On Thu, Aug 25, 2011 at 12:15 AM, Clemens Ladisch clemens@ladisch.dewrote:
Raymond Toy wrote:
I'm working on an application where I'd like to have relatively fixed
timing
between calls to snd_pcm_writei.
Why?
It's a bit complicated. A different process generates the audio and places the data in a shared memory area. My process sends signals the other process to generate more data, I read the shared memory area (for the previously generated data). Sometime later, the other process fills the shared area with new samples. Thus, if the timing between calls is not fairly regular, I'll either reread the same data or miss the new data. (I'm not in control of how this works. I just have to deal with what I'm given.)
[...] I see is that most calls to snd_pcm_writei are spaced about 50 ms apart. This makes some sense, but I was expecting the calls to be about 42.67 ms apart. However, about every 6th call, the time is just 200 us or so. Why is that?
In theory, when the device is configured for a specific period size, its interrupts (and therefore the application wakeups) will arrive spaced at this interval (modulo any scheduling delays).
However, if you're using the dmix plugin, the period size of your application's device will not necessarily be identical with the period size of the hardware device. To see the actual period size, look into /proc/asound/card?/pcm0p/sub0/hw_params.
It says:
access: MMAP_INTERLEAVED format: S16_LE subformat: STD channels: 2 rate: 44100 (44100/1) period_size: 44096 buffer_size: 88192
Ray