[linux-audio-user] Is everyone sick of interrupts yet?

Lee Revell rlrevell at joe-job.com
Tue Sep 28 20:05:28 EDT 2004


On Tue, 2004-09-28 at 18:05, Mark Knecht wrote:
> Lee Revell wrote:
> Under Windows XP, using Pro Tools on the same laptop, I can create many 
> 'xrun' equivalents by moving the mouse at the wrong time, so I'm very 
> sinsitive to it.
> 

XP is just not designed for low latency.  I would not expect OS X to
have the same problem.

> Shared interrupts are only slightly worse than not shared. With shared 
> both devices have to be interrogated even if only one generated the 
> interrupt. this has to take longer. However, with good IRQ ahndler code 
> it should not be an issue.

Correct.  I have measured the delay, is it a matter of a few
nanoseconds.  Way too small to be an issue _if_ the irq handling system
is properly designed for low latency, as is the case with OSX and Linux
2.6 with the VP patches.  The 2.4 low latency kernel is more of a hack.

I have also measured the delay when the USB irq handler is shared with
the sound card.  With Ingo's latency tracer you can see the actual flow
of control when the USB irq interrupts the soundcard irq.  It's very
small, something like 50-100 usecs.  To keep this in perspective when
using jack at 32 frames, 48KHZ, you have 333 usecs to respond to the
interrupt.

Lee



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