Here is 3.8 usbmon with jackd set at a period of 512.
This is what the jackd manual page has to say about these things:
-p, --period int
Specify the number of frames between JACK process() calls. This
value must be a power of 2, and the default is 1024. If you
need low latency, set -p as low as you can go without seeing
xruns. A larger period size yields higher latency, but makes
xruns less likely. The JACK capture latency in seconds is
--period divided by --rate.
(this is what is set to 256 or 512 etc..)
-- rate is set to 44100 (in the last capture - sometimes it is set to
48000)
Her e is some more (user-level) documentation:
https://help.ubuntu.com/community/HowToJACKConfiguration
As you can see from this Ubuntu documentation, a normal/good setting is
considered to be a period of 128 (and many people try to use 32 or 64).
512 is really too high for realtime audio purposes.
What is strange is that 3.5 starts fine down to these low period sizes,
even though performance might be degraded (with xruns or whatever), but
3.8 does not even start - the device seems to lock up with lights
flashing on and off.
** Attachment added: "3.8_mon.out"
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1191603/+attachment/3718564/+files/3.8_mon.out
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https://bugs.launchpad.net/bugs/1191603
Title:
raring-updates: regression: 3.8.0-24.35 causes sporadic failure of USB
audio devices
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