CalculateIOThreadTimeConstraints on Mountain Lion
CalculateIOThreadTimeConstraints on Mountain Lion
- Subject: CalculateIOThreadTimeConstraints on Mountain Lion
- From: Stéphane Letz <email@hidden>
- Date: Thu, 09 May 2013 10:25:05 +0200
Hi,
Our audio code was using HP_Device::CalculateIOThreadTimeConstraints on SnowLeopard to compute the real-time audio thread "computation" quantum. This value was typically higher with small buffer size (like 500 us for a buffer size of 64 frames), and lower for bigger buffer size (like 100 for a buffer sire of 256 frames and up). The logic as we understood was that real-time audio threads with a small buffer size would not want to be re-scheduled too often to be sure to proceed in time.
Now it seems that this logic has been changed on Mountain Lion and values are not the same anymore (maybe Lion already ?). Getting the computation quantum value from the real-time CoreAudio thread, we see values like 250 us for buffers size between 64 and 256 frames, 500 us for a buffer size of 512 frames, and 1000 us for buffer size of 1024 frames and up. this on a recent Core i7 (4 physical cores, 8 logical).
How are those values computed? Do there depend of the number of core on the machine? It there any code somewhere to look at (like the HP_Device::CalculateIOThreadTimeConstraints that was part of Apple publicly available code?)
Thanks.
Stéphane Letz
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