[heap] Limit the number of parallel evacuation tasks near OOM.

Launching too many parallel tasks near OOM increases risk of allocation
failure during GC and OOM crash.

Bug: chromium:824214, v8:7605
Change-Id: I336d1f01e4005fb2a8e16ef92f40532b8ed83f2c
TBR: mlippautz@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/983919
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52282}

src/heap/heap.cc

@@ -1997,7 +1997,13 @@ int Heap::NumberOfScavengeTasks() {
   const int num_scavenge_tasks =
       static_cast<int>(new_space()->TotalCapacity()) / MB;
   static int num_cores = V8::GetCurrentPlatform()->NumberOfWorkerThreads() + 1;
-  return Max(1, Min(Min(num_scavenge_tasks, kMaxScavengerTasks), num_cores));
+  int tasks =
+      Max(1, Min(Min(num_scavenge_tasks, kMaxScavengerTasks), num_cores));
+  if (!CanExpandOldGeneration(static_cast<size_t>(tasks * Page::kPageSize))) {
+    // Optimize for memory usage near the heap limit.
+    tasks = 1;
+  }
+  return tasks;
 }
 
 void Heap::Scavenge() {

src/heap/mark-compact.cc

@@ -473,7 +473,14 @@ int NumberOfAvailableCores() {
 
 int MarkCompactCollectorBase::NumberOfParallelCompactionTasks(int pages) {
   DCHECK_GT(pages, 0);
-  return FLAG_parallel_compaction ? Min(NumberOfAvailableCores(), pages) : 1;
+  int tasks =
+      FLAG_parallel_compaction ? Min(NumberOfAvailableCores(), pages) : 1;
+  if (!heap_->CanExpandOldGeneration(
+          static_cast<size_t>(tasks * Page::kPageSize))) {
+    // Optimize for memory usage near the heap limit.
+    tasks = 1;
+  }
+  return tasks;
 }
 
 int MarkCompactCollectorBase::NumberOfParallelPointerUpdateTasks(int pages,