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

content/browser/download/parallel_download_job.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/download/parallel_download_job.h"
 
 #include "base/memory/ptr_util.h"
 #include "content/browser/download/download_create_info.h"
 #include "content/browser/download/download_stats.h"
 #include "content/browser/download/parallel_download_utils.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/storage_partition.h"
 
 namespace content {
 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(
     DownloadItemImpl* download_item,
     std::unique_ptr<DownloadRequestHandleInterface> request_handle,
     const DownloadCreateInfo& create_info)
     : DownloadJobImpl(download_item, std::move(request_handle)),
       initial_request_offset_(create_info.offset),
       content_length_(create_info.total_bytes),
       requests_sent_(false),
@@ skipping 113 matching lines @@
   // of the workers finish their job.
   DownloadItem::ReceivedSlices slices_to_download =
       FindSlicesToDownload(download_item_->GetReceivedSlices());
 
   DCHECK(!slices_to_download.empty());
   int64_t first_slice_offset = slices_to_download[0].offset;
   DCHECK_LE(initial_request_offset_, first_slice_offset);
 
   // 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());
   DCHECK_EQ(slices_to_download.back().received_bytes,
             DownloadSaveInfo::kLengthFullContent);
 
   ForkSubRequests(slices_to_download);
   RecordParallelDownloadRequestCount(
       static_cast<int>(slices_to_download.size()));
   requests_sent_ = true;
@@ skipping 41 matching lines @@
   // download request.
   download_params->set_referrer(Referrer(download_item_->GetReferrerUrl(),
                                          blink::WebReferrerPolicyAlways));
   // Send the request.
   worker->SendRequest(std::move(download_params));
   DCHECK(workers_.find(offset) == workers_.end());
   workers_[offset] = std::move(worker);
 }
 
 }  // namespace content