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Date: Mon, 23 Nov 2009 14:42:38 +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
Message-ID: <20091123144238.GA10275@srcf.ucam.org>
References: <ac3eb2510911181130l6f25642cy1d5332f79a112cf9@mail.gmail.com> <20091118194053.GB12944@srcf.ucam.org> <ac3eb2510911181147ufeacb58u5194b0e1e5fd86d6@mail.gmail.com> <20091118195342.GA13627@srcf.ucam.org> <ac3eb2510911181203r1be5c3dajdacec94f196d550c@mail.gmail.com> <20091118200712.GA14026@srcf.ucam.org> <20091122233749.GA9699@ucw.cz> <20091123141754.GE8742@kernel.dk> <20091123142557.GA10084@srcf.ucam.org> <20091123143140.GG8742@kernel.dk>
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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@srcf.ucam.org
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