mSleepTimeUs与其在systrace的表现

1、mSleepTimeUs

表示要sleep的时间，一般是等于mIdleSleepTimeUs，而 mIdleSleepTimeUs是idleSleepTimeUs()的返回。

uint32_t AudioFlinger::MixerThread::idleSleepTimeUs() const

{

return (uint32_t)(((mNormalFrameCount * 1000) / mSampleRate) * 1000) / 2;

}

大概就是等于out_write一次数据的时间的一半。比如mNormalFrameCount = 1920， mSampleRate = 48000， 那idleSleepTimeUs = 1920/4800/2 = 20ms，但是mNormalFrameCount还不一定

等于hal层返回的framecount，mNormalFrameCount = multiplier * mFrameCount;

mFrameCount 才是hal层返回的framecount，mFrameCount = mBufferSize / mFrameSize;

2、mActiveSleepTimeUs

mActiveSleepTimeUs = activeSleepTimeUs();

uint32_t AudioFlinger::PlaybackThread::activeSleepTimeUs() const

{

return (uint32_t)((uint32_t)((mNormalFrameCount * 1000) / mSampleRate) * 1000);

}

所以mActiveSleepTimeUs会等于idleSleepTimeUs的2倍，大概是40ms。

3、sleepTimeShift

一个移位参数，主要用于计算实际mSleepTimeUs的值， mSleepTimeUs 不超过kMaxThreadSleepTimeShift（即static const uint32_t kMaxThreadSleepTimeShift = 2;）

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