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

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Message-ID: <20091123144238.GA10275@srcf.ucam.org>
Date:	Mon, 23 Nov 2009 14:42:38 +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 03:31:40PM +0100, Jens Axboe wrote:

> Well, take a look at laptop mode. A timer per-io is probably
> unavoidable, but doing it at IO completion could mean a big decrease in
> timer activity as opposed to doing it for each incoming IO. And since
> you are looking at when the disk is idle, it makes a lot more sense to
> me to do that when the we complete a request (and have no further
> pending IO) rather than on incoming IO.

Right. The current implementation I have does a del_timer() at 
submission (which should be moved to post-merge) - that should be cheap 
in the common case of a new command being submitted when there's already 
commands outstanding. There's then a mod_timer() at completion time. 
That's still a certain amount of expense, but it should be less.

> Your biggest performance issue here is going to be sync IO, since the
> disk will go idle very briefly before being kicked into action again.

Ok, I'll try to benchmark that.

The alternative (polling) method would be something much like Kay 
suggested - either add an extra field to stat or an extra sysfs file, 
then invalidate that on submission and set to jiffies on completion. 
It's not ideal from a wakeups perspective, but it's pretty low impact on 
the kernel side.

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