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

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Message-ID: <20080417092751.GX12774@kernel.dk>
Date:	Thu, 17 Apr 2008 11:27:55 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Paolo Valente <paolo.valente@...more.it>
Cc:	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND][RFC] BFQ I/O Scheduler

On Thu, Apr 17 2008, Paolo Valente wrote:
> Jens Axboe ha scritto:
> >
> >
> >I was thinking about that too. Generally I've been opposed to doing
> >scheduling decisions on anything but time, since that is always
> >relevant. When to hand out slices and to what process, that algorithm is
> >really basic in CFQ and could do with an improvement.
> >
> >  
> 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.

-- 
Jens Axboe

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