[alsa-devel] Default device XRUNs with perriods=2

[Alan Horstmann]

Hi,

I am trying to debug/fix some problems with Portaudio on Linux platform, and 
would appreciate assistance from Alsa experts.

The 'default' pcm on the test systems here (Suse 9 and SLED10), is standard 
for the configuration and includes dmix and plug conversion, with fixed 
period of 1024 at 48000.  Pulseaudio is not on these systems.

When playing data (using a polling mechanism) at 48000 both 3 (or more) and 2 
periods of buffer work fine; with the fixed period of 1024 the lowest latency 
is 2 * 1024 / 48000 = 42.6ms.

However, at 44100, 2 periods does not run properly at all, but 3 does.  The 
periods in this case are (940, 941) due to the rate conversion.

What I see (captured on another machine) from start of the stream is one 
21.3ms period of good audio, a discontinuity followed by some variable amount 
of out-of-sync audio, and a section of silence.

I believe that the discontinuity corresponds to the point at which 
snd_pcm_poll_descriptors_revents() returns POLLERR and portaudio detects the 
POLLERR and restarts the stream, so that after a certain delay there is 
another 21.3ms of audio, POLLERR, now no out-of-sync audio, just silence, 
before it all repeats.  So it stutters only outputting 21.3ms bursts of 
audio.

The CPU usage during this is about 2%.  I am suspecting some 
mis-configuration, or is it wrong to expect this to work at 2 periods due to 
the rate conversion etc?

Any pointers etc would be really appreciated.  This is the info dump:-

ACCESS:  MMAP_INTERLEAVED
FORMAT:  FLOAT_LE
SUBFORMAT:  STD
SAMPLE_BITS: 32
FRAME_BITS: 64
CHANNELS: 2
RATE: 44100
PERIOD_TIME: (21333 21334)
PERIOD_SIZE: (940 941)
PERIOD_BYTES: (7520 7528)
PERIODS: 2
BUFFER_TIME: (42653 42654)
BUFFER_SIZE: 1881
BUFFER_BYTES: 15048
TICK_TIME: 0

start_mode: DATA
xrun_mode: STOP
tstamp_mode: MMAP
period_step: 1
sleep_min: 0
avail_min: 940
xfer_align: 1
silence_threshold: 0
silence_size: 4235635449541951488
boundary: 4235635449541951488
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : FLOAT_LE
  subformat    : STD
  channels     : 2
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 32
  buffer_size  : 1881
  period_size  : 940
  period_time  : 21333
  tick_time    : 0
  tstamp_mode  : MMAP
  period_step  : 1
  sleep_min    : 0
  avail_min    : 940
  xfer_align   : 1
  start_threshold  : 940
  stop_threshold   : 1881
  silence_threshold: 0
  silence_size : 4235635449541951488
  boundary     : 4235635449541951488


Regards

Alan