[LAU] Analysing (debugging) USB port performance

[Lorenzo Sutton]

Hi LAU,

I have anecdotally, but recurrently observed on laptops - including a 
rather recent one - that there is always a 'best' USB port for external 
sound cards.

For instance, on my latest machine with a decent 'realtime audio' 
configuration/set-up (real-time kernel, /etc/limits stuff, 'performance' 
CPU governor, (wireless) network switched off), I'm able to have a 
pleasant xrun-free session recording in Ardour including a bunch of 
tracks with effects playing at 64 frames and period of 3 with a 
relatively cheap card (UMC202) on one of the USB ports.
On the other hand, in the exact same conditions I get incidental xruns 
at even 128 frames and xrun instability at 64 frames on the other USB ports.

I wonder:

1. Is there a more scientific (well, precise at least) method to assess 
this USB port performance? What to test or look into?
2. Is there a way to change (e.g. improve the not-so-good USB port 
performance) OS/software wise, or is this usually hard-wired in notebooks?
   2a. Are IRQs relevant on laptops and if so can a whole USB port (or 
the device attached to it) be optimised from the OS?

Of course I _can_ live with one 'good sound-card port' on a laptop but 
I'm quite curious about people's experiences and the gurus' wisdom - 
albeit on my former machine this was the left-side port which was closer 
to where the sound-card usually sits, now it's on the right, too bad! :)

Hopefully other LAU have mused about such USB-related mysteries in the 
past...

Lorenzo.