IO Proc Jitter
IO Proc Jitter
- Subject: IO Proc Jitter
- From: EJ Campbell <email@hidden>
- Date: Mon, 11 Nov 2002 21:42:04 -0800
My IO Proc is not always called at a consistent rate. When recording
from an iMic at 48khz, my IO proc is normally called once every 11
milliseconds. But occasionally the IOProc is delayed for approximately
four or five milliseconds. Unlike with the priority inversion I
experienced earlier (thankfully vanquished by switching to Mach
semaphores), HAL eventually catches up, and no packets are lost. For
example, here's what two periods of jitter looked like (the numbers
represent the time between calls to my IO Proc in milliseconds):
11 11 11 15 8 14 8 14 7 9 11 11 11
11 11 11 16 7 14 7 14 7 14 8 8 14 7 11 16 8 14 8 8 11 11 13 9 10 11 16
7 9 11 11 11
I'm not doing anything that requires extremely low latency, so this
jitter is acceptable and can be smoothed over with buffering, but I'd
like to know its cause and whether there is anything I can do to
prevent it. I've noticed that when my machine is not busy writing to
disk, the jitter is much less frequent, but still occurs. It is exactly
the same whether I use the Sound Manager API or utilize Core Audio
directly. Finally, I've checked to make sure that I'm not spending an
unusually long time in any of my IO Procs, so I don't think the problem
is on my end.
Any thoughts?
Thanks,
EJ
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