Add the metric to record percentage improvement for parallel download.

content/browser/download/download_stats.cc

Index: content/browser/download/download_stats.cc
diff --git a/content/browser/download/download_stats.cc b/content/browser/download/download_stats.cc
index 80be40f4e3e4849ac5ddbd5696f55f516f334690..87561929d2974eacbfc09c62bef95d089d35be96 100644
--- a/content/browser/download/download_stats.cc
+++ b/content/browser/download/download_stats.cc
@@ -818,11 +818,32 @@ void RecordParallelizableDownloadStats(
     return;
 
   if (bytes_downloaded_with_parallel_streams > 0) {
+    int64_t bandwidth_with_parallel_streams = CalculateBandwidthBytesPerSecond(
+        bytes_downloaded_with_parallel_streams, time_with_parallel_streams);
     RecordBandwidthMetric(
         "Download.ParallelizableDownloadBandwidth."
         "WithParallelRequestsMultipleStreams",
-        CalculateBandwidthBytesPerSecond(bytes_downloaded_with_parallel_streams,
-                                         time_with_parallel_streams));
+        bandwidth_with_parallel_streams);
+
+    if (bandwidth_without_parallel_streams) {
+      int bandwidth_ratio_percentage =
+          (100.0 * bandwidth_with_parallel_streams) /
+          bandwidth_without_parallel_streams;
+      UMA_HISTOGRAM_CUSTOM_COUNTS(
+          "Download.ParallelDownload.BandwidthRatioPercentage",
+          bandwidth_ratio_percentage, 0, 400, 101);
+      base::TimeDelta total_time =
+          time_with_parallel_streams + time_without_parallel_streams;
+      size_t total_size = bytes_downloaded_with_parallel_streams +
+                          bytes_downloaded_without_parallel_streams;
+      base::TimeDelta improved_time = base::TimeDelta::FromSecondsD(
+          static_cast<double>(total_size) / bandwidth_with_parallel_streams);
+      int time_ratio_percentage =

[qinmin, 2017/05/10 18:46:47]

This doesn't seem right.
When we say that parallel download improves the download time, shouldn't we
compare actual download time with the theoretical download time without parallel
requests?

So it should be total_time/(total_size/bandwidth_without_parallel_streams)?

[xingliu, 2017/05/10 20:08:06]

Done. Makes sense.

+          100.0 * improved_time.InSecondsF() / total_time.InSecondsF();
+      UMA_HISTOGRAM_CUSTOM_COUNTS(
+          "Download.ParallelDownload.TotalTimeRatioPercentage",
+          time_ratio_percentage, 0, 200, 101);
+    }
   }
 
   base::TimeDelta time_saved;