Re: [RESEND][RFC] BFQ I/O Scheduler

[Aaron Carroll]

Jens Axboe wrote:
> > Maybe there is also another middle-ground solution. I'll try to sketch 
> > it out:
> > . use sectors instead of time
> > . impose a penalty to each thread in proportion to the distance between 
> > its disk requests
> > . reduce the maximum budget of each thread as a function of this seek 
> > penalty so as to prevent the thread from stealing more than a given time 
> > slice (the simple mechanism to limit per-thread budget is already 
> > implemented in bfq).
> >
> > By doing so, both fairness and time isolation should be guaranteed.
> > Finally, this policy should be safe in that, given the maximum time used 
> > by a seeky thread to consume its maximum budget on a reference disk, the 
> > time used on any faster disk should be shorter.
> >
> > Does it seem reasonable?
>
> Not for CFQ, that will stay time based. The problem with #2 above is
> that it then quickly turns to various heuristics, which is just
> impossible to tune for general behaviour. Or it just falls apart for
> other real life situations.

Like SSD or hardware RAID.  Time-slices have the nice property of fairness
irrespective of the underlying hardware characteristics.
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