Implement video frame acknowledgements in the chromoting protocol.

remoting/host/capture_scheduler.cc

 // Copyright (c) 2011 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 "remoting/host/capture_scheduler.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "base/sys_info.h"
 #include "base/time/default_tick_clock.h"
 #include "base/time/time.h"
+#include "remoting/proto/video.pb.h"
 
 namespace {
 
 // Number of samples to average the most recent capture and encode time
 // over.
 const int kStatisticsWindow = 3;
 
 // The hard limit is 30fps or 33ms per recording cycle.
 const int64 kDefaultMinimumIntervalMs = 33;
 
 // Controls how much CPU time we can use for encode and capture.
 // Range of this value is between 0 to 1. 0 means using 0% of of all CPUs
 // available while 1 means using 100% of all CPUs available.
 const double kRecordingCpuConsumption = 0.5;
 
-// Maximum number of frames that can be processed simultaneously.
+// Maximum number of captured frames in the encoding queue. Currently capturer
+// implementations do not allow to keep more than 2 DesktopFrame objects.
+static const int kMaxFramesInEncodingQueue = 2;
+
+// Maximum number of frames that haven't been acknowledged. It's used only when

[Wez, 2015/02/21 03:12:02]

nit: Suggest "Maximum number of unacknowledged frames. Ignored if the client
does not support ACKs."

[Sergey Ulanov, 2015/02/23 17:35:38]

Done.

+// the client support ACKs. This values was chosen experimentally, using

[Wez, 2015/02/21 03:12:02]

nit: "This" doesn't go with "values" - "This value" or "These values"

[Sergey Ulanov, 2015/02/23 17:35:38]

Done.

+// synthetic performance tests (see ProtocolPerfTest), to maximize frame rate,
+// while keeping round-trip latency low.
+static const int kMaxUnacknowledgedFrames = 4;
+
+// Maximum number of frames that can be processed simultaneously. It's used only

[Wez, 2015/02/21 03:12:02]

Need to be clear about where this limit comes from, i.e. what does "processing"
a frame mean? Is it "processed" once we get the send completion, or something
else?

[Sergey Ulanov, 2015/02/23 17:35:38]

This is a legacy value and will need to be removed in the future. Added TODO to
remove it.

+// in the case when the client doesn't support ACKs.
 static const int kMaxPendingFrames = 2;
 
 }  // namespace
 
 namespace remoting {
 
 // We assume that the number of available cores is constant.
 CaptureScheduler::CaptureScheduler(const base::Closure& capture_closure)
     : capture_closure_(capture_closure),
+      acks_supported_(false),
       tick_clock_(new base::DefaultTickClock()),
       capture_timer_(new base::Timer(false, false)),
       minimum_interval_(
           base::TimeDelta::FromMilliseconds(kDefaultMinimumIntervalMs)),
       num_of_processors_(base::SysInfo::NumberOfProcessors()),
       capture_time_(kStatisticsWindow),
       encode_time_(kStatisticsWindow),
-      pending_frames_(0),
+      num_encoding_frames_(0),
+      num_sending_frames_(0),
+      num_unacknowledged_frames_(0),
       capture_pending_(false),
-      is_paused_(false) {
+      is_paused_(false),
+      next_frame_id_(0) {
   DCHECK(num_of_processors_);
 }
 
 CaptureScheduler::~CaptureScheduler() {
 }
 
 void CaptureScheduler::Start() {
   DCHECK(CalledOnValidThread());
 
   ScheduleNextCapture();
@@ skipping 13 matching lines @@
   }
 }
 
 void CaptureScheduler::OnCaptureCompleted() {
   DCHECK(CalledOnValidThread());
 
   capture_pending_ = false;
   capture_time_.Record(
       (tick_clock_->NowTicks() - last_capture_started_time_).InMilliseconds());
 
+  ++num_encoding_frames_;
+
+  ScheduleNextCapture();
+}
+
+void CaptureScheduler::OnFrameEncoded(VideoPacket* packet) {
+  DCHECK(CalledOnValidThread());
+
+  // Set packet_id for the outgoing packet.
+  packet->set_frame_id(next_frame_id_);
+  ++next_frame_id_;
+
+  // Update internal stats.
+  encode_time_.Record(packet->encode_time_ms());
+
+  --num_encoding_frames_;
+  ++num_sending_frames_;
+  ++num_unacknowledged_frames_;
+
   ScheduleNextCapture();
 }
 
 void CaptureScheduler::OnFrameSent() {
   DCHECK(CalledOnValidThread());
 
-  // Decrement the pending capture count.
-  pending_frames_--;
-  DCHECK_GE(pending_frames_, 0);
+  --num_sending_frames_;
+  DCHECK_GE(num_sending_frames_, 0);
 
   ScheduleNextCapture();
 }
 
-void CaptureScheduler::OnFrameEncoded(base::TimeDelta encode_time) {
+void CaptureScheduler::ProcessVideoAck(scoped_ptr<VideoAck> video_ack) {
   DCHECK(CalledOnValidThread());
 
-  encode_time_.Record(encode_time.InMilliseconds());
+  // Host always sets |frame_id| field to indicated that it expects ACK from the
+  // client. It's assumed that the client doesn't support ACKs until the first
+  // ACK message is received.
+  acks_supported_ = true;
+
+  --num_unacknowledged_frames_;
+  DCHECK_GE(num_unacknowledged_frames_, 0);
+
   ScheduleNextCapture();
 }
 
 void CaptureScheduler::SetTickClockForTest(
     scoped_ptr<base::TickClock> tick_clock) {
   tick_clock_ = tick_clock.Pass();
 }
+
 void CaptureScheduler::SetTimerForTest(scoped_ptr<base::Timer> timer) {
   capture_timer_ = timer.Pass();
 }
+
 void CaptureScheduler::SetNumOfProcessorsForTest(int num_of_processors) {
   num_of_processors_ = num_of_processors;
 }
 
 void CaptureScheduler::ScheduleNextCapture() {
   DCHECK(CalledOnValidThread());
 
-  if (is_paused_ || pending_frames_ >= kMaxPendingFrames || capture_pending_)
+  if (is_paused_ || capture_pending_ ||
+      num_encoding_frames_ >= kMaxFramesInEncodingQueue) {
     return;
+  }
+
+  if (acks_supported_) {
+    if (num_encoding_frames_ + num_unacknowledged_frames_ >=
+        kMaxUnacknowledgedFrames) {
+      return;
+    }
+  } else {
+    if (num_encoding_frames_ + num_sending_frames_ >= kMaxPendingFrames) {
+      return;
+    }
+  }
 
   // Delay by an amount chosen such that if capture and encode times
   // continue to follow the averages, then we'll consume the target
   // fraction of CPU across all cores.
   base::TimeDelta delay =
       std::max(minimum_interval_,
                base::TimeDelta::FromMilliseconds(
                    (capture_time_.Average() + encode_time_.Average()) /
                    (kRecordingCpuConsumption * num_of_processors_)));
 
   // Account for the time that has passed since the last capture.
   delay = std::max(base::TimeDelta(), delay - (tick_clock_->NowTicks() -
                                                last_capture_started_time_));
 
   capture_timer_->Start(
       FROM_HERE, delay,
       base::Bind(&CaptureScheduler::CaptureNextFrame, base::Unretained(this)));
 }
 
 void CaptureScheduler::CaptureNextFrame() {
   DCHECK(CalledOnValidThread());
   DCHECK(!is_paused_);
   DCHECK(!capture_pending_);
 
-  pending_frames_++;
-  DCHECK_LE(pending_frames_, kMaxPendingFrames);
-
   capture_pending_ = true;
   last_capture_started_time_ = tick_clock_->NowTicks();
   capture_closure_.Run();
 }
 
 }  // namespace remoting