linux-kernel - Re: [RESEND][RFC] BFQ I/O Scheduler

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Message-ID: <48072421.60307@cse.unsw.edu.au>
Date:	Thu, 17 Apr 2008 20:19:13 +1000
From:	Aaron Carroll <aaronc@....unsw.edu.au>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Paolo Valente <paolo.valente@...more.it>,
	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND][RFC] BFQ I/O Scheduler

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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