linux-kernel - Re: Where is RLIMIT_RT

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Message-Id: <1153146757.24228.28.camel@idefix.homelinux.org>
Date:	Tue, 18 Jul 2006 00:32:36 +1000
From:	Jean-Marc Valin <Jean-Marc.Valin@...erbrooke.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Lee Revell <rlrevell@...-job.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: Where is RLIMIT_RT_CPU?

> Have you thought about using something like Xen?  Have a virtual machine
> that you can even give root access to users, and still have control of
> the actual physical machine.

What does Xen have to do with that? I don't want to run stuff in another
virtual machine, just on my desktop. Also, no matter how good the
virtualization is, if the host eats CPU, the guest doesn't have it. 

> One issue I think you might have is what exactly is a CPU limit?  If the
> system is idle, and you have an app that goes into a busy loop, do you
> kill it after it hits the limit, even if it isn't RT?  Or do you just
> force it to schedule?  Or do you consider idle a special case?  Do you
> want just the apps to be limited, or all the apps that belong to a
> specific user.

The standard was is to simply demote ill-behaved tasks to SCHED_OTHER or
suspend them temporarily, i.e. limit the percentage of CPU they can get
over a certain period of time.

> >From this thread, it seems your goal is to have a single console that
> users can log into and run a RT thread for audio but still not be able
> to lock up the entire system. Right?  So having an RT limit for this use
> might actually be beneficial.  But this is a very rare case, and if you
> are the only one needing this type of feature, then it will likely not
> make it into the kernel.  

Rare case? So you never use a VoIP app (sure they can work as non-rt,
but you get much better quality as latency goes down)? How about media
players that don't skip? There are lots of uses for low-latency (i.e. rt
scheduling) on a desktop. Pro audio people are the first to benefit, but
all users can see an improvement.

> But it if turns out that lots of people like
> this feature, and want it, then it might have a chance, if there is no
> other way to accomplish it.

There *are* lots of people who want it and who have been complaining for
quite a while (see threads on linux-audio-dev). All that's missing is a
tiny bit, which makes it even more frustrating.

> Currently, it looks like you can use either Xen or just stick to one of
> the patches you mentioned earlier.

Sure, I can patch my kernel, but then the distros will just keep on
ignoring the problem, which is bad.

	Jean-Marc
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