output/alsa: non-blocking mode

Use SND_PCM_NONBLOCK, and perform all snd_pcm_writei() calls in the IOThread. Use a lockless queue to copy data from the OutputThread to the IOThread. This rather major change aims to improve MPD's internal latency. All waits are now under MPD's control, instead of blocking inside libasound2. As a side effect, an output's filter is now decoupled from the actual device I/O, which solves a major latency problem with the conversion filter on slow CPUs and small period buffers. See: https://bugs.musicpd.org/view.php?id=3900

output/alsa: non-blocking mode
b1c7649e · Max Kellermann · 853740f1 · b1c7649e · b1c7649e · b1c7649e
Commit b1c7649e authored Jan 24, 2017 by Max Kellermann
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Makefile.am Makefile.am +1 -0

NEWS NEWS +2 -0

AlsaOutputPlugin.cxx src/output/plugins/AlsaOutputPlugin.cxx +0 -0

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--- a/Makefile.am
+++ b/Makefile.am
@@ -1413,6 +1413,7 @@ libmixer_plugins_a_CPPFLAGS = $(AM_CPPFLAGS) \

 if ENABLE_ALSA
 liboutput_plugins_a_SOURCES += \
+	$(ALSA_SOURCES) \
 	src/output/plugins/AlsaOutputPlugin.cxx \
 	src/output/plugins/AlsaOutputPlugin.hxx
 libmixer_plugins_a_SOURCES += \

--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,8 @@ ver 0.21 (not yet released)
 * protocol
  - "tagtypes" can be used to hide tags
  - "find" and "search" can sort
+* output
+  - alsa: non-blocking mode

 ver 0.20.5 (not yet released)
 * tags

--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx