manner.
> 
> a. BFQ employs WF2Q+ for fairness and CFQ employes weighted round robin.
> b. BFQ uses the budget (sector count) as notion of service and CFQ uses
>    time slices.
> c. BFQ supports hierarchical fair queuing and CFQ does not.  
> 
> We are looking forward for implementation of point C. Fabio seems to
> thinking of supporting time slice as a service (B). It seems like
> convergence of CFQ and BFQ except the point A (WF2Q+ vs weighted round
> robin). 
> 
> It looks like WF2Q+ provides tighter service bound and bfq guys mention
> that they have been able to ensure throughput while ensuring tighter 
> bounds. If that's the case, does that mean BFQ is a replacement for CFQ
> down the line?
>   

BFQ started from CFQ, extending it in the way you correctly describe,
so it is indeed very similar.  There are also some minor changes to
locking, cic handling, hw_tag detection and to the CIC_SEEKY heuristic.

The two schedulers share similar goals, and in my opinion BFQ can be
considered, in the long term, a CFQ replacement; *but* before talking
about replacing CFQ we have to consider that:

  - it *needs* review and testing; we've done our best, but for sure
    it's not enough; review and testing are never enough;
  - the service domain fairness, which was one of our objectives, requires
    some extra complexity; the mechanisms we used and the design choices
    we've made may not fit all the needs, or may not be as generic as the
    simpler CFQ's ones;
  - CFQ has years of history behind and has been tuned for a wider
    variety of environments than the ones we've been able to test.

If time-based fairness is considered more robust and the loss of
service-domain fairness is not a problem, then the two schedulers can
be made even more similar.