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change audio-mixer throttling to integral-based linear throttle/backoff

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This commit is contained in:
Zach Pomerantz 2017-01-24 21:27:59 +00:00
parent b028d84aab
commit 2bc725e2be
2 changed files with 43 additions and 27 deletions
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66
assignment-client/src/audio/AudioMixer.cpp
View file

@ -294,6 +294,7 @@ void AudioMixer::sendStatsPacket() {
statsObject["threads"] = _slavePool.numThreads();
statsObject["trailing_mix_ratio"] = _trailingMixRatio;
statsObject["throttling_ratio"] = _throttlingRatio;
statsObject["avg_streams_per_frame"] = (float)_stats.sumStreams / (float)_numStatFrames;
@ -410,7 +411,7 @@ void AudioMixer::start() {
{
auto timer = _sleepTiming.timer();
auto frameDuration = timeFrame(frameTimestamp);
throttle(frameDuration);
throttle(frameDuration, frame);
}
auto frameTimer = _frameTiming.timer();
@ -473,34 +474,49 @@ std::chrono::microseconds AudioMixer::timeFrame(p_high_resolution_clock::time_po
return duration;
}
void AudioMixer::throttle(std::chrono::microseconds duration) {
// throttle using a basic PI controller, keeping duration under 9000 us
const int TARGET = 9000;
const float PROPORTIONAL_TERM = -0.0f;
const float INTEGRAL_TERM = -0.0f;
void AudioMixer::throttle(std::chrono::microseconds duration, int frame) {
// throttle using a modified integral controller
const float FRAME_TIME = 10000.0f;
float mixRatio = duration.count() / FRAME_TIME;
// error term is the fraction of a frame away from the target duration
float error = (TARGET - duration.count()) / 10000.0f;
// constants are determined based on a "regular" 16-CPU EC2 server
if (_throttlingRatio == 0.0f && error > 0) {
// if we are not throttling nor struggling, reset the controller and continue
if (error > 0) {
_throttlingIntegral = 0.0f;
return;
} else {
qDebug() << "audio-mixer is struggling - throttling";
// target different mix and backoff ratios (they also have different backoff rates)
// this is to prevent oscillation, and encourage throttling to find a steady state
const float TARGET = 0.9f;
// on a "regular" machine with 100 avatars, this is the largest value where
// - overthrottling can be recovered
// - oscillations will not occur after the recovery
const float BACKOFF_TARGET = 0.47f;
// the mixer is known to struggle at about 80 on a "regular" machine
// so throttle 2/80 the streams to ensure smooth audio (throttling is linear)
const float THROTTLE_RATE = 2 / 80.0f;
const float BACKOFF_RATE = THROTTLE_RATE / 4;
// recovery should be bounded so that large changes in user count is a tolerable experience
// throttling is linear, so most cases will not need a full recovery
const int RECOVERY_TIME = 180;
// weight more recent frames to determine if throttling is necessary,
const int TRAILING_FRAMES = (int)(100 * RECOVERY_TIME * BACKOFF_RATE);
const float CURRENT_FRAME_RATIO = 1.0f / TRAILING_FRAMES;
const float PREVIOUS_FRAMES_RATIO = 1.0f - CURRENT_FRAME_RATIO;
_trailingMixRatio = PREVIOUS_FRAMES_RATIO * _trailingMixRatio + CURRENT_FRAME_RATIO * mixRatio;
if (frame % TRAILING_FRAMES == 0) {
if (_trailingMixRatio > TARGET) {
_throttlingRatio += THROTTLE_RATE;
_throttlingRatio = std::min(_throttlingRatio, 1.0f);
qDebug("audio-mixer is struggling (%f mix/sleep) - throttling %f of streams",
(double)_trailingMixRatio, (double)_throttlingRatio);
} else if (_throttlingRatio > 0.0f && _trailingMixRatio <= BACKOFF_TARGET) {
_throttlingRatio -= BACKOFF_RATE;
_throttlingRatio = std::max(_throttlingRatio, 0.0f);
qDebug("audio-mixer is recovering (%f mix/sleep) - throttling %f of streams",
(double)_trailingMixRatio, (double)_throttlingRatio);
}
}
_throttlingIntegral += error;
_throttlingIntegral = INTEGRAL_TERM == 0.0f ? 1.0f : std::max(_throttlingIntegral, 1 / INTEGRAL_TERM);
_throttlingRatio = PROPORTIONAL_TERM * error + INTEGRAL_TERM * _throttlingIntegral;
_throttlingRatio = glm::clamp(_throttlingRatio, 0.0f, 1.0f);
if (_throttlingRatio == 0.0f) {
qDebug() << "audio-mixer is recovered - no longer throttling";
}
}
int AudioMixer::prepareFrame(const SharedNodePointer& node, unsigned int frame) {

4
assignment-client/src/audio/AudioMixer.h
View file

@ -73,7 +73,7 @@ private slots:
private:
// mixing helpers
std::chrono::microseconds timeFrame(p_high_resolution_clock::time_point& timestamp);
void throttle(std::chrono::microseconds frameDuration);
void throttle(std::chrono::microseconds frameDuration, int frame);
// pop a frame from any streams on the node
// returns the number of available streams
int prepareFrame(const SharedNodePointer& node, unsigned int frame);
@ -84,7 +84,7 @@ private:
void parseSettingsObject(const QJsonObject& settingsObject);
float _throttlingIntegral { 0.0f };
float _trailingMixRatio { 0.0f };
float _throttlingRatio { 0.0f };
int _numStatFrames { 0 };