linux-kernel - Re: [PATCH, v10 3/3] cgroups: introduce timer slack controller

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Message-ID: <20111017145952.GB15769@srcf.ucam.org>
Date:	Mon, 17 Oct 2011 15:59:52 +0100
From:	Matthew Garrett <mjg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Lennart Poettering <mzxreary@...inter.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	jacob.jun.pan@...ux.intel.com, linux-kernel@...r.kernel.org,
	Matt Helsley <matthltc@...ibm.com>, linux-api@...r.kernel.org,
	Kay Sievers <kay.sievers@...y.org>, harald@...hat.com,
	david@...ar.dk, greg@...ah.com
Subject: Re: [PATCH, v10 3/3] cgroups: introduce timer slack controller

On Mon, Oct 17, 2011 at 04:49:29PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-10-17 at 15:40 +0100, Matthew Garrett wrote:
> > On Mon, Oct 17, 2011 at 04:28:27PM +0200, Peter Zijlstra wrote:
> > > An XDamage and repaint from the X client, after which your copy will
> > > complete and you get what you asked for?
> > 
> > An XDamage and then an asynchronous RPC call to the remote server to 
> > identify the contents of the next frame before drawing them, plus some 
> > sort of new synchronisation mechanism for blocking the X query until 
> > that point?
> 
> Why would this be a problem? 
> 
> I mean, why would getting a copy of an otherwise invisible surface be a
> performance sensitive path? If the compositor needs the surface it would
> make it visible and send the XDamage once and keep it visible henceforth
> until the time it again becomes invisible, at which point you have to
> stop updates again.

I'm not saying that it's a problem. I'm saying that your approach 
changes behavioural semantics in a way that may violate application 
expectations just as surely as changing the timer behaviour does. 
There's no free approach.

> > Timers are a resource. People want to manage that resource. cgroups are 
> > a convenient mechanism for managing resources.
> 
> Yes, and a ball is round (unless you're a USA-ian, in which case they're
> ovoid), what's your point?

If there's no reason to want to manage that resource, why do we support 
timer slack at all?

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