Re: [linux-audio-dev] mux concept paper

From: Lee Revell <rlrevell@email-addr-hidden-job.com>
Date: Thu Feb 17 2005 - 19:11:57 EET

On Thu, 2005-02-17 at 16:33 +0000, Steve Harris wrote:
> > Besides, I wrote a patch to eliminate the /proc/cpuinfo hack, it's low
> > prio but should be in JACK eventually. But like Paul says this has
> > absolutely nothing to do with the sound.
>
> Actually it can be on laptops with CPU scaling. JACK gets sufficiently
> confused on mine that it tells ALSA to restart the card constantly,
> causing all kinds of noise.
>
> I have to lock the CPU to the maximum frequency to stop it happening.

That thread kind of petered out on jackit-devel without a resolution. I
have some more information on the problem.

As many of you know the issue is that some CPU's frequency scaling
implementations causes the TSC (which JACK and many other apps use
because it's by far the cheapest high res timer on x86) to change speed,
confusing JACK severely. According to the "experts" on LKML
(specifically George Anzinger from MontaVista) these systems are BROKEN
- IOW this behavior violates some spec. Newer machines (P4 and later)
are guaranteed not to have the problem.

The normal means for dealing with broken hardware, when you don't want
to make everyone take a performance hit due to a few buggy systems, is
to use a blacklist.

Anyway this is an issue for jackit-devel. The short answer is to
disable frequency scaling when you're doing audio work. Which is common
sense anyway.

Lee
Received on Thu Feb 17 20:15:20 2005

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