Don't create parallel request if download is about to complete

content/browser/download/parallel_download_job.cc

Index: content/browser/download/parallel_download_job.cc
diff --git a/content/browser/download/parallel_download_job.cc b/content/browser/download/parallel_download_job.cc
index 2b4ae47d85a7f6ea39e4ca7009779c00497d2bb6..46e58ea058fc22486a086f7c611b1a1261561318 100644
--- a/content/browser/download/parallel_download_job.cc
+++ b/content/browser/download/parallel_download_job.cc
@@ -16,6 +16,10 @@ namespace {
 
 const int kVerboseLevel = 1;
 
+// The minimum remaining download time for parallel request creation.
+// TODO(qinmin): make this finch configurable.
+const int kMinimumRemainingTimeInSecondsForParallelRequestCreation = 10;
+
 }  // namespace
 
 ParallelDownloadJob::ParallelDownloadJob(
@@ -149,15 +153,23 @@ void ParallelDownloadJob::BuildParallelRequests() {
 
   // Create more slices for a new download. The initial request may generate
   // a received slice.
-  if (slices_to_download.size() <= 1 &&
-      initial_request_offset_ <= first_slice_offset) {
-    // TODO(qinmin): Check the size of the last slice. If it is huge, we can
-    // split it into N pieces and pass the last N-1 pieces to different workers.
-    // Otherwise, just fork |slices_to_download.size()| number of workers.
-    slices_to_download = FindSlicesForRemainingContent(
-        first_slice_offset,
-        content_length_ - first_slice_offset + initial_request_offset_,
-        GetParallelRequestCount(), GetMinSliceSize());
+  if (slices_to_download.size() <= 1 && download_item_->GetTotalBytes() > 0) {
+    int64_t current_speed = download_item_->CurrentSpeed() == 0

[David Trainor- moved to gerrit, 2017/04/10 20:10:00]

std::max(1, download_item_->CurrentSpeed())?  Also current_bytes_per_second?

[qinmin, 2017/04/10 21:43:39]

Done.

+                                ? 1
+                                : download_item_->CurrentSpeed();
+    int64_t remaining_bytes =
+        download_item_->GetTotalBytes() - download_item_->GetReceivedBytes();
+    if (remaining_bytes / current_speed >

[David Trainor- moved to gerrit, 2017/04/10 20:10:00]

Do we want an UMA stat for when we decide/don't decide to do this?  Maybe even a
histogram of typical seconds left?

[qinmin, 2017/04/10 21:43:39]

will add the decion about parallel download creation as an enum after xingliu@'s
change is submitted.

Added a metric to record the remaining time.

+        kMinimumRemainingTimeInSecondsForParallelRequestCreation) {
+      // TODO(qinmin): Check the size of the last slice. If it is huge, we can
+      // split it into N pieces and pass the last N-1 pieces to different
+      // workers. Otherwise, just fork |slices_to_download.size()| number of
+      // workers.
+      slices_to_download = FindSlicesForRemainingContent(
+          first_slice_offset,
+          content_length_ - first_slice_offset + initial_request_offset_,
+          GetParallelRequestCount(), GetMinSliceSize());
+    }
   }
 
   DCHECK(!slices_to_download.empty());