Tweak compaction candidate selection to avoid keeping page with low occupancy around.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index e9cbb52ba94e654270ce59b6b99f81420b76a839..67818d261eb21722cd41b0bff7d68a3939349200 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1025,23 +1025,28 @@ class Heap {
   // Performs garbage collection operation.
   // Returns whether there is a chance that another major GC could
   // collect more garbage.
-  bool CollectGarbage(AllocationSpace space, GarbageCollector collector);
+  bool CollectGarbage(AllocationSpace space,
+                      GarbageCollector collector,
+                      const char* gc_reason,
+                      const char* collector_reason);
 
   // Performs garbage collection operation.
   // Returns whether there is a chance that another major GC could
   // collect more garbage.
-  inline bool CollectGarbage(AllocationSpace space);
+  inline bool CollectGarbage(AllocationSpace space,
+                             const char* gc_reason = NULL);
 
   static const int kNoGCFlags = 0;
   static const int kMakeHeapIterableMask = 1;
+  static const int kReduceMemoryFootprintMask = 2;
 
   // Performs a full garbage collection.  If (flags & kMakeHeapIterableMask) is
   // non-zero, then the slower precise sweeper is used, which leaves the heap
   // in a state where we can iterate over the heap visiting all objects.
-  void CollectAllGarbage(int flags);
+  void CollectAllGarbage(int flags, const char* gc_reason = NULL);
 
   // Last hope GC, should try to squeeze as much as possible.
-  void CollectAllAvailableGarbage();
+  void CollectAllAvailableGarbage(const char* gc_reason = NULL);
 
   // Check whether the heap is currently iterable.
   bool IsHeapIterable();
@@ -1741,7 +1746,8 @@ class Heap {
   }
 
   // Checks whether a global GC is necessary
-  GarbageCollector SelectGarbageCollector(AllocationSpace space);
+  GarbageCollector SelectGarbageCollector(AllocationSpace space,
+                                          const char** reason);
 
   // Performs garbage collection
   // Returns whether there is a chance another major GC could
@@ -2364,7 +2370,9 @@ class GCTracer BASE_EMBEDDED {
     double start_time_;
   };
 
-  explicit GCTracer(Heap* heap);
+  explicit GCTracer(Heap* heap,
+                    const char* gc_reason,
+                    const char* collector_reason);
   ~GCTracer();
 
   // Sets the collector.
@@ -2387,9 +2395,17 @@ class GCTracer BASE_EMBEDDED {
   // Returns size of object in heap (in MB).
   inline double SizeOfHeapObjects();
 
-  double start_time_;  // Timestamp set in the constructor.
-  intptr_t start_size_;  // Size of objects in heap set in constructor.
-  GarbageCollector collector_;  // Type of collector.
+  // Timestamp set in the constructor.
+  double start_time_;
+
+  // Size of objects in heap set in constructor.
+  intptr_t start_object_size_;
+
+  // Size of memory allocated from OS set in constructor.
+  intptr_t start_memory_size_;
+
+  // Type of collector.
+  GarbageCollector collector_;
 
   // A count (including this one, e.g. the first collection is 1) of the
   // number of garbage collections.
@@ -2424,6 +2440,9 @@ class GCTracer BASE_EMBEDDED {
   double steps_took_since_last_gc_;
 
   Heap* heap_;
+
+  const char* gc_reason_;
+  const char* collector_reason_;
 };