linux-kernel - Re: [RFC, PATCH] cfq-iosched: remove redundant queuing detection code

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Message-ID: <x49fx8m5q7w.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 10 Nov 2009 10:55:47 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	aaronc@...ato.unsw.edu.au
Subject: Re: [RFC, PATCH] cfq-iosched: remove redundant queuing detection code

Jens Axboe <jens.axboe@...cle.com> writes:

>> Why don't we simply use the value configured via the queue_depth sysfs
>> file?
>
> First of all, that only covers SCSI. We could do that by having the tag
> on/off functions set the same flag. But even for such devices, actual
> tag depth is dependent upon what other devices are on the controller
> (since it's often a shared map) and may not even be statically
> detectable in the sense that actual depth is only really seen when the
> device returns busy on a queue attempt.
>
> In most cases it would work fine, but the dynamic detection is more
> reliable. The sysfs setting in reality is max setting.

OK, thanks for the patient explanation, Jens!

Cheers,
Jeff
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