Bug 984698 - Reduce the amount of audio prerolled when we are not also decoding video in the same file. r=padenot

content/media/MediaDecoderStateMachine.cpp

@@ -74,6 +74,13 @@ static const uint32_t LOW_AUDIO_USECS = 300000;
 // less than the low audio threshold.
 const int64_t AMPLE_AUDIO_USECS = 1000000;
 
+// When we're only playing audio and we don't have a video stream, we divide
+// AMPLE_AUDIO_USECS and LOW_AUDIO_USECS by the following value. This reduces
+// the amount of decoded audio we buffer, reducing our memory usage. We only
+// need to decode far ahead when we're decoding video using software decoding,
+// as otherwise a long video decode could cause an audio underrun.
+const int64_t NO_VIDEO_AMPLE_AUDIO_DIVISOR = 8;
+
 // Maximum number of bytes we'll allocate and write at once to the audio
 // hardware when the audio stream contains missing frames and we're
 // writing silence in order to fill the gap. We limit our silence-writes
@@ -1856,6 +1863,14 @@ nsresult MediaDecoderStateMachine::DecodeMetadata()
                              mDecoder.get(), mStartTime, mEndTime, GetDuration(),
                              mTransportSeekable, mMediaSeekable));
 
+  if (HasAudio() && !HasVideo()) {
+    // We're playing audio only. We don't need to worry about slow video
+    // decodes causing audio underruns, so don't buffer so much audio in
+    // order to reduce memory usage.
+    mAmpleAudioThresholdUsecs /= NO_VIDEO_AMPLE_AUDIO_DIVISOR;
+    mLowAudioThresholdUsecs /= NO_VIDEO_AMPLE_AUDIO_DIVISOR;
+  }
+
   // Inform the element that we've loaded the metadata and the first frame,
   // setting the default framebuffer size for audioavailable events.  Also,
   // if there is audio, let the MozAudioAvailable event manager know about