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

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Message-ID: <20091123141754.GE8742@kernel.dk>
Date:	Mon, 23 Nov 2009 15:17:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	Kay Sievers <kay.sievers@...y.org>,
	David Zeuthen <david@...ar.dk>, linux-kernel@...r.kernel.org,
	linux-hotplug@...r.kernel.org
Subject: Re: [PATCH] [RFC] Add support for uevents on block device idle
	changes

On Mon, Nov 23 2009, Pavel Machek wrote:
> On Wed 2009-11-18 20:07:12, Matthew Garrett wrote:
> > On Wed, Nov 18, 2009 at 09:03:21PM +0100, Kay Sievers wrote:
> > 
> > > Sure, but what's wrong with reading that file every 50 seconds? Almost
> > > all boxes poll for media changes of optical drives and usb card
> > > readers anyway, so it's not that we are not doing stuff like this
> > > already.
> > 
> > We poll for media because there's no event-based way of avoiding it - in 
> > this case there is.
> 
> ...when you add overhead to every disk operation. I'd say that polling
> once in 50 seconds is preferable to that.

I have to agree, doing a mod_timer() on every single IO is going to suck
big time. I went to great lengths to avoid doing that even for timeout
detection. So that's pretty much a non-starter to begin with.

Additionally, as Bart also wrote, you are not doing this in the right
place. You want to do this post-merge, not for each incoming IO. Have
you looked at laptop mode? Looks like you are essentially re-inventing
that, but in a bad way.

-- 
Jens Axboe

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