[LAD] discussing HZ (kernel timer frequency) and midi timing

Dominique Michel dominique.michel at citycable.ch
Sat May 5 09:32:53 UTC 2007

Le Fri, 4 May 2007 12:18:25 +0200,
Florian Schmidt <mista.tapas at gmx.net> a écrit :

> On Friday 04 May 2007, Mulyadi Santosa wrote:
> > Hi
> >
> > Sorry for late reply..
> >
> > > I have
> > > dev.rtc.max-user-freq = 8192
> > > in my sysctl.conf.
> > > Before seq24 timing seemed to be changing between laid back and forward.
> > > Haven't tested since setting.
> > > Do
> > > cat /proc/sys/dev/rtc/max-user-freq
> > > to see your setting now.
> > > echo 8192 > /proc/sys/dev/rtc/max-user-freq
> > > changes setting.
> >
> > Thank you, even this small hint gave me a valuable direction. It makes
> > me believe there are many applications that do rely on non-kernel-timer
> > source.. in this case RTC.
> >
> > May I ask, could seq24 reports somekind of note playing delay? or xruns?
> seq24 does _not_ use RTC for timing. It uses an approach based on sleep() (in 
> some way or the other), so setting the rtc-max-frequency does nothing for 
> seq24 performance. To improve seq24 performance over the vanilla kernel you 
> need a kernel with the system timer frequency set back to something like 
> 1000Hz (vanilla has 200Hz, which is too unprecise for MIDI timing based on 
> sleeping short amounts of time)..
> Flo

I get recently in troubles with my rt kernel. On
http://rt.wiki.kernel.org/index.php/RT_PREEMPT_HOWTO, they write at "Since rt
patch 2.6.18-rt6 you will probably have to activate ACPI option to activate
high resolution timer. Since the TSC timer on PC platforms, as used in the
previous versions, are now marked as unsuitable for hrt mode due to many lacks
of functionalities and reliabilties, you will need i.E. pm_timer as provided by
ACPI to use as clock source." 

When doing this, my box just hang from time to time. The problem I have is

1) My box just hang when it hang and I don't get this problem with a vanilla,
desktop preemption and the same ACPI setting. With the same rt kernel but
without ACPI support, it doesn't hang. Older rt kernel as 2.6.18-rt6 are also
hanging when ACPI is enabled, so it is in fact an old kernel issue between ACPI
and my motherboard. The problem is I don't know how to reproduce it and get some
debug information. I even done a kernel bug report, but without other
possibility as to wait some time (from a few hours to a few days) in order to
reproduce it, and without any useful error messages, I doubt at this issue will
never been resolved.

2) More relevant to this list: I am not sure if I understand well this matter.
They write "i.E pm_timer". Is it an alternative way to get the same
functionality but without ACPI?


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