gh-712: Improve peak jitter calculations

- Extract JitterMeter class.
- Remove `min_jitter` metric (it is almost always zero or very
  close to it).
- Replace `max_jitter` with `peak_jitter`, which is similar to
  maximum, but tries to exclude harmless spikes to reduce latency.

See comments in JitterMeter for details on the algorithm.

src/internal_modules/roc_audio/jitter_meter.cpp

@@ -0,0 +1,151 @@
+/*
+ * Copyright (c) 2024 Roc Streaming authors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+#include "roc_audio/jitter_meter.h"
+#include "roc_core/panic.h"
+
+namespace roc {
+namespace audio {
+
+bool JitterConfig::deduce_defaults(audio::LatencyTunerProfile latency_profile) {
+    if (jitter_window == 0) {
+        if (latency_profile == audio::LatencyTunerProfile_Responsive) {
+            jitter_window = 10000;
+        } else {
+            jitter_window = 30000;
+        }
+    }
+
+    if (peak_quantile_window == 0) {
+        peak_quantile_window = jitter_window / 5;
+    }
+
+    if (envelope_resistance_coeff == 0) {
+        if (latency_profile == audio::LatencyTunerProfile_Responsive) {
+            envelope_resistance_coeff = 0.05;
+        } else {
+            envelope_resistance_coeff = 0.1;
+        }
+    }
+
+    return true;
+}
+
+JitterMeter::JitterMeter(const JitterConfig& config, core::IArena& arena)
+    : config_(config)
+    , jitter_window_(arena, config.jitter_window)
+    , smooth_jitter_window_(arena, config.envelope_smoothing_window_len)
+    , envelope_window_(arena, config.peak_quantile_window, config.peak_quantile_coeff)
+    , peak_window_(arena, config.jitter_window)
+    , capacitor_charge_(0)
+    , capacitor_discharge_resistance_(0)
+    , capacitor_discharge_iteration_(0) {
+}
+
+const JitterMetrics& JitterMeter::metrics() const {
+    return metrics_;
+}
+
+void JitterMeter::update_jitter(const core::nanoseconds_t jitter) {
+    // Moving average of jitter.
+    jitter_window_.add(jitter);
+
+    // Update current value of jitter envelope based on current value of jitter.
+    // Envelope is computed based on smoothed jitter + a peak detector with capacitor.
+    smooth_jitter_window_.add(jitter);
+    const core::nanoseconds_t jitter_envelope =
+        update_envelope_(smooth_jitter_window_.mov_max(), jitter_window_.mov_avg());
+
+    // Quantile of envelope.
+    envelope_window_.add(jitter_envelope);
+    // Moving maximum of quantile of envelope.
+    peak_window_.add(envelope_window_.mov_quantile());
+
+    metrics_.mean_jitter = jitter_window_.mov_avg();
+    metrics_.peak_jitter = peak_window_.mov_max();
+    metrics_.curr_jitter = jitter;
+    metrics_.curr_envelope = jitter_envelope;
+}
+
+// This function calculates jitter envelope using a peak detector with capacitor.
+//
+// The quantile of jitter envelope is used as the value for `peak_jitter` metric.
+// LatencyTuner selects target latency based on its value. We want find lowest
+// possible peak jitter and target latency that are safe (don't cause disruptions).
+//
+// The function tries to achieve two goals:
+//
+//  - The quantile of envelope (e.g. 90% of values) should be above regular repeating
+//    spikes, typical for wireless networks, and should ignore occasional exceptions
+//    if they're not too high and not too frequent.
+//
+//  - The quantile of envelope should be however increased if occasional spike is
+//    really high, which is often a predictor of increasing network load
+//    (i.e. if spike is abnormally high, chances are that more high spikes follows).
+//
+// The role of the capacitor is to amplify the impact of jitter spikes of certain
+// kinds. Without it, spikes would be too thin to be reliably detected by quantile.
+//
+// Typical jitter envelope before applying capacitor:
+//
+//   ------------------------------------- maximum (too high)
+//     |╲
+//     ||          |╲        |╲
+//   --||----------||--------||----------- quantile (too low)
+//   __||______|╲__||__|╲____||__|╲____
+//
+// And after applying capacitor:
+//
+//     |╲_
+//   --|  |_-------|╲_-------|╲----------- quantile (good)
+//     |    ╲      |  ╲_     |  ╲_
+//   __|     ╲_|╲__|    ╲____|    ╲____
+//
+core::nanoseconds_t JitterMeter::update_envelope_(const core::nanoseconds_t cur_jitter,
+                                                  const core::nanoseconds_t avg_jitter) {
+    // `capacitor_charge_` represents current envelope value.
+    // Each step we either instantly re-charge capacitor if we see a peak, or slowly
+    // discharge it until it reaches zero or we see next peek.
+
+    if (capacitor_charge_ < cur_jitter) {
+        // If current jitter is higher than capacitor charge, instantly re-charge
+        // capacitor. The charge is set to the jitter value, and the resistance to
+        // discharging is proportional to the value of the jitter related to average.
+        //
+        // Peaks that are significantly higher than average cause very slow discharging,
+        // and hence have bigger impact on the envelope's quantile.
+        //
+        // Peaks that are not so high discharge quicker, but if they are frequent enough,
+        // capacitor value is constantly re-charged and keeps high. Hence, frequent peeks
+        // also have bigger impact on the envelope's quantile.
+        //
+        // Peaks that are neither high nor frequent have small impact on the quantile.
+        capacitor_charge_ = cur_jitter;
+        capacitor_discharge_resistance_ = std::pow((double)cur_jitter / avg_jitter,
+                                                   config_.envelope_resistance_exponent)
+            * config_.envelope_resistance_coeff;
+        capacitor_discharge_iteration_ = 0;
+    } else if (capacitor_charge_ > 0) {
+        // No peak detected, continue discharging (exponentionally).
+        capacitor_charge_ =
+            core::nanoseconds_t(capacitor_charge_
+                                * std::exp(-capacitor_discharge_iteration_
+                                           / capacitor_discharge_resistance_));
+        capacitor_discharge_iteration_++;
+    }
+
+    if (capacitor_charge_ < 0) {
+        // Fully discharged. Normally doesn't happen.
+        capacitor_charge_ = 0;
+    }
+
+    return capacitor_charge_;
+}
+
+} // namespace audio
+} // namespace roc

src/internal_modules/roc_audio/jitter_meter.h

@@ -0,0 +1,168 @@
+/*
+ * Copyright (c) 2024 Roc Streaming authors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+//! @file roc_audio/jitter_meter.h
+//! @brief Jitter metrics calculator.
+
+#ifndef ROC_AUDIO_JITTER_METER_H_
+#define ROC_AUDIO_JITTER_METER_H_
+
+#include "roc_audio/latency_config.h"
+#include "roc_core/iarena.h"
+#include "roc_core/noncopyable.h"
+#include "roc_core/time.h"
+#include "roc_stat/mov_aggregate.h"
+#include "roc_stat/mov_quantile.h"
+
+namespace roc {
+namespace audio {
+
+//! Jitter meter parameters.
+//!
+//! Mean jitter is calculated as moving average of last `jitter_window` packets.
+//!
+//! Peak jitter calculation is performed in several steps:
+//!
+//!  1. Calculate jitter envelope - a curve that outlines jitter extremes.
+//!     Envelope calculation is based on a smoothing window
+//!     (`envelope_smoothing_window_len`) and a peak detector with capacitor
+//!     (`envelope_resistance_exponent`, `envelope_resistance_coeff`).
+//!
+//!  2. Calculate moving quantile of the envelope - a line above certain percentage
+//!     of the envelope values across moving window (`peak_quantile_coeff`,
+//!     `peak_quantile_window`).
+//!
+//!  3. Calculate moving maximum of the envelope's quantile across last `jitter_window`
+//!     samples. This is the resulting peak jitter.
+struct JitterConfig {
+    //! Number of packets for calculating long-term jitter sliding statistics.
+    //! @remarks
+    //!  Increase this value if you want slower and smoother reaction.
+    //!  Peak jitter is not decreased until jitter envelope is low enough
+    //!  during this window.
+    //! @note
+    //!  Default value is about a few minutes.
+    size_t jitter_window;
+
+    //! Number of packets in small smoothing window to calculate jitter envelope.
+    //! @remarks
+    //!  The larger is this value, the rougher is jitter envelope.
+    //! @note
+    //!  Default value is a few packets.
+    size_t envelope_smoothing_window_len;
+
+    //! Exponent coefficient of capacitor resistance used in jitter envelope.
+    //! @note
+    //!  Capacitor discharge speed is (peak ^ exp) * coeff, where `peak` is
+    //!  the jitter peak size relative to the average jitter, `exp` is
+    //!  `envelope_resistance_exponent`, and `coeff` is `envelope_resistance_coeff`.
+    //! @remarks
+    //!  Increase this value to make impact to the peak jitter of high spikes much
+    //!  stronger than impact of low spikes.
+    double envelope_resistance_exponent;
+
+    //! Linear coefficient of capacitor resistance used in jitter envelope.
+    //! @note
+    //!  Capacitor discharge speed is (peak ^ exp) * coeff, where `peak` is
+    //!  the jitter peak size relative to the average jitter, `exp` is
+    //!  `envelope_resistance_exponent`, and `coeff` is `envelope_resistance_coeff`.
+    //! @remarks
+    //!  Increase this value to make impact to the peak jitter of frequent spikes
+    //!  stronger than impact of rare spikes.
+    double envelope_resistance_coeff;
+
+    //! Number of packets for calculating envelope quantile.
+    //! @remarks
+    //!  This window size is used to calculate moving quantile of the envelope.
+    //! @note
+    //!  This value is the compromise between reaction speed to the increased
+    //!  jitter and ability to distinguish rare spikes from frequent ones.
+    //!  If you increase this value, we can detect and cut out more spikes that
+    //!  are harmless, but we react to the relevant spikes a bit slower.
+    size_t peak_quantile_window;
+
+    //! Coefficient of envelope quantile from 0 to 1.
+    //! @remarks
+    //!  Defines percentage of the envelope that we want to cut out.
+    //! @note
+    //!  E.g. value 0.9 means that we want to draw a line that is above 90%
+    //!  of all envelope values across the quantile window.
+    double peak_quantile_coeff;
+
+    JitterConfig()
+        : jitter_window(0)
+        , envelope_smoothing_window_len(10)
+        , envelope_resistance_exponent(6)
+        , envelope_resistance_coeff(0)
+        , peak_quantile_window(0)
+        , peak_quantile_coeff(0.90) {
+    }
+
+    //! Automatically fill missing settings.
+    ROC_ATTR_NODISCARD bool deduce_defaults(audio::LatencyTunerProfile latency_profile);
+};
+
+//! Jitter metrics.
+struct JitterMetrics {
+    //! Moving average of the jitter.
+    core::nanoseconds_t mean_jitter;
+
+    //! Moving peak value of the jitter.
+    //! @remarks
+    //!  This metric is similar to moving maximum, but excludes short rate spikes
+    //!  that are considered harmless.
+    core::nanoseconds_t peak_jitter;
+
+    //! Last jitter value.
+    core::nanoseconds_t curr_jitter;
+
+    //! Last jitter envelope value.
+    core::nanoseconds_t curr_envelope;
+
+    JitterMetrics()
+        : mean_jitter(0)
+        , peak_jitter(0)
+        , curr_jitter(0)
+        , curr_envelope(0) {
+    }
+};
+
+//! Jitter metrics calculator.
+class JitterMeter : public core::NonCopyable<JitterMeter> {
+public:
+    //! Initialize.
+    JitterMeter(const JitterConfig& config, core::IArena& arena);
+
+    //! Get updated jitter metrics.
+    const JitterMetrics& metrics() const;
+
+    //! Update jitter metrics based on the jitter value for newly received packet.
+    void update_jitter(core::nanoseconds_t jitter);
+
+private:
+    core::nanoseconds_t update_envelope_(core::nanoseconds_t cur_jitter,
+                                         core::nanoseconds_t avg_jitter);
+
+    const JitterConfig config_;
+
+    JitterMetrics metrics_;
+
+    stat::MovAggregate<core::nanoseconds_t> jitter_window_;
+    stat::MovAggregate<core::nanoseconds_t> smooth_jitter_window_;
+    stat::MovQuantile<core::nanoseconds_t> envelope_window_;
+    stat::MovAggregate<core::nanoseconds_t> peak_window_;
+
+    core::nanoseconds_t capacitor_charge_;
+    double capacitor_discharge_resistance_;
+    double capacitor_discharge_iteration_;
+};
+
+} // namespace audio
+} // namespace roc
+
+#endif // ROC_AUDIO_JITTER_METER_H_

src/internal_modules/roc_audio/latency_tuner.cpp

@@ -198,7 +198,7 @@ bool LatencyTuner::update_stream() {
     roc_panic_if(init_status_ != status::StatusOK);
 
     if (enable_latency_adjustment_ && latency_is_adaptive_ && has_metrics_) {
-        update_target_latency_(link_metrics_.max_jitter, link_metrics_.jitter,
+        update_target_latency_(link_metrics_.peak_jitter, link_metrics_.mean_jitter,
                                latency_metrics_.fec_block_duration);
     }
 
@@ -350,7 +350,7 @@ void LatencyTuner::compute_scaling_(packet::stream_timestamp_diff_t actual_laten
 //    NB: After the increasement the new latency target value must not be greater than
 //        upper threshold in any circumstances.
 //
-void LatencyTuner::update_target_latency_(const core::nanoseconds_t max_jitter_ns,
+void LatencyTuner::update_target_latency_(const core::nanoseconds_t peak_jitter_ns,
                                           const core::nanoseconds_t mean_jitter_ns,
                                           const core::nanoseconds_t fec_block_ns) {
     // If there is no active timeout, check if evaluated target latency is
@@ -360,7 +360,7 @@ void LatencyTuner::update_target_latency_(const core::nanoseconds_t max_jitter_n
         // jitter statistics. Later we'll use this value only for decision making if it
         // worth changing or we rather keep the current latency target untouched.
         const core::nanoseconds_t estimate = std::max(
-            std::max(core::nanoseconds_t(max_jitter_ns * max_jitter_overhead_),
+            std::max(core::nanoseconds_t(peak_jitter_ns * max_jitter_overhead_),
                      core::nanoseconds_t(mean_jitter_ns * mean_jitter_overhead_)),
             fec_block_ns);
 
@@ -527,7 +527,7 @@ void LatencyTuner::periodic_report_() {
             "latency tuner:"
             " e2e_latency=%ld(%.3fms) niq_latency=%ld(%.3fms) target_latency=%ld(%.3fms)"
             " stale=%ld(%.3fms)"
-            " packets(lost/exp)=%ld/%ld jitter(avg/min/max)=%.3fms/%.3fms/%.3fms"
+            " packets(lost/exp)=%ld/%ld jitter(mean/peak)=%.3fms/%.3fms"
             " fec_blk=%.3fms"
             " fe=%.6f eff_fe=%.6f fe_stbl=%s",
             // e2e_latency, niq_latency, target_latency
@@ -540,9 +540,8 @@ void LatencyTuner::periodic_report_() {
             // packets
             (long)link_metrics_.lost_packets, (long)link_metrics_.expected_packets,
             // jitter
-            (double)link_metrics_.jitter / core::Millisecond,
-            (double)link_metrics_.min_jitter / core::Millisecond,
-            (double)link_metrics_.max_jitter / core::Millisecond,
+            (double)link_metrics_.mean_jitter / core::Millisecond,
+            (double)link_metrics_.peak_jitter / core::Millisecond,
             // fec_blk
             (double)latency_metrics_.fec_block_duration / core::Millisecond,
             // fe, eff_fe, fe_stbl

src/internal_modules/roc_audio/latency_tuner.h

@@ -90,7 +90,7 @@ class LatencyTuner : public core::NonCopyable<> {
     bool check_actual_latency_(packet::stream_timestamp_diff_t latency);
     void compute_scaling_(packet::stream_timestamp_diff_t latency);
 
-    void update_target_latency_(core::nanoseconds_t max_jitter_ns,
+    void update_target_latency_(core::nanoseconds_t peak_jitter_ns,
                                 core::nanoseconds_t mean_jitter_ns,
                                 core::nanoseconds_t fec_block_ns);
     void try_decrease_latency_(core::nanoseconds_t estimate,