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

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Message-ID: <20091123142557.GA10084@srcf.ucam.org>
Date:	Mon, 23 Nov 2009 14:25:57 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Pavel Machek <pavel@....cz>, 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 at 03:17:54PM +0100, Jens Axboe wrote:

> 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.

It's conditional on a (default off) setting, so it's not a hit unless 
the user requests it. But yeah, the performance hit is obviously a 
concern. It may be that polling is the least bad way of doing this.

> 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.

Right, that's mostly down to my having no familiarity with the block 
layer at all :) I can fix that up easily enough, but if a deferrable 
timer is going to be too expensive then it'll need some rethinking 
anyway.

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