Reduce CPU usage of download shelf UI. (both MD and pre-MD shelves)

chrome/browser/ui/views/download/download_item_view.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/views/download/download_item_view.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <vector>
@@ skipping 223 matching lines @@
   if (sampling_event_.get())
     sampling_event_->CreateUserDecisionEvent(ExperienceSamplingEvent::kIgnore);
 }
 
 // Progress animation handlers.
 
 void DownloadItemView::StartDownloadProgress() {
   if (progress_timer_.IsRunning())
     return;
   progress_start_time_ = base::TimeTicks::Now();
-  progress_timer_.Start(
-      FROM_HERE,
-      base::TimeDelta::FromMilliseconds(DownloadShelf::kProgressRateMs),
-      base::Bind(&DownloadItemView::SchedulePaint, base::Unretained(this)));
+  progress_timer_.Start(FROM_HERE, base::TimeDelta::FromMilliseconds(
+                                       DownloadShelf::kProgressRateMs),
+                        base::Bind(&DownloadItemView::ProgressTimerFired,
+                                   base::Unretained(this)));
 }
 
 void DownloadItemView::StopDownloadProgress() {
   if (!progress_timer_.IsRunning())
     return;
   previous_progress_elapsed_ += base::TimeTicks::Now() - progress_start_time_;
   progress_start_time_ = base::TimeTicks();
   progress_timer_.Stop();
 }
 
@@ skipping 12 matching lines @@
   if (!model_.ShouldShowInShelf()) {
     shelf_->RemoveDownloadView(this);  // This will delete us!
     return;
   }
 
   if (IsShowingWarningDialog() && !model_.IsDangerous()) {
     // We have been approved.
     ClearWarningDialog();
   } else if (!IsShowingWarningDialog() && model_.IsDangerous()) {
     ShowWarningDialog();
     // Force the shelf to layout again as our size has changed.

[sky, 2016/01/06 21:47:21]

If you hit this, don't you need to SchedulePaint on the shelf?

[Evan Stade, 2016/01/07 00:06:05]

Indeed. ClearWarningDialog() does this but ShowWarningDialog() does not, for
reasons unknown. I've refactored a bit to share more code (and repeat things
like UpdateAccessibleName() less often).

     shelf_->Layout();
     SchedulePaint();
   } else {
     base::string16 status_text = model_.GetStatusText();
     switch (download()->GetState()) {
       case DownloadItem::IN_PROGRESS:
         download()->IsPaused() ?
             StopDownloadProgress() : StartDownloadProgress();
+        SchedulePaint();
         LoadIconIfItemPathChanged();
         break;
       case DownloadItem::INTERRUPTED:
         StopDownloadProgress();
         complete_animation_.reset(new gfx::SlideAnimation(this));
         complete_animation_->SetSlideDuration(kInterruptedAnimationDurationMs);
         complete_animation_->SetTweenType(gfx::Tween::LINEAR);
         complete_animation_->Show();
         SchedulePaint();
         LoadIcon();
@@ skipping 23 matching lines @@
     status_text_ = status_text;
   }
 
   base::string16 new_tip = model_.GetTooltipText(font_list_, kTooltipMaxWidth);
   if (new_tip != tooltip_text_) {
     tooltip_text_ = new_tip;
     TooltipTextChanged();
   }
 
   UpdateAccessibleName();
-
-  // We use the parent's (DownloadShelfView's) SchedulePaint, since there
-  // are spaces between each DownloadItemView that the parent is responsible
-  // for painting.
-  shelf_->SchedulePaint();
 }
 
 void DownloadItemView::OnDownloadDestroyed(DownloadItem* download) {
   shelf_->RemoveDownloadView(this);  // This will delete us!
 }
 
 void DownloadItemView::OnDownloadOpened(DownloadItem* download) {
   disabled_while_opening_ = true;
   SetEnabled(false);
   base::MessageLoop::current()->task_runner()->PostDelayedTask(
@@ skipping 1044 matching lines @@
 void DownloadItemView::AnimateStateTransition(State from, State to,
                                               gfx::SlideAnimation* animation) {
   if (from == NORMAL && to == HOT) {
     animation->Show();
   } else if (from == HOT && to == NORMAL) {
     animation->Hide();
   } else if (from != to) {
     animation->Reset((to == HOT) ? 1.0 : 0.0);
   }
 }
+
+void DownloadItemView::ProgressTimerFired() {
+  // Only repaint for the indeterminate size case. Otherwise, we'll repaint only
+  // when there's an update notified via OnDownloadUpdated().
+  if (model_.PercentComplete() < 0)
+    SchedulePaint();

[Evan Stade, 2016/01/06 19:34:10]

I reverted the change for indeterminate size downloads, but CPU usage is pretty
high again in that case.

I tried only scheduling a paint for the progress rect but it didn't seem to make
a difference. Further down the line I want to try painting the progress
indicator on its own layer and only invalidating that layer, but that will
require more investigation so I'll leave it for a later date.

If we can't find a way to optimize this animation, I would contend we should
change the animation to something that looks good at 2fps because in its current
state it seems wasteful of CPU (and battery/electricity) as no actual
information is being conveyed at 30fps that isn't conveyed at 2fps.

[asanka, 2016/01/06 19:44:28]

Understood. I agree that we aren't conveying 30fps of information and I think we
could  increase kProgressRateMs substantially without anyone noticing. But I
think our cutoff would be less than 500ms.

This change addresses my concern that we were depending on the byte count update
timer frequency for a UI animation. I'll defer to others on what a good
animation rate is.

+}