Apple CoreAudio - 爲什麼AUGraphStop（）需要25ms才能完成？

Question

我們的音頻應用程序是使用含有混頻器單元，一個轉換器單元和輸出單元AUGraphs。這是一個實時應用程序，因此性能是一個高度關注的問題。

的問題已被標記了其中AUGraphStop（）需要25毫秒即可完成主線程上，我們的探查表明，花費這段時間的睡眠。誰能解釋爲什麼會發生這種情況？它是等待下一個零交叉點，還是等待一個更多的緩衝區渲染？我已經嘗試了幾種解決方法，包括髮送一個沉默的渲染幀（並設置kAudioUnitRenderAction_OutputIsSilence標誌），然後嘗試停止，並調用kAudioUnitRenderAction_PostRender通知回調內的AUGraphStop（）（這有混合結果，並在閱讀後它似乎不是一個推薦的方法）。

我試圖降低每片幀的數量，但它似乎不有所作爲。我也試圖消除所有的音頻單元除了輸出節點，以試圖縮小問題到特定的單位，但AUGraphStop的成本（）仍維持在25毫秒。

下面是我們如何在初始化圖形：

//Configure converter/mixer/output unit descriptors 
AudioComponentDescription OutputUnitDesc = { kAudioUnitType_Output ,kAudioUnitSubType_DefaultOutput, kAudioUnitManufacturer_Apple }; 
AudioComponentDescription MixerUnitDesc = { kAudioUnitType_Mixer, kAudioUnitSubType_MatrixMixer, kAudioUnitManufacturer_Apple }; 
AudioComponentDescription ConverterUnitDesc = { kAudioUnitType_FormatConverter, kAudioUnitSubType_AUConverter, kAudioUnitManufacturer_Apple }; 

//Create graph 
CA_ERR(NewAUGraph(&mAudioGraph)); 

//Add converter/mixer/output nodes 
CA_ERR(AUGraphAddNode(mAudioGraph, &OutputUnitDesc, &mOutputNode)); 
CA_ERR(AUGraphAddNode(mAudioGraph, &MixerUnitDesc, &mMixerNode)); 
CA_ERR(AUGraphAddNode(mAudioGraph, &ConverterUnitDesc, &mConverterNode)); 

//Connect nodes 
CA_ERR(AUGraphConnectNodeInput(mAudioGraph, mConverterNode, 0, mMixerNode, 0)); 
CA_ERR(AUGraphConnectNodeInput(mAudioGraph, mMixerNode, 0, mOutputNode, 0)); 

//Open the graph (instantiates the units) 
CA_ERR(AUGraphOpen(mAudioGraph)); 

//Get the created units 
CA_ERR(AUGraphNodeInfo(mAudioGraph, mOutputNode, NULL, &mOutputUnit)); 
CA_ERR(AUGraphNodeInfo(mAudioGraph, mMixerNode, NULL, &mMixerUnit)); 
CA_ERR(AUGraphNodeInfo(mAudioGraph, mConverterNode, NULL, &mConverterUnit)); 

//Setup stream format description 
mStreamDesc.mFormatID = kAudioFormatLinearPCM; 
mStreamDesc.mFormatFlags = kLinearPCMFormatFlagIsPacked | kAudioFormatFlagIsSignedInteger; 
mStreamDesc.mChannelsPerFrame = Header->nChannels; 
mStreamDesc.mSampleRate = (Float64)Header->nSamplesPerSec; 
mStreamDesc.mBitsPerChannel = Header->wBitsPerSample; 
mStreamDesc.mBytesPerFrame = (Header->wBitsPerSample >> 3) * Header->nChannels; 
mStreamDesc.mFramesPerPacket = 1; 
mStreamDesc.mBytesPerPacket = mStreamDesc.mBytesPerFrame * mStreamDesc.mFramesPerPacket; 
mStreamDesc.mReserved = 0; 

//Set data endianness according to file type - TODO: Get endianness from header 
AudioSystem::FileType FileType = mSample->GetFiletype(); 

if(FileType == AudioSystem::WAV) 
     mStreamDesc.mFormatFlags |= kAudioFormatFlagsNativeEndian; 
else if(FileType == AudioSystem::OGG) 
     mStreamDesc.mFormatFlags |= kLinearPCMFormatFlagIsBigEndian; 

//Configure number of input/output busses for mixer unit 
int NumChannelsIn = Header->nChannels; 
int NumChannelsOut = (int)AudioSystem::GetOutputChannelConfig(); 
CA_ERR(AudioUnitSetProperty(mMixerUnit, kAudioUnitProperty_BusCount, kAudioUnitScope_Input, 0, &NumChannelsIn, sizeof(u32))); 
CA_ERR(AudioUnitSetProperty(mMixerUnit, kAudioUnitProperty_BusCount, kAudioUnitScope_Output, 0, &NumChannelsOut, sizeof(u32))); 

//Set render callback 
AURenderCallbackStruct callback = { AudioRenderCallback, this }; 
CA_ERR(AUGraphSetNodeInputCallback(mAudioGraph, mConverterNode, 0, &callback)); 

//Set stream format to something native to CoreAudio 
AudioStreamBasicDescription OutputDesc = {0}; 
UInt32 Size = sizeof(AudioStreamBasicDescription); 
CA_ERR(AudioUnitGetProperty(mMixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &OutputDesc, &Size)); 

//Set num output channels 
OutputDesc.mChannelsPerFrame = (int)AudioSystem::GetOutputChannelConfig(); 

//Set stream format 
CA_ERR(AudioUnitSetProperty(mConverterUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &mStreamDesc, sizeof(AudioStreamBasicDescription))); 
CA_ERR(AudioUnitSetProperty(mConverterUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 0, &OutputDesc, sizeof(AudioStreamBasicDescription))); 
CA_ERR(AudioUnitSetProperty(mMixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &OutputDesc, sizeof(AudioStreamBasicDescription))); 
CA_ERR(AudioUnitSetProperty(mMixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 0, &OutputDesc, sizeof(AudioStreamBasicDescription))); 
CA_ERR(AudioUnitSetProperty(mOutputUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &OutputDesc, sizeof(AudioStreamBasicDescription))); 

//Initialise graph 
CA_ERR(AUGraphInitialize(mAudioGraph)); 

//Set notification callback 
CA_ERR(AUGraphAddRenderNotify(mAudioGraph, AudioNotifyCallback, this)); 

//Set global mixer volume 
CA_ERR(AudioUnitSetParameter(mMixerUnit, kMatrixMixerParam_Volume, kAudioUnitScope_Global, 0xFFFFFFFF, 1.0, 0)); 

//Set input channel volumes 
for(int i = 0; i < Header->nChannels; i++) 
{ 
     CA_ERR(AudioUnitSetParameter(mMixerUnit, kMatrixMixerParam_Volume, kAudioUnitScope_Input, i, 1.0, 0)); 
} 

//Set output channel volumes 
for(int i = 0; i < (int)AudioSystem::GetOutputChannelConfig(); i++) 
{ 
     CA_ERR(AudioUnitSetParameter(mMixerUnit, kMatrixMixerParam_Volume, kAudioUnitScope_Output, i, 1.0, 0)); 
} 

圖形啓動和停止使用AUGraphStart（）和AUGraphStop（），沒什麼特別的。下面是使用Shark Profiler捕獲的callstack：

| | | | | | | | | + 1.3%, AudioUnitGraph::Stop(), AudioToolbox 
| | | | | | | | | | + 1.3%, AudioOutputUnitStop, AudioUnit 
| | | | | | | | | | | + 1.3%, CallComponentDispatch, CarbonCore 
| | | | | | | | | | | | + 1.3%, DefaultOutputAUEntry, CoreAudio 
| | | | | | | | | | | | | + 1.3%, AUHALEntry, CoreAudio 
| | | | | | | | | | | | | | + 1.3%, usleep$UNIX2003, libSystem.B.dylib 
| | | | | | | | | | | | | | | + 1.3%, nanosleep$UNIX2003, libSystem.B.dylib 
| | | | | | | | | | | | | | | | 1.3%, __semwait_signal, libSystem.B.dylib 

它花費了整個AUGraphStop（）睡覺！

任何線索？

Answer 1

如果您已經達到每片的最小可配置幀數，AUHAL中的這25ms很可能是硬件約束的表現（訪問和/或（重新）配置某些音頻DAC需要多長時間控制寄存器或狀態，和/或一些音頻放大器控制狀態），這是由鎖定此硬件訪問的iOS音頻驅動程序線程中的固定延遲嚴重處理的。但這只是一個瘋狂的猜測。

對於硬實時應用程序，我可能會考慮將AUGraphStop（）（以及AUGraphStart（））移出主UI線程，併爲AU工作處理您自己的線程信號。