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

[Aaron Carroll <aaronc@cse.unsw.edu.au>]

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.