Re: [PATCH] [RFC] Add support for uevents on block device idle

[Matthew Garrett <mjg59@srcf.ucam.org>]

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