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

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Message-ID: <20091119141634.GA311@srcf.ucam.org>
Date:	Thu, 19 Nov 2009 14:16:34 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	David Zeuthen <david@...ar.dk>, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, linux-hotplug@...r.kernel.org
Subject: Re: [PATCH] [RFC] Add support for uevents on block device idle
	changes

On Thu, Nov 19, 2009 at 02:29:29PM +0100, Kay Sievers wrote:
> On Thu, Nov 19, 2009 at 14:01, Matthew Garrett <mjg@...hat.com> wrote:
> > On Thu, Nov 19, 2009 at 12:09:30PM +0100, Kay Sievers wrote:
> >> On Wed, Nov 18, 2009 at 22:33, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> >> > My use cases are on the order of a second.
> >>
> >> Ok, what's the specific use case, which should be triggered after a
> >> second? I thought you were thinking about disk spindown or similar.
> >
> > The first is altering ALPM policy. ALPM will be initiated by the host if
> > the number of queued requests hits zero - if there's no hysteresis
> > implemented, then that can result in a significant performance hit. We
> > don't need /much/ hysteresis, but it's the difference between a 50%
> > performance hit and not having that.
> 
> Can't that logic live entirely in the kernel, instead of being a
> rather generic userspace event interface (with the current limitation
> to a single user)?

It could, but it seems a bit of a hack. It'd still also require the 
timer to be in the kernel, so we might as well expose that to userspace.

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