RemoteBitrateEstimatorAbsSendTime起始階段探測帶寬失敗

使用RemoteBitrateEstimatorAbsSendTime的時候發現剛剛開始會出現大量的Probe failed, sent at這樣的打印，然後打印等級是Info（這個等級的打印說明問題不大），但是還是看了一下代碼瞭解一下是什麼原因導致的。這個是因爲收到的包過於密集導致的，出現大量的接收或者發送的包間隔不到1毫秒（ComputeClusters函數）。

• 當且僅當發送間隔和接收間隔都大於等於1毫秒的時候，這個變量num_above_min_delta纔會自加。正確的包都會使變量count自加。
• 當num_above_min_delta的個數小於一半count的個數的時候就會打印一下我們看到的這種log
• 爲什麼會出現這樣的情況呢，具體閱讀平滑發送模塊代碼PacedSender就知道了，這個是因爲一開始堆積的數據量大和起始發送碼率大pacing_bitrate_kbps_，然後單個包較小，單次發送的包個數就多（音頻包的優先級高，會一次性發送所有的音頻包，視頻包會平滑發送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;
}