Frame Scheduling

API and Timing Model

Frame Scheduler Development

Life of a Pixel shows how a client Present request is integrated into a Scenic frame.

Present2 Best Practices Examples

These examples show how the API can be used for different use cases. The examples assume that clients using the API have listeners registered for the OnFramePresented and OnScenicEvent event callbacks and that session is an initialized Scenic Session channel. For examples of how to set up scenic, see Scenic examples.

Example 1

The simplest type of application creates and presents a new update every time a previous one has been presented. This is reasonable for applications with small workloads (takes less than a frame to create a frame) and no reqirements to minimize latency.


void main() {
  PresentNewFrame();
}

void PresentNewFrame() {
  // Create a new update and enquee it.
  CreateAndEnqueueNewUpdate();

  // Flush enqueued events.
  Present2Args args;
  args.set_requested_presentation_time(0);
  args.set_acquire_fences({});
  args.set_release_fences({});
  args.set_requested_prediction_span(0);
  session->Present2(std::move(args), /*callback=*/[]{});
}

void OnFramePresented(...) {
  PresentNewFrame();
}

void CreateAndEnqueueNewUpdate() {
   // Enqueue commands to update the session
}


Example 2

This example demonstrates how to write an application with small input-driven updates and, where minimizing latency is important. It creates a new small update upon receiving an input and immediately calls Present2 attempting to keep latency low as possible. This approach should only be used for very small workloads since it creates some unnecessary work by not batching update creation.


int64 num_calls_left_ = 0;
bool update_pending_ = false;

main() {
  session->RequestPresentationTimes(/*requested_prediction_span=*/0,
                     /*callback=*/[this](FuturePresentationTimes future_times){
    UpdateNumCallsLeft(future_times.remaining_presents_in_flight_allowed);
  };}
}

void UpdateNumCallsLeft(int64_t num_calls_left){
  num_calls_left_ = num_calls_left;
}

void OnScenicEvent(Event event) {
  if (IsInputEvent(event)) {
    CreateAndEnqueueUpdate(std::move(event));
    if (num_calls_left_ > 0) {
      PresentFrame();
    } else {
      update_pending_ = true;
    }
  }
}

void PresentFrame() {
  --num_calls_left_;
  update_pending_ = false;

  Present2Args args;
  args.set_requested_presentation_time(0);
  args.set_acquire_fences({});
  args.set_release_fences({});
  args.set_requested_prediction_span(0);
  session->Present2(std::move(args),
                    /*callback=*/[this](FuturePresentationTimes future_times){
    UpdateNumCallsLeft(future_times.remaining_presents_in_flight_allowed);
  };);
}

void OnFramePresented(FramePresentedInfo info) {
    UpdateNumCallsLeft(info.num_presents_allowed);
  if (frame_pending_ && num_calls_left_ > 0) {
     PresentFrame();
  }
}


Example 3

This example demonstrates how to write an input-driven application that batches inputs.


struct TargetTimes {
  zx_time_t latch_point;
  zx_time_t presentation_time;
};

int64 num_calls_left_ = 0;
bool update_pending_ = false;
bool frame_pending_ = false;
zx_time_t last_targeted_time_ = 0;
std::vector<Event> unhandled_input_events_;
async_dispatcher dispatcher_;

void UpdateNumCallsLeft(int64_t num_calls_left){
  num_calls_left_ = num_calls_left;
}

zx_duration_t UpdateCreationTime() {
  // Return a prediction for how long an update could take to create.
}

TargetTimes FindNextPresentationTime(
                      std::vector<PresentationInfo> future_presentations) {
  // Select the next future time to target.
  zx_time_t now = time.Now();
  for(auto times : future_presentations) {
    if (times.latch_point > now + UpdateCreationTime()
        && times.presentation_time > last_targeted_time_) {
      return {times.latch_point, times.presentation_time};
    }
  }

  // This should never be reached.
  return {now, now};
}

void CreateAndEnqueueNewUpdate(std::vector<Event> input_events) {
   // Enqueue commands to update the session.
}

void OnScenicEvent(Event event) {
  if (IsInputEvent(event)) {
    unhandled_input_events_.push_back(std::move(event));
    RequestNewFrame();
  }
}

void RequestNewFrame() {
  if (update_pending_) {
    return;
  } else {
    update_pending_ = true;
    ScheduleNextFrame();
  }
}

void PresentFrame() {
  present_pending_ = false;

  session->RequestPresentationTimes(/*requested_prediction_span=*/0,
                    /*callback=*/[this](FuturePresentationTimes future_times){
    if (future_times.remaining_presents_in_flight_allowed > 0) {
      // No present calls left. Need to wait to be returned some by previous
      // frames being completed in OnFramePresented(). This could happen when
      // Scenic gets overwhelmed or stalled for some reason.
      present_pending_ = true;
      return;
    }

    TargetTimes target_times =
          FindNextPresentationTime(future_times.future_presentations);
    last_targeted_time_ = target_time.presentation_time;


    // Wait until slightly before the deadline to start creating the update.
    zx_time_t wakeup_time = target_times.latch_point - UpdateCreationTime();
    async::PostTaskForTime(
      dispatcher_,
      [this, presentation_time] {
        update_pending_ = false;
        present_pending_ = false;

        CreateAndEnqueueUpdate(std::move(unhandled_input_events_));

        Present2Args args;
        // We subtract a bit from our requested time (1 ms in this example)
        // for two reasons:
        // 1. Future presentation times aren't guaranteed to be entirely
        // accurate due to hardware vsync drift and other factors.
        // 2. A presetnt call is guaranteed to be presented "at or later than"
        // the requested presentation time.
        // This guards against against `Present2` calls getting accidentally
        // delayed for an entire frame.
        args.set_requested_presentation_time(
              target_times.presentation_time - 1'000'000);
        args.set_acquire_fences({});
        args.set_release_fences({});
        args.set_requested_prediction_span(0);
        session->Present2(std::move(args), /*callback=*/[]{};);
      },
      wakeup_time);
  };}
}

void OnFramePresented(FramePresentedInfo info) {
  if (frame_pending_ && info.num_presents_allowed > 0) {
     PresentFrame();
  }
}