Bitrate controller for VideoToolbox encoder.

webrtc/modules/video_coding/bitrate_adjuster.cc

+/*
+ *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/modules/video_coding/include/bitrate_adjuster.h"
+
+#include "webrtc/base/checks.h"
+#include "webrtc/base/logging.h"
+#include "webrtc/system_wrappers/include/clock.h"
+
+namespace webrtc {
+
+// Update bitrate at most once every second.
+const uint32_t BitrateAdjuster::kBitrateUpdateIntervalMs = 1000;
+
+// Update bitrate at most once every 30 frames.
+const uint32_t BitrateAdjuster::kBitrateUpdateFrameInterval = 30;
+
+// 5 percent of original.
+const uint32_t BitrateAdjuster::kBitrateTolerancePct = 5;
+
+BitrateAdjuster::BitrateAdjuster(Clock* clock)
+    : clock_(clock),
+      bitrate_tracker_(1.5 * kBitrateUpdateIntervalMs, 8 * 1000) {

[pbos-webrtc, 2016/02/22 15:44:18]

What#s 8? constant please

[tkchin_webrtc, 2016/02/22 18:23:18]

Done.

+  Reset();
+}
+
+void BitrateAdjuster::SetTargetBitrateBps(uint32_t bitrate_bps) {
+  rtc::CritScope cs(&crit_);
+  target_bitrate_bps_ = bitrate_bps;
+  // If we haven't had time to determine what adjustment is required yet then
+  // just use the target.
+  adjusted_bitrate_bps_ = target_bitrate_bps_;
+}
+
+uint32_t BitrateAdjuster::GetTargetBitrateBps() const {
+  rtc::CritScope cs(&crit_);
+  return target_bitrate_bps_;
+}
+
+uint32_t BitrateAdjuster::GetAdjustedBitrateBps() const {
+  rtc::CritScope cs(&crit_);
+  return adjusted_bitrate_bps_;
+}
+
+uint32_t BitrateAdjuster::GetEstimatedBitrateBps() {
+  rtc::CritScope cs(&crit_);
+  return bitrate_tracker_.Rate(clock_->TimeInMilliseconds());
+}
+
+uint32_t BitrateAdjuster::GetMinAdjustedBitrateBps() const {
+  rtc::CritScope cs(&crit_);
+  // Half of target bitrate.
+  return target_bitrate_bps_ / 2;
+}
+
+uint32_t BitrateAdjuster::GetMaxAdjustedBitrateBps() const {
+  rtc::CritScope cs(&crit_);
+  // 95% of target bitrate. In practice we know that the VideoToolbox encoder
+  // on iOS overshoots regularly. Setting it to 95% seems to reduce
+  // oscillations.
+  return .95 * target_bitrate_bps_;

[pbos-webrtc, 2016/02/22 15:44:18]

Is it intentional that this thing can only operate on encoders that overshoot?
If they undershoot we'll need to compensate as well, no? If this is only used
when overshooting, then please update the name.

[tkchin_webrtc, 2016/02/22 18:23:18]

As we talked about before, we wanted this for the iOS encoder specifically
first. As far as I know, the Android hardware encoder also regularly overshoots
so this should still be usable.

This can be used for both over and undershooting, there's just currently no way
to set it. The values are used purely for clamping. I had them as getters to
that it'd be easy to update the class later.

We can add a setter in a subsequent CL. I don't want to add any more to this CL
given that the the setter behavior is not being used at this point of time.

[pbos-webrtc, 2016/02/22 18:37:19]

But this reads to me like it's allowed to clamp only between 0.95 and 0.5, why
can't it go to or above 1.0?

[tkchin_webrtc, 2016/02/23 00:05:27]

Done.

+}
+
+void BitrateAdjuster::Update(size_t frame_size) {
+  rtc::CritScope cs(&crit_);
+  uint32_t current_time_ms = clock_->TimeInMilliseconds();
+  bitrate_tracker_.Update(frame_size, current_time_ms);
+  UpdateBitrate(current_time_ms);
+}
+
+// Only safe to call this after Update calls have stopped
+void BitrateAdjuster::Reset() {
+  rtc::CritScope cs(&crit_);
+  target_bitrate_bps_ = 0;
+  adjusted_bitrate_bps_ = 0;
+  last_bitrate_update_time_ms_ = 0;
+  frames_since_last_update_ = 0;
+  bitrate_tracker_.Reset();
+}
+
+void BitrateAdjuster::UpdateBitrate(uint32_t current_time_ms) {
+  rtc::CritScope cs(&crit_);
+  uint32_t time_since_last_update_ms =
+      current_time_ms - last_bitrate_update_time_ms_;
+  // Don't attempt to update bitrate unless enough time and frames have passed.
+  ++frames_since_last_update_;
+  if (time_since_last_update_ms < kBitrateUpdateIntervalMs ||
+      frames_since_last_update_ < kBitrateUpdateFrameInterval) {
+    return;
+  }
+  float estimated_bitrate_bps = bitrate_tracker_.Rate(current_time_ms);
+  float target_bitrate_bps = target_bitrate_bps_;
+  float error = target_bitrate_bps - estimated_bitrate_bps;
+
+  // Adjust if we've overshot by any amount or if we've undershot too much.
+  if (estimated_bitrate_bps > target_bitrate_bps ||
+      error * 100 > kBitrateTolerancePct * target_bitrate_bps) {

[pbos-webrtc, 2016/02/22 15:44:18]

make kBitrateTolerance not be in percent.

[tkchin_webrtc, 2016/02/22 18:23:18]

why? Do you have a particular value in mind?

[pbos-webrtc, 2016/02/22 18:37:19]

Just make it 0.05 instead of 5 to avoid the multiplications etc.

[tkchin_webrtc, 2016/02/23 00:05:27]

Done.

+    // Adjust the bitrate by a fraction of the error.
+    float adjustment = .5 * error;
+    float adjusted_bitrate_bps = target_bitrate_bps + adjustment;
+
+    // Clamp the adjustment to [0.5 * target, 0.95 * target].
+    float min_bitrate_bps = GetMinAdjustedBitrateBps();

[pbos-webrtc, 2016/02/22 18:37:19]

This takes crit_ recursively, can you put EXCLUSIVE_LOCKS_REQUIRED(crit_) on the
clamping rates instead of taking the lock inside?

[tkchin_webrtc, 2016/02/23 00:05:27]

Done. But now requires friend class for test.

[pbos-webrtc, 2016/02/23 15:04:40]

Can't you just make sure it's clamped from the outside? Loop and
overuse/underuse for 10s and see that it hits but doesn't go above/below the
threshold?

[tkchin_webrtc, 2016/02/23 17:03:20]

Done.

+    float max_bitrate_bps = GetMaxAdjustedBitrateBps();
+    adjusted_bitrate_bps = std::max(adjusted_bitrate_bps, min_bitrate_bps);
+    adjusted_bitrate_bps = std::min(adjusted_bitrate_bps, max_bitrate_bps);
+
+    // Set the adjustment if it's not already set.
+    float last_adjusted_bitrate_bps = adjusted_bitrate_bps_;
+    if (adjusted_bitrate_bps != last_adjusted_bitrate_bps) {
+      LOG(LS_INFO) << "Adjusting encoder bitrate:"
+                      << "\n  target_bitrate:"
+                      << static_cast<uint32_t>(target_bitrate_bps)
+                      << "\n  estimated_bitrate:"
+                      << static_cast<uint32_t>(estimated_bitrate_bps)
+                      << "\n  last_adjusted_bitrate:"
+                      << static_cast<uint32_t>(last_adjusted_bitrate_bps)
+                      << "\n  adjusted_bitrate:"
+                      << static_cast<uint32_t>(adjusted_bitrate_bps);
+      adjusted_bitrate_bps_ = adjusted_bitrate_bps;
+    }
+  }
+  last_bitrate_update_time_ms_ = current_time_ms;
+  frames_since_last_update_ = 0;
+}
+
+}  // namespace webrtc