[Jack-Devel] Android xrun questions

I am testing Jack on the original Nexus 7 and recently released Nexus 7 2.
The new Nexus 7 is the focus of my development. My audio device is a USB
Native Instruments Komplete Audio 6, which I have been testing at 96K
sampling rate. Android has been modified to include glib and to run Jack
software as root. This enables real time priorities and memory locking.
Jack addresses the USB device using the ALSA driver.

Current testing indicates Jack is stable on Nexus 7 2 at 96k with buffers
set at 512, periods 2. When I set this to 256/3, I see a repeating pattern
of xruns and watchdog resets. Every second I see 13 xruns, plus or minus 1
and every 15 seconds a watchdog reset. My supervisory software restarts
jackd and the pattern repeats. This occurs indefinitely and is 100%
reproducible.

I presume I am now looking for  periodic event that occurs every 75 usecs
and delays Jack processing. Then either it interrupts processing for a
longer period of time every 200 iterations or there is a different problem
that occurs every 15000 usecs. I will likely take this to a Google forum
soon.

So now the Jack questions.

1. There is something that puzzles me regarding my diagnostic output. When
testing against Nexus 7, I see occasional xruns at 96k, buffers 512,
periods 3. My diagnostics display jack_get_xrun_delayed_usecs. This returns
something that seems sensible to me, e.g. 4.924 ms, which matches the
return from jack_get_max_delayed_usecs. However, in the failure scenario on
Nexus 7 2, when an xrun occurs jack_get_xrun_delayed_usecs returns values
such as 0.031 ms, 0.160 ms, even 0.0 ms. Can someone assist ne in
interpreting these results?

2. I am curious about the recommendation for 3 periods when using a USB
device. It seems an obvious precaution when using USB 1.0 as any given
buffer can be sourced from 2 USB packets with a 1 ms delay between. But
does that recommendation still apply to a USB 2.0 device thqt is generating
packets every 125 usecs? If so, what is the rationale?

3. Of course recommendations regarding isolating my problem events are
welcome.

Thanks in advance,

Brad Justice

Testing details
===========
Android version: 4.3.2.1
Kernel version: 3.4.0
Jack priority: 8 (Google recommends running audio threads at RT priority 2
or 3. Running Jack at priority 60 had no effect on test results.)

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