Automatic resolution throttling for screen capture pipeline.

media/capture/video_capture_oracle.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/capture/video_capture_oracle.h"
 
 #include <algorithm>
 
 #include "base/format_macros.h"
+#include "base/numerics/safe_conversions.h"
 #include "base/strings/stringprintf.h"
 
 namespace media {
 
 namespace {
 
 // This value controls how many redundant, timer-base captures occur when the
 // content is static. Redundantly capturing the same frame allows iterative
 // quality enhancement, and also allows the buffer to fill in "buffered mode".
 //
 // TODO(nick): Controlling this here is a hack and a layering violation, since
 // it's a strategy specific to the WebRTC consumer, and probably just papers
 // over some frame dropping and quality bugs. It should either be controlled at
 // a higher level, or else redundant frame generation should be pushed down
 // further into the WebRTC encoding stack.
 const int kNumRedundantCapturesOfStaticContent = 200;
 
+// The half-life of data points provided to the accumulator used when evaluating
+// the recent utilization of the buffer pool.  This value is based on a
+// simulation, and reacts quickly to change to avoid depleting the buffer pool
+// (which would cause hard frame drops).
+const int kBufferUtilizationEvaluationMicros = 200000;
+
+// The half-life of data points provided to the accumulator used when evaluating
+// the recent resource utilization of the consumer.  The trade-off made here is
+// reaction time versus over-reacting to outlier data points.
+const int kConsumerCapabilityEvaluationMicros = 1000000;
+
+// The minimum amount of time that must pass between changes to the capture
+// size.  This throttles the rate of size changes, to avoid stressing consumers
+// and to allow the end-to-end system sufficient time to stabilize before
+// re-evaluating the capture size.
+const int kMinSizeChangePeriodMicros = 3000000;
+
+// The maximum amount of time that may elapse without a feedback update.  Any
+// longer, and currently-accumulated feedback is not considered recent enough to
+// base decisions off of.  This prevents changes to the capture size when there
+// is an unexpected pause in events.
+const int kMaxTimeSinceLastFeedbackUpdateMicros = 1000000;
+
+// The amount of additional time, since content animation was last detected, to
+// continue being extra-careful about increasing the capture size.  This is used
+// to prevent breif periods of non-animating content from throwing off the
+// heuristics that decide whether to increase the capture size.
+const int kDebouncingPeriodForAnimatedContentMicros = 3000000;

[hubbe, 2015/06/25 21:57:43]

All of these constants are a bit hard to read (too many zeroes)
Perhaps use millis? (Or seconds?)

[miu, 2015/07/01 22:11:47]

Done.  Sort of.  It all comes back down to all those extra division operations. 
;-)  If you feel strongly about this, I can change it again, but how about my
solution of just adding a little comment to the right of each for clarification?

+
+// When content is animating, this is the length of time the system must be
+// contiguously under-utilized before increasing the capture size.
+const int kProvingPeriodForAnimatedContentMicros = 30000000;
+
 // Given the amount of time between frames, compare to the expected amount of
 // time between frames at |frame_rate| and return the fractional difference.
 double FractionFromExpectedFrameRate(base::TimeDelta delta, int frame_rate) {
   DCHECK_GT(frame_rate, 0);
   const base::TimeDelta expected_delta =
       base::TimeDelta::FromSeconds(1) / frame_rate;
   return (delta - expected_delta).InMillisecondsF() /
       expected_delta.InMillisecondsF();
 }
 
+// Returns the next-higher TimeTicks value.
+// TODO(miu): Patch FeedbackSignalAccumulator reset behavior and remove this
+// hack.
+base::TimeTicks JustAfter(base::TimeTicks t) {
+  return t + base::TimeDelta::FromMicroseconds(1);
+}
+
+// Returns true if updates have been accumulated by |accumulator| for a
+// sufficient amount of time and the latest update was fairly recent, relative
+// to |now|.
+bool HasSufficientRecentFeedback(const FeedbackSignalAccumulator& accumulator,
+                                 base::TimeTicks now) {
+  const base::TimeDelta amount_of_history =
+      accumulator.update_time() - accumulator.reset_time();
+  return (amount_of_history.InMicroseconds() >= kMinSizeChangePeriodMicros) &&
+      ((now - accumulator.update_time()).InMicroseconds() <=
+           kMaxTimeSinceLastFeedbackUpdateMicros);
+}
+
 }  // anonymous namespace
 
-VideoCaptureOracle::VideoCaptureOracle(base::TimeDelta min_capture_period)
+VideoCaptureOracle::VideoCaptureOracle(
+    base::TimeDelta min_capture_period,
+    const gfx::Size& max_frame_size,
+    media::ResolutionChangePolicy resolution_change_policy)
     : next_frame_number_(0),
       last_successfully_delivered_frame_number_(-1),
       num_frames_pending_(0),
       smoothing_sampler_(min_capture_period,
                          kNumRedundantCapturesOfStaticContent),
-      content_sampler_(min_capture_period) {
-}
+      content_sampler_(min_capture_period),
+      resolution_chooser_(max_frame_size, resolution_change_policy),
+      buffer_pool_utilization_(base::TimeDelta::FromMicroseconds(
+          kBufferUtilizationEvaluationMicros)),
+      estimated_capable_area_(base::TimeDelta::FromMicroseconds(
+          kConsumerCapabilityEvaluationMicros)) {}
 
 VideoCaptureOracle::~VideoCaptureOracle() {}
 
+void VideoCaptureOracle::SetSourceSize(const gfx::Size& source_size) {
+  resolution_chooser_.SetSourceSize(source_size);
+  // If the |resolution_chooser_| computed a new capture size, that will become
+  // visible via a future call to ObserveEventAndDecideCapture().
+}
+
 bool VideoCaptureOracle::ObserveEventAndDecideCapture(
     Event event,
     const gfx::Rect& damage_rect,
     base::TimeTicks event_time) {
   DCHECK_GE(event, 0);
   DCHECK_LT(event, kNumEvents);
   if (event_time < last_event_time_[event]) {
     LOG(WARNING) << "Event time is not monotonically non-decreasing.  "
                  << "Deciding not to capture this frame.";
     return false;
   }
   last_event_time_[event] = event_time;
 
   bool should_sample = false;
   duration_of_next_frame_ = base::TimeDelta();
   switch (event) {
-    case kCompositorUpdate:
+    case kCompositorUpdate: {
       smoothing_sampler_.ConsiderPresentationEvent(event_time);
+      const bool had_proposal = content_sampler_.HasProposal();
       content_sampler_.ConsiderPresentationEvent(damage_rect, event_time);
       if (content_sampler_.HasProposal()) {
+        VLOG_IF(1, !had_proposal) << "Content sampler now detects animation.";
         should_sample = content_sampler_.ShouldSample();
         if (should_sample) {
           event_time = content_sampler_.frame_timestamp();
           duration_of_next_frame_ = content_sampler_.sampling_period();
         }
+        last_time_animation_was_detected_ = event_time;
       } else {
+        VLOG_IF(1, had_proposal) << "Content sampler detects animation ended.";
         should_sample = smoothing_sampler_.ShouldSample();
         if (should_sample)
           duration_of_next_frame_ = smoothing_sampler_.min_capture_period();
       }
       break;
+    }
+
     case kTimerPoll:
       // While the timer is firing, only allow a sampling if there are none
       // currently in-progress.
       if (num_frames_pending_ == 0) {
         should_sample = smoothing_sampler_.IsOverdueForSamplingAt(event_time);
         if (should_sample)
           duration_of_next_frame_ = smoothing_sampler_.min_capture_period();
       }
       break;
+
     case kNumEvents:
       NOTREACHED();
       break;
   }
 
+  if (!should_sample)
+    return false;
+
+  // Update |capture_size_| and reset all feedback signal accumulators if
+  // either: 1) this is the first frame; or 2) |resolution_chooser_| has an
+  // updated capture size and sufficient time has passed since the last size
+  // change.
+  if (next_frame_number_ == 0) {
+    CommitCaptureSizeAndReset(event_time - duration_of_next_frame_);
+  } else if (capture_size_ != resolution_chooser_.capture_size()) {
+    const base::TimeDelta time_since_last_change =
+        event_time - buffer_pool_utilization_.reset_time();
+    if (time_since_last_change.InMicroseconds() >= kMinSizeChangePeriodMicros)
+      CommitCaptureSizeAndReset(GetFrameTimestamp(next_frame_number_ - 1));
+  }
+
   SetFrameTimestamp(next_frame_number_, event_time);
-  return should_sample;
+  return true;
 }
 
-int VideoCaptureOracle::RecordCapture() {
+int VideoCaptureOracle::RecordCapture(double pool_utilization) {
+  DCHECK(std::isfinite(pool_utilization) && pool_utilization >= 0.0);
+
   smoothing_sampler_.RecordSample();
-  content_sampler_.RecordSample(GetFrameTimestamp(next_frame_number_));
+  const base::TimeTicks timestamp = GetFrameTimestamp(next_frame_number_);
+  content_sampler_.RecordSample(timestamp);
+
+  buffer_pool_utilization_.Update(pool_utilization, timestamp);
+  AnalyzeAndAdjust(timestamp);
+
   num_frames_pending_++;
   return next_frame_number_++;
 }
 
+void VideoCaptureOracle::RecordWillNotCapture(double pool_utilization) {
+  DCHECK(std::isfinite(pool_utilization) && pool_utilization >= 0.0);
+
+  VLOG(1) << "Client rejects proposal to capture frame (at #"
+          << next_frame_number_ << ").";
+
+  const base::TimeTicks timestamp = GetFrameTimestamp(next_frame_number_);
+  buffer_pool_utilization_.Update(pool_utilization, timestamp);
+  AnalyzeAndAdjust(timestamp);
+
+  // Note: Do not advance |next_frame_number_| since it will be re-used for the
+  // next capture proposal.
+}
+
 bool VideoCaptureOracle::CompleteCapture(int frame_number,
                                          bool capture_was_successful,
                                          base::TimeTicks* frame_timestamp) {
   num_frames_pending_--;
+  DCHECK_GE(num_frames_pending_, 0);
 
   // Drop frame if previously delivered frame number is higher.
   if (last_successfully_delivered_frame_number_ > frame_number) {
     LOG_IF(WARNING, capture_was_successful)
         << "Out of order frame delivery detected (have #" << frame_number
         << ", last was #" << last_successfully_delivered_frame_number_
         << ").  Dropping frame.";
     return false;
   }
 
+  if (!IsFrameInRecentHistory(frame_number)) {
+    LOG(WARNING) << "Very old capture being ignored: frame #" << frame_number;
+    return false;
+  }
+
   if (!capture_was_successful) {
     VLOG(2) << "Capture of frame #" << frame_number << " was not successful.";
     return false;
   }
 
   DCHECK_NE(last_successfully_delivered_frame_number_, frame_number);
   last_successfully_delivered_frame_number_ = frame_number;
 
   *frame_timestamp = GetFrameTimestamp(frame_number);
 
   // If enabled, log a measurement of how this frame timestamp has incremented
   // in relation to an ideal increment.
-  if (VLOG_IS_ON(2) && frame_number > 0) {
+  if (VLOG_IS_ON(3) && frame_number > 0) {
     const base::TimeDelta delta =
         *frame_timestamp - GetFrameTimestamp(frame_number - 1);
     if (content_sampler_.HasProposal()) {
       const double estimated_frame_rate =
           1000000.0 / content_sampler_.detected_period().InMicroseconds();
       const int rounded_frame_rate =
           static_cast<int>(estimated_frame_rate + 0.5);
-      VLOG(2) << base::StringPrintf(
+      VLOG_STREAM(3) << base::StringPrintf(
           "Captured #%d: delta=%" PRId64 " usec"
           ", now locked into {%s}, %+0.1f%% slower than %d FPS",
           frame_number,
           delta.InMicroseconds(),
           content_sampler_.detected_region().ToString().c_str(),
           100.0 * FractionFromExpectedFrameRate(delta, rounded_frame_rate),
           rounded_frame_rate);
     } else {
-      VLOG(2) << base::StringPrintf(
+      VLOG_STREAM(3) << base::StringPrintf(
           "Captured #%d: delta=%" PRId64 " usec"
           ", d/30fps=%+0.1f%%, d/25fps=%+0.1f%%, d/24fps=%+0.1f%%",
           frame_number,
           delta.InMicroseconds(),
           100.0 * FractionFromExpectedFrameRate(delta, 30),
           100.0 * FractionFromExpectedFrameRate(delta, 25),
           100.0 * FractionFromExpectedFrameRate(delta, 24));
     }
   }
 
-  return !frame_timestamp->is_null();
+  return true;
+}
+
+void VideoCaptureOracle::RecordConsumerFeedback(int frame_number,
+                                                double resource_utilization) {
+  if (!std::isfinite(resource_utilization)) {
+    LOG(DFATAL) << "Non-finite utilization provided by consumer for frame #"
+                << frame_number << ": " << resource_utilization;
+    return;
+  }
+  if (resource_utilization <= 0.0)
+    return;  // Non-positive values are normal, meaning N/A.
+
+  if (!IsFrameInRecentHistory(frame_number)) {
+    VLOG(1) << "Very old frame feedback being ignored: frame #" << frame_number;
+    return;
+  }
+  const base::TimeTicks timestamp = GetFrameTimestamp(frame_number);
+
+  // Translate the utilization metric to be in terms of the capable frame area
+  // and update the feedback accumulators.  Research suggests utilization is at
+  // most linearly proportional to area, and typically is sublinear.  Either
+  // way, the end-to-end system should converge to the right place using the
+  // more-conservative assumption (linear).
+  const int area_at_full_utilization =
+      base::saturated_cast<int>(capture_size_.GetArea() / resource_utilization);
+  estimated_capable_area_.Update(area_at_full_utilization, timestamp);
 }
 
 base::TimeTicks VideoCaptureOracle::GetFrameTimestamp(int frame_number) const {
-  DCHECK_LE(frame_number, next_frame_number_);
-  DCHECK_LT(next_frame_number_ - frame_number, kMaxFrameTimestamps);
+  DCHECK(IsFrameInRecentHistory(frame_number));
   return frame_timestamps_[frame_number % kMaxFrameTimestamps];
 }
 
 void VideoCaptureOracle::SetFrameTimestamp(int frame_number,
                                            base::TimeTicks timestamp) {
+  DCHECK(IsFrameInRecentHistory(frame_number));
   frame_timestamps_[frame_number % kMaxFrameTimestamps] = timestamp;
 }
 
+bool VideoCaptureOracle::IsFrameInRecentHistory(int frame_number) const {
+  return ((next_frame_number_ - frame_number) < kMaxFrameTimestamps &&
+          frame_number <= next_frame_number_ &&
+          frame_number >= 0);
+}
+
+void VideoCaptureOracle::CommitCaptureSizeAndReset(
+    base::TimeTicks last_frame_time) {
+  capture_size_ = resolution_chooser_.capture_size();
+  VLOG(2) << "Now proposing a capture size of " << capture_size_.ToString();
+
+  // Reset each short-term feedback accumulator with a stable-state starting
+  // value.
+  const base::TimeTicks ignore_before_time = JustAfter(last_frame_time);
+  buffer_pool_utilization_.Reset(1.0, ignore_before_time);
+  estimated_capable_area_.Reset(capture_size_.GetArea(), ignore_before_time);
+
+  // With the new capture size, erase any prior conclusion about the end-to-end
+  // system being under-utilized.
+  start_time_of_underutilization_ = base::TimeTicks();
+}
+
+void VideoCaptureOracle::AnalyzeAndAdjust(const base::TimeTicks analyze_time) {
+  const int decreased_area = AnalyzeForDecreasedArea(analyze_time);
+  if (decreased_area > 0) {
+    resolution_chooser_.SetTargetFrameArea(decreased_area);
+    return;
+  }
+
+  const int increased_area = AnalyzeForIncreasedArea(analyze_time);
+  if (increased_area > 0) {
+    resolution_chooser_.SetTargetFrameArea(increased_area);
+    return;
+  }
+
+  // Explicitly set the target frame area to the current capture area.  This
+  // cancels-out the results of a previous call to this method, where the
+  // |resolution_chooser_| may have been instructed to increase or decrease the
+  // capture size.  Conditions may have changed since then which indicate no
+  // change should be committed (via CommitCaptureSizeAndReset()).
+  resolution_chooser_.SetTargetFrameArea(capture_size_.GetArea());
+}
+
+int VideoCaptureOracle::AnalyzeForDecreasedArea(base::TimeTicks analyze_time) {
+  const int current_area = capture_size_.GetArea();
+  DCHECK_GT(current_area, 0);
+
+  // Translate the recent-average buffer pool utilization to be in terms of
+  // "capable number of pixels per frame," for an apples-to-apples comparison
+  // below.
+  int buffer_capable_area;
+  if (HasSufficientRecentFeedback(buffer_pool_utilization_, analyze_time) &&
+      buffer_pool_utilization_.current() > 1.0) {
+    // This calculation is hand-wavy, but seems to work well in a variety of
+    // situations.
+    buffer_capable_area = static_cast<int>(
+        current_area / buffer_pool_utilization_.current());
+  } else {
+    buffer_capable_area = current_area;
+  }
+
+  int consumer_capable_area;
+  if (HasSufficientRecentFeedback(estimated_capable_area_, analyze_time)) {
+    consumer_capable_area =
+        base::saturated_cast<int>(estimated_capable_area_.current());
+  } else {
+    consumer_capable_area = current_area;
+  }
+
+  // If either of the "capable areas" is less than the current capture area,
+  // decrease the capture area by AT LEAST one step.
+  int decreased_area = -1;
+  const int capable_area = std::min(buffer_capable_area, consumer_capable_area);
+  if (capable_area < current_area) {
+    decreased_area = std::min(
+        capable_area,
+        resolution_chooser_.FindSmallerFrameSize(current_area, 1).GetArea());
+    start_time_of_underutilization_ = base::TimeTicks();
+    VLOG(2) << "Proposing a "
+            << (100.0 * (current_area - decreased_area) / current_area)
+            << "% decrease in capture area.  :-(";
+  }
+
+  // Always log the capability interpretations at verbose logging level 3.  At
+  // level 2, only log when when proposing a decreased area.
+  VLOG(decreased_area == -1 ? 3 : 2)
+      << "Capability of pool=" << (100.0 * buffer_capable_area / current_area)
+      << "%, consumer=" << (100.0 * consumer_capable_area / current_area)
+      << '%';
+
+  return decreased_area;
+}
+
+int VideoCaptureOracle::AnalyzeForIncreasedArea(base::TimeTicks analyze_time) {
+  // Compute what one step up in capture size/area would be.  If the current
+  // area is already at the maximum, no further analysis is necessary.
+  const int current_area = capture_size_.GetArea();
+  const int increased_area =
+      resolution_chooser_.FindLargerFrameSize(current_area, 1).GetArea();
+  if (increased_area <= current_area)
+    return -1;
+
+  // Determine whether the buffer pool could handle an increase in area.
+  if (!HasSufficientRecentFeedback(buffer_pool_utilization_, analyze_time))
+    return -1;
+  if (buffer_pool_utilization_.current() > 0.0) {
+    const int buffer_capable_area = base::saturated_cast<int>(
+        current_area / buffer_pool_utilization_.current());
+    if (buffer_capable_area < increased_area) {
+      start_time_of_underutilization_ = base::TimeTicks();
+      return -1;  // Buffer pool is not under-utilized.
+    }
+  }
+
+  // Determine whether the consumer could handle an increase in area.
+  if (HasSufficientRecentFeedback(estimated_capable_area_, analyze_time)) {
+    if (estimated_capable_area_.current() < increased_area) {
+      start_time_of_underutilization_ = base::TimeTicks();
+      return -1;  // Consumer is not under-utilized.
+    }
+  } else if (estimated_capable_area_.update_time() ==
+             estimated_capable_area_.reset_time()) {
+    // The consumer does not provide any feedback.  In this case, the consumer's
+    // capability isn't a consideration.
+  } else {
+    // Consumer is providing feedback, but hasn't reported it recently.  Just in
+    // case it's stalled, don't make things worse by increasing the capture
+    // area.
+    start_time_of_underutilization_ = base::TimeTicks();
+    return -1;
+  }
+
+  // At this point, the system is currently under-utilized.  Reset the start
+  // time if the system was not under-utilized when the last analysis was made.
+  if (start_time_of_underutilization_.is_null())
+    start_time_of_underutilization_ = analyze_time;
+
+  // While content is animating, require a "proving period" of contiguous
+  // under-utilization before increasing the capture area.  This will mitigate
+  // the risk of causing frames to be dropped when increasing the load.  If
+  // content is not animating, be aggressive about increasing the capture area,
+  // to improve the quality of non-animating content (where frame drops are not
+  // much of a concern).
+  if ((analyze_time - last_time_animation_was_detected_).InMicroseconds() <
+          kDebouncingPeriodForAnimatedContentMicros) {
+    if ((analyze_time - start_time_of_underutilization_).InMicroseconds() <
+            kProvingPeriodForAnimatedContentMicros) {
+      // Content is animating and the system has not been contiguously
+      // under-utilizated for long enough.
+      return -1;
+    }
+  }
+
+  VLOG(2) << "Proposing a "
+          << (100.0 * (increased_area - current_area) / current_area)
+          << "% increase in capture area.  :-)";
+
+  return increased_area;
+}
+
 }  // namespace media