Make sweeper threads respect the max_available_threads constraint.

BUG=

Review URL: https://codereview.chromium.org/916103005

Cr-Commit-Position: refs/heads/master@{#27091}

src/heap/heap.cc

@@ -142,7 +142,8 @@ Heap::Heap()
       external_string_table_(this),
       chunks_queued_for_free_(NULL),
       gc_callbacks_depth_(0),
-      deserialization_complete_(false) {
+      deserialization_complete_(false),
+      concurrent_sweeping_enabled_(false) {
 // Allow build-time customization of the max semispace size. Building
 // V8 with snapshots and a non-default max semispace size is much
 // easier if you can define it as part of the build environment.
@@ -5447,6 +5448,9 @@ bool Heap::SetUp() {
     if (!ConfigureHeapDefault()) return false;
   }
 
+  concurrent_sweeping_enabled_ =
+      FLAG_concurrent_sweeping && isolate_->max_available_threads() > 1;
+
   base::CallOnce(&initialize_gc_once, &InitializeGCOnce);
 
   MarkMapPointersAsEncoded(false);

src/heap/heap.h

@@ -1316,6 +1316,8 @@ class Heap {
   // Returns the current sweep generation.
   int sweep_generation() { return sweep_generation_; }
 
+  bool concurrent_sweeping_enabled() { return concurrent_sweeping_enabled_; }
+
   inline Isolate* isolate();
 
   void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
@@ -2131,6 +2133,8 @@ class Heap {
 
   bool deserialization_complete_;
 
+  bool concurrent_sweeping_enabled_;
+
   friend class AlwaysAllocateScope;
   friend class Deserializer;
   friend class Factory;

src/heap/incremental-marking.cc

@@ -935,7 +935,7 @@ intptr_t IncrementalMarking::Step(intptr_t allocated_bytes,
     if (state_ == SWEEPING) {
       if (heap_->mark_compact_collector()->sweeping_in_progress() &&
           (heap_->mark_compact_collector()->IsSweepingCompleted() ||
-           !FLAG_concurrent_sweeping)) {
+           !heap_->concurrent_sweeping_enabled())) {
         heap_->mark_compact_collector()->EnsureSweepingCompleted();
       }
       if (!heap_->mark_compact_collector()->sweeping_in_progress()) {

src/heap/mark-compact.cc

@@ -481,12 +481,12 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
 
   // If sweeping is not completed or not running at all, we try to complete it
   // here.
-  if (!FLAG_concurrent_sweeping || !IsSweepingCompleted()) {
+  if (!heap()->concurrent_sweeping_enabled() || !IsSweepingCompleted()) {
     SweepInParallel(heap()->paged_space(OLD_DATA_SPACE), 0);
     SweepInParallel(heap()->paged_space(OLD_POINTER_SPACE), 0);
   }
   // Wait twice for both jobs.
-  if (FLAG_concurrent_sweeping) {
+  if (heap()->concurrent_sweeping_enabled()) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
@@ -4371,7 +4371,7 @@ void MarkCompactCollector::SweepSpaces() {
       SweepSpace(heap()->old_data_space(), CONCURRENT_SWEEPING);
     }
     sweeping_in_progress_ = true;
-    if (FLAG_concurrent_sweeping) {
+    if (heap()->concurrent_sweeping_enabled()) {
       StartSweeperThreads();
     }
   }