[Cast] In Audio/VideoSender, drop frames when too-long a duration is in-flight.

media/cast/sender/audio_sender.cc

 // Copyright 2014 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/cast/sender/audio_sender.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "media/cast/cast_defines.h"
@@ skipping 72 matching lines @@
 
 void AudioSender::InsertAudio(scoped_ptr<AudioBus> audio_bus,
                               const base::TimeTicks& recorded_time) {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
   if (cast_initialization_status_ != STATUS_AUDIO_INITIALIZED) {
     NOTREACHED();
     return;
   }
   DCHECK(audio_encoder_.get()) << "Invalid internal state";
 
-  if (AreTooManyFramesInFlight()) {
+  if (ShouldDropNextFrame(recorded_time)) {
     VLOG(1) << "Dropping frame due to too many frames currently in-flight.";
     return;
   }
 
   audio_encoder_->InsertAudio(audio_bus.Pass(), recorded_time);
 }
 
 void AudioSender::SendEncodedAudioFrame(
     scoped_ptr<EncodedFrame> encoded_frame) {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
 
   const uint32 frame_id = encoded_frame->frame_id;
 
   const bool is_first_frame_to_be_sent = last_send_time_.is_null();
   last_send_time_ = cast_environment_->Clock()->NowTicks();
   last_sent_frame_id_ = frame_id;
   // If this is the first frame about to be sent, fake the value of
   // |latest_acked_frame_id_| to indicate the receiver starts out all caught up.
   // Also, schedule the periodic frame re-send checks.
   if (is_first_frame_to_be_sent) {
     latest_acked_frame_id_ = frame_id - 1;
+    frame_id_to_rtp_timestamp_[latest_acked_frame_id_ & 0xff] =
+        encoded_frame->rtp_timestamp;

[hubbe, 2014/08/28 17:34:45]

why don't we save the reference time here instead?

[miu, 2014/08/28 18:57:58]

We never save the reference time (see line 126 below).  I didn't want to
introduce a second [redundant] frame_id_to_capture_time_ map.

[hubbe, 2014/08/28 19:03:10]

I think another map would be better than translating timestamps to rtp and back.

Also, why are we setting frame_id_to_rtp_timestamp_ in two places?
Wouldn't it be sufficeint to just set it on line 110 or something?

[miu, 2014/08/28 20:37:15]

Done.  This led me to move a few "common members" into FrameSender, and then I
was able to trivially get rid of RtpTimestampHelper.  Moar LOC deleted!  :)
I removed this.  I think, in some earlier code patch, I was afraid of the
possibility of this element getting accessed the first time
ShouldDropNextFrame() was called.

     ScheduleNextResendCheck();
   }
 
   cast_environment_->Logging()->InsertEncodedFrameEvent(
       last_send_time_, FRAME_ENCODED, AUDIO_EVENT, encoded_frame->rtp_timestamp,
       frame_id, static_cast<int>(encoded_frame->data.size()),
       encoded_frame->dependency == EncodedFrame::KEY,
       configured_encoder_bitrate_);
   // Only use lowest 8 bits as key.
   frame_id_to_rtp_timestamp_[frame_id & 0xff] = encoded_frame->rtp_timestamp;
@@ skipping 112 matching lines @@
     std::vector<uint32> cancel_sending_frames;
     while (latest_acked_frame_id_ != cast_feedback.ack_frame_id) {
       latest_acked_frame_id_++;
       cancel_sending_frames.push_back(latest_acked_frame_id_);
     }
     transport_sender_->CancelSendingFrames(ssrc_, cancel_sending_frames);
     latest_acked_frame_id_ = cast_feedback.ack_frame_id;
   }
 }
 
-bool AudioSender::AreTooManyFramesInFlight() const {
+bool AudioSender::ShouldDropNextFrame(base::TimeTicks capture_time) const {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
   int frames_in_flight = 0;
+  base::TimeDelta duration_in_flight;
   if (!last_send_time_.is_null()) {
-    frames_in_flight +=
+    frames_in_flight =
         static_cast<int32>(last_sent_frame_id_ - latest_acked_frame_id_);
+    if (frames_in_flight > 0) {
+      const uint32 oldest_unacked_frame_id = latest_acked_frame_id_ + 1;
+      base::TimeTicks oldest_unacked_capture_time;
+      if (rtp_timestamp_helper_.EstimateTimeTicks(
+              frame_id_to_rtp_timestamp_[oldest_unacked_frame_id & 0xff],
+              &oldest_unacked_capture_time)) {
+        duration_in_flight = capture_time - oldest_unacked_capture_time;
+      }
+    }
   }
   VLOG(2) << frames_in_flight
           << " frames in flight; last sent: " << last_sent_frame_id_
-          << " latest acked: " << latest_acked_frame_id_;
-  return frames_in_flight >= max_unacked_frames_;
+          << "; latest acked: " << latest_acked_frame_id_
+          << "; duration in flight: "
+          << duration_in_flight.InMicroseconds() << " usec ("
+          << (target_playout_delay_ > base::TimeDelta() ?
+                  100 * duration_in_flight / target_playout_delay_ :
+                  kint64max) << "%)";
+  return frames_in_flight >= max_unacked_frames_ ||
+      duration_in_flight >= target_playout_delay_;
 }
 
 void AudioSender::ResendForKickstart() {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
   DCHECK(!last_send_time_.is_null());
   VLOG(1) << "Resending last packet of frame " << last_sent_frame_id_
           << " to kick-start.";
   last_send_time_ = cast_environment_->Clock()->NowTicks();
   transport_sender_->ResendFrameForKickstart(ssrc_, last_sent_frame_id_);
 }
 
 }  // namespace cast
 }  // namespace media