Re: [Jack-Devel] Jack on ARM with low-latency USB

On 11/27/2013 03:49 PM, Fabian Schlieper wrote:
> Hi Jeremy,
> 
> Yes mad sampling rate -r4441000 is a typo ;)
> 
> Good hint with the jack_bufsize! Distortions temporarly disappear when 
> increasing the buffer size and setting it back to 128 (jack_bufsize 256 & 
> jack_bufsize 128)
> Altough, distortions start again after some time.
> 

Hello Fabian,

Then something is having a hard time catching up because this looks like
that xruns appear after a while.

> Here are some more details about the system:
> Linux Kernel: 3.8.4-rt2 #4 SMP PREEMPT RT Tue Nov 12 11:59:47 CET 2013 
> armv7l GNU/Linux 
> CPU governors is performance.
> 

3.8 might have USB related issues. That's one of the reasons I stick
with 3.4. Not sure if the USB issues are related to your specific issue
though.

> Results from https://github.com/raboof/realtimeconfigquickscan all good 
> except:
> 
> Checking access to the high precision event timer... not found - not good
> /dev/hpet not found.
> Checking access to the real-time clock... not found - not good
> /dev/rtc not found.
> Checking if kernel system timer is set to 1000 hz... not found - not good
> Try setting CONFIG_HZ to 1000
> 
> AFAIK the timers are only relevant for MIDI, so this should not cause any 
> issues here.
> 

No. In fact, this whole section of the realtimeconfigquickscan script is
a bit outdated. Afaik barely any Linux MIDI applications are using
/dev/rtc or /dev/hpet anymore.

> Content of rtirq.conf:
> 
> RTIRQ_NAME_LIST="rtc snd usb i8042"
> # Highest priority.
> RTIRQ_PRIO_HIGH=99
> # Priority decrease step.
> RTIRQ_PRIO_DECR=2
> # Lowest priority.
> RTIRQ_PRIO_LOW=51
> # Whether to reset all IRQ threads to SCHED_OTHER.
> RTIRQ_RESET_ALL=1
> # On kernel configurations that support it,
> # which services should be NOT threaded
> # (space separated list).
> RTIRQ_NON_THREADED="rtc snd"
> # Process names which will be forced to the
> # highest realtime priority range (99-91)
> # (space separated list, from highest to lower priority).
> # RTIRQ_HIGH_LIST="timer"
> 
> Running the script sets the IRQ thread for ehci_usb RT priority to -96, 
> jackd has -81 and the jack client process -70.
> The client applies a FIR convolution on the signal. Thats simple though, 
> keeping CPU usage at 0.x%.
> After starting jackd, im settings the process priority:
> sudo chrt --fifo -p 80 `pidof jackd`
> Although, I'm not 100% sure if this is actually necessary, and passing the -
> P80 is enough?
> 

Yes, that's enough, afaik it does almost exactly the same thing as the
chrt incantation.

> There's only one single ehci USB bus (which shares the IRQ with an ohci USB 
> OTG bus). No chance to get ethernet and audio on different buses.
>

Maybe you could try unbinding the driver from the OTG controller.

> What kernel version and realtime patch are you running on the Cubieboard?

3.4.61-rt77

> Did you enable full preemption?

Yes.

 And did you do any further optimizations?
>

I've tweaked the Ext4 parameters, disabled all unneeded servers, set
CPU's to performance, slightly overclocked the CPU's. And it's a
headless installation, no X. I should check if I did any other
optimizations. If so I'll report back.

Jeremy

> Thanks & Regards
> Fabian
> 
> 
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