linux-kernel - Re: [PATCH] [RFC] Add support for uevents on block device idle changes

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Message-ID: <20091118195342.GA13627@srcf.ucam.org>
Date:	Wed, 18 Nov 2009 19:53:42 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	David Zeuthen <david@...ar.dk>, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, linux-hotplug@...r.kernel.org
Subject: Re: [PATCH] [RFC] Add support for uevents on block device idle
	changes

On Wed, Nov 18, 2009 at 08:47:37PM +0100, Kay Sievers wrote:
> On Wed, Nov 18, 2009 at 20:40, Matthew Garrett <mjg@...hat.com> wrote:
> > On Wed, Nov 18, 2009 at 08:30:07PM +0100, Kay Sievers wrote:
> >
> >> Wouldn't it be good enough, if we add a file "idle_since" which
> >> contains the time of the actual disk idle time, and userspace can
> >> schedule a re-examination of that value at the actual end of the idle
> >> time it is looking for?
> >
> > That would require either polling or waking up a userspace application
> > on every disk access. Doing it in-kernel involves only a single timer
> > wakeup for every active/idle transition.
> 
> How would it? If you look for, like a 60 seconds timeout, and the file
> contains 20, you schedule a wakeup in 40 seconds. If the file after
> the 40 seconds contains 60, you reached your idle timeout exactly at
> that moment, if it's less, then you re-calculate and start from the
> beginning.

How is that not polling? You'll repeatedly read a file looking for a 
value that may never appear - imagine the case where you're waiting for 
60 seconds of idleness, but the disk always becomes active again after 
50.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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