Use all available workers for concurrent marking.

R=ulan@chromium.org

Bug: chromium:812178
Change-Id: I35a727cb6c663bbd5f1beab98324e5d1b1ecf5c7
Reviewed-on: https://chromium-review.googlesource.com/918663
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51458}

src/heap/concurrent-marking.cc

@@ -506,14 +506,10 @@ void ConcurrentMarking::ScheduleTasks() {
   base::LockGuard<base::Mutex> guard(&pending_lock_);
   DCHECK_EQ(0, pending_task_count_);
   if (task_count_ == 0) {
-    // TODO(ulan): Increase the number of tasks for platforms that benefit
-    // from it.
-    task_count_ =
-        (static_cast<int>(
-             V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()) +
-         1) /
-        2;
-    task_count_ = Max(Min(task_count_, kMaxTasks), 1);
+    task_count_ = Max(
+        1, Min(kMaxTasks,
+               static_cast<int>(V8::GetCurrentPlatform()
+                                    ->NumberOfAvailableBackgroundThreads())));
   }
   // Task id 0 is for the main thread.
   for (int i = 1; i <= task_count_; i++) {

src/heap/concurrent-marking.h

@@ -53,7 +53,7 @@ class ConcurrentMarking {
     COMPLETE_TASKS_FOR_TESTING,
   };
 
-  static constexpr int kMaxTasks = 4;
+  static constexpr int kMaxTasks = 8;
   using MarkingWorklist = Worklist<HeapObject*, 64 /* segment size */>;
 
   ConcurrentMarking(Heap* heap, MarkingWorklist* shared,