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Date: Mon, 23 Nov 2009 14:25:57 +0000
From: Matthew Garrett <mjg59@srcf.ucam.org>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: Pavel Machek <pavel@ucw.cz>, Kay Sievers <kay.sievers@vrfy.org>,
       David Zeuthen <david@fubar.dk>, linux-kernel@vger.kernel.org,
       linux-hotplug@vger.kernel.org
Subject: Re: [PATCH] [RFC] Add support for uevents on block device idle
	changes
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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@srcf.ucam.org
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