[LAU] rtirq

Joakim Hernberg jhernberg at alchemy.lu
Fri Mar 20 08:50:19 UTC 2015

On Thu, 19 Mar 2015 10:02:57 +0100
Ralf Mardorf <ralf.mardorf at alice-dsl.net> wrote:

> More interesting would be to get more information about the timer
> issue. I'm using hr timer for MIDI, but when I tried to start jackd
> with hpet, I always got messages similar to "not enough timers
> available" (I can't start an audio session right now, I will post the
> correct messages at another time). If we remove RTC from the config,
> do we need to replace it with something else? Using hr timer for MIDI
> reduced MIDI jitter on my machine, but I never had an hr timer entry
> in rtirq.

AFAIK, once hrtimers are enabled in a kernel, everything runs off that
by default.  You can easily see if your kernel is using the rtc,
just cat /proc/interrupts, if the line ending in rtc0 doesn't show that
it's been used much, it's not used.  On my system it shows that it's
fired once on cpu0.  I also am fairly certain that JACK uses hpet or a
similar high quality timer by default if it's available on the system.
IMO it's just legacy info that hangs around on the net.

I'd also really like to do away with the 1000Hz ticker and start using
NOHZ on my rt kernels, but I haven't changed that yet, just to
accommodate the people talking of MIDI jitter...

I'd be very happy if someone concerned about midi jitter could test an
alternative kernel to see how the changes would affect the matter.



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