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input/alsa: set period_size=buffer_size/4 

This way, we have four periods instead of the default of two.  With
only two periods, we don't get woken up often enough, and we
frequently encounter buffer overruns.  With four periods, we have more
time to breathe, and the buffer overruns magically disappear.
This commit is contained in:
Max Kellermann 2017-01-14 21:46:17 +01:00
parent d1f3a87c08
commit 7e8b448985
2 changed files with 17 additions and 21 deletions
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1
NEWS
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@ -1,6 +1,7 @@
ver 0.20.2 (not yet released)
* input
- alsa: fix crash bug
- alsa: fix buffer overruns
* decoder
- flac: add options "probesize" and "analyzeduration"
* resampler

37
src/input/plugins/AlsaInputPlugin.cxx
View File

@ -57,15 +57,6 @@ static constexpr unsigned int default_rate = 44100; // cd quality
static constexpr size_t ALSA_MAX_BUFFERED = default_rate * default_channels * 2;
static constexpr size_t ALSA_RESUME_AT = ALSA_MAX_BUFFERED / 2;
/**
* This value should be the same as the read buffer size defined in
* PcmDecoderPlugin.cxx:pcm_stream_decode().
* We use it to calculate how many audio frames to buffer in the alsa driver
* before reading from the device. snd_pcm_readi() blocks until that many
* frames are ready.
*/
static constexpr size_t read_buffer_size = 4096;
class AlsaInputStream final
: public AsyncInputStream,
MultiSocketMonitor, DeferredMonitor {
@ -344,18 +335,22 @@ ConfigureCapture(snd_pcm_t *capture_handle,
(unsigned)period_size_min, (unsigned)period_size_max,
period_time_min, period_time_max);
/* period needs to be big enough so that poll() doesn't fire too often,
* but small enough that buffer overruns don't occur if Read() is not
* invoked often enough.
* the calculation here is empirical; however all measurements were
* done using 44100:16:2. When we extend this plugin to support
* other audio formats then this may need to be revisited */
snd_pcm_uframes_t period = read_buffer_size * 2;
int direction = -1;
if ((err = snd_pcm_hw_params_set_period_size_near(capture_handle, hw_params,
&period, &direction)) < 0)
throw FormatRuntimeError("Cannot set period size (%s)",
snd_strerror(err));
/* choose the maximum possible buffer_size ... */
snd_pcm_hw_params_set_buffer_size(capture_handle, hw_params,
buffer_size_max);
/* ... and calculate the period_size to have four periods in
one buffer; this way, we get woken up often enough to avoid
buffer overruns, but not too often */
snd_pcm_uframes_t buffer_size;
if (snd_pcm_hw_params_get_buffer_size(hw_params, &buffer_size) == 0) {
snd_pcm_uframes_t period_size = buffer_size / 4;
int direction = -1;
if ((err = snd_pcm_hw_params_set_period_size_near(capture_handle, hw_params,
&period_size, &direction)) < 0)
throw FormatRuntimeError("Cannot set period size (%s)",
snd_strerror(err));
}
if ((err = snd_pcm_hw_params(capture_handle, hw_params)) < 0)
throw FormatRuntimeError("Cannot set parameters (%s)",