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

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Message-ID: <20080417071012.GP12774@kernel.dk>
Date:	Thu, 17 Apr 2008 09:10:12 +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:
> Pavel Machek ha scritto:
> >
> >>In the first type of tests, to achieve a higher throughput than CFQ
> >>(with the default 100 ms time slice), the maximum budget for BFQ
> >>had to be set to at least 4k sectors.  Using the same value for the
> >>    
> >
> >Hmm, 4k sectors is ~40 seconds worst case, no? That's quite long...
> >  
> Actually, in the worst case among our tests, the aggregate throughput 
> with 4k sectors was ~ 20 MB/s, hence the time for 4k sectors ~ 4k * 512 
> / 20M = 100 ms.

That's not worse case, it is pretty close to BEST case. Worst case is 4k
of sectors, with each being a 512b IO and causing a full stroke seek.
For that type of workload, even a modern sata hard drive will be doing
500kb/sec or less. That's rougly a thousand sectors per seconds, so ~4
seconds worst case for 4k sectors.

-- 
Jens Axboe

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