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

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Message-ID: <20080417084815.GA31835@atrey.karlin.mff.cuni.cz>
Date:	Thu, 17 Apr 2008 10:48:16 +0200
From:	Pavel Machek <pavel@....cz>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Paolo Valente <paolo.valente@...more.it>,
	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.

One seek is still 10msec on modern drive, right? So 4k seeks =
40seconds, no? 4seconds would correspond to 1msec per seek, which
seems low.

writes with O_SYNC could force full seek on each request, right?
									Pavel
-- 
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