RemoteBitrateEstimatorAbsSendTime起始阶段探测带宽失败

使用RemoteBitrateEstimatorAbsSendTime的时候发现刚刚开始会出现大量的Probe failed, sent at这样的打印，然后打印等级是Info（这个等级的打印说明问题不大），但是还是看了一下代码了解一下是什么原因导致的。这个是因为收到的包过于密集导致的，出现大量的接收或者发送的包间隔不到1毫秒（ComputeClusters函数）。

• 当且仅当发送间隔和接收间隔都大于等于1毫秒的时候，这个变量num_above_min_delta才会自加。正确的包都会使变量count自加。
• 当num_above_min_delta的个数小于一半count的个数的时候就会打印一下我们看到的这种log
• 为什么会出现这样的情况呢，具体阅读平滑发送模块代码PacedSender就知道了，这个是因为一开始堆积的数据量大和起始发送码率大pacing_bitrate_kbps_，然后单个包较小，单次发送的包个数就多（音频包的优先级高，会一次性发送所有的音频包，视频包会平滑发送PacedSender::SendPacket）

void RemoteBitrateEstimatorAbsSendTime::ComputeClusters(
    std::list<Cluster>* clusters) const {
  Cluster current;
  int64_t prev_send_time = -1;
  int64_t prev_recv_time = -1;
  for (std::list<Probe>::const_iterator it = probes_.begin();
       it != probes_.end();
       ++it) {
    if (prev_send_time >= 0) {
      int send_delta_ms = it->send_time_ms - prev_send_time;
      int recv_delta_ms = it->recv_time_ms - prev_recv_time;
      if (send_delta_ms >= 1 && recv_delta_ms >= 1) {
        ++current.num_above_min_delta;
      }
      if (!IsWithinClusterBounds(send_delta_ms, current)) {
        if (current.count >= kMinClusterSize)
          AddCluster(clusters, &current);
        current = Cluster();
      }
      current.send_mean_ms += send_delta_ms;
      current.recv_mean_ms += recv_delta_ms;
      current.mean_size += it->payload_size;
      ++current.count;
    }
    prev_send_time = it->send_time_ms;
    prev_recv_time = it->recv_time_ms;
  }
  if (current.count >= kMinClusterSize)
    AddCluster(clusters, &current);
}

std::list<Cluster>::const_iterator
RemoteBitrateEstimatorAbsSendTime::FindBestProbe(
    const std::list<Cluster>& clusters) const {
  int highest_probe_bitrate_bps = 0;
  std::list<Cluster>::const_iterator best_it = clusters.end();
  for (std::list<Cluster>::const_iterator it = clusters.begin();
       it != clusters.end();
       ++it) {
    if (it->send_mean_ms == 0 || it->recv_mean_ms == 0)
      continue;
    if (it->num_above_min_delta > it->count / 2 &&
        (it->recv_mean_ms - it->send_mean_ms <= 2.0f &&
         it->send_mean_ms - it->recv_mean_ms <= 5.0f)) {
      int probe_bitrate_bps =
          std::min(it->GetSendBitrateBps(), it->GetRecvBitrateBps());
      if (probe_bitrate_bps > highest_probe_bitrate_bps) {
        highest_probe_bitrate_bps = probe_bitrate_bps;
        best_it = it;
      }
    } else {
      int send_bitrate_bps = it->mean_size * 8 * 1000 / it->send_mean_ms;
      int recv_bitrate_bps = it->mean_size * 8 * 1000 / it->recv_mean_ms;
      LOG(LS_INFO) << "Probe failed, sent at " << send_bitrate_bps
                   << " bps, received at " << recv_bitrate_bps
                   << " bps. Mean send delta: " << it->send_mean_ms
                   << " ms, mean recv delta: " << it->recv_mean_ms
                   << " ms, num probes: " << it->count;
      break;
    }
  }
  return best_it;
}