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

Jeremy Jongepier <jeremy <at> autostatic.com> writes:

> 
> On 11/21/2013 05:17 PM, Fabian Schlieper wrote:
> > Hi everyone,
> > 
> > I'm trying to build a low-latency audio processor on ODROID-X2 with an 
> > external USB sound card (Behringer UCA-202).
> > I can't get jack working stable with a period size of 128 samples.
> > 
> > I compiled a real-time kernel (with full preemption) and installed the 
rtirq 
> > script.
> > Command line starting jack:
> > jackd -R -P80 -p16 -t2000 -dalsa -dhw:$USB_SOUND_CARD_INDEX,0 -p128 -n3 
-
> > r4441000 -i2 -o2 -s
> > 
> > It's running without audio glitches, but after some time (or when other 
USB 
> > devices are heavily active) the sound gets disturbed and keeps crackling 
until 
> > jack is restarted.
> > 
> > I know I could increase the period size, but I want to have low-latency, 
Is 
> > 128 period size even realistic with USB sound?
> > 
> > Thanks for any comments and hints.
> 
> Hello Fabian,
> 
> What other optimizations did you do besides running a real-time kernel
> and installing rtirq? Which RT kernel are you using? And could you post
> the contents of /etc/default/rtirq? What software are you using to test?
> And I assume -r4441000 is a typo :) And you don't need the -R option,
> it's a JACK default.
> 
> Other things you could try:
> * Setting CPU scaling governor to performance
> * Overclocking the CPU
> * Check if the USB controller shares its IRQ or not and on which bus it
> sits. For instance on my Cubieboard the ethernet controller sits on the
> same bus as the audio codec. If network traffic goes up (ssh -X and then
> opening Hydrogen for example) audio can get distorted.
> * Increase IO by using a good SD card and Ext4 with the noatime and
> nobarrier flags and the rootflags=data=writeback kernel parameter.
> 
> You can try to get clean audio again by reseting the buffer size with
> jack_bufsize 128. And yes, -p128 should be realistic, I can run a
> UCG-102 with -p64 on a Cubieboard2 (Allwinner A20).
> 
> Best,
> 
> Jeremy
> 
> 
> 
> 
> Jack-Devel mailing list
> Jack-Devel <at> lists.jackaudio.org
> http://lists.jackaudio.org/listinfo.cgi/jack-devel-jackaudio.org
> 



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.

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.

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.

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?

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.

What kernel version and realtime patch are you running on the Cubieboard?
Did you enable full preemption? And did you do any further optimizations?

Thanks & Regards
Fabian

1385563799.13668_0.ltw:2,a <loom.20131127T154748-259 at post dot gmane dot org>