[heap] Spawn parallel scavenging task per page in the from space

This makes the heuristic for computing the number of parallel tasks
in Scavenger consistent with that in Mark-Compactor.

The patch helps mobile devices where even 1 MB new space can take
10ms to scavenge.

Change-Id: I979de5e8485b93808ea079af2756f53d9b720e10
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1685612Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62566}

src/heap/scavenger.cc

@@ -362,11 +362,9 @@ void ScavengerCollector::MergeSurvivingNewLargeObjects(
 
 int ScavengerCollector::NumberOfScavengeTasks() {
   if (!FLAG_parallel_scavenge) return 1;
-  const int num_scavenge_tasks =
-      static_cast<int>(heap_->new_space()->TotalCapacity()) / MB;
-  static int num_cores = V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
-  int tasks =
-      Max(1, Min(Min(num_scavenge_tasks, kMaxScavengerTasks), num_cores));
+  const int num_pages = heap_->new_space()->from_space().pages_used();
+  const int num_cores = V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
+  int tasks = Max(1, Min(Min(num_pages, kMaxScavengerTasks), num_cores));
   if (!heap_->CanExpandOldGeneration(
           static_cast<size_t>(tasks * Page::kPageSize))) {
     // Optimize for memory usage near the heap limit.

src/heap/spaces.cc

@@ -2826,6 +2826,7 @@ void SemiSpace::Swap(SemiSpace* from, SemiSpace* to) {
   std::swap(from->current_page_, to->current_page_);
   std::swap(from->external_backing_store_bytes_,
             to->external_backing_store_bytes_);
+  std::swap(from->pages_used_, to->pages_used_);
 
   to->FixPagesFlags(saved_to_space_flags, Page::kCopyOnFlipFlagsMask);
   from->FixPagesFlags(0, 0);