Only sweep one page eagerly unless we are running out of space.

src/spaces.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1539 matching lines @@
 
   // Evacuation candidates are swept by evacuator.  Needs to return a valid
   // result before _and_ after evacuation has finished.
   static bool ShouldBeSweptLazily(Page* p) {
     return !p->IsEvacuationCandidate() &&
            !p->IsFlagSet(Page::RESCAN_ON_EVACUATION) &&
            !p->WasSweptPrecisely();
   }
 
   void SetPagesToSweep(Page* first) {
+    if (first == &anchor_) first = NULL;
     first_unswept_page_ = first;
   }
 
   bool AdvanceSweeper(intptr_t bytes_to_sweep);
 
   bool IsSweepingComplete() {
     return !first_unswept_page_->is_valid();
   }
 
   Page* FirstPage() { return anchor_.next_page(); }
   Page* LastPage() { return anchor_.prev_page(); }
 
-  bool IsFragmented(Page* p) {
+  // Returns zero for pages that have so little fragmentation that it is not
+  // worth defragmenting them.  Otherwise a positive integer that gives an
+  // estimate of fragmentation on an arbitrary scale.
+  int Fragmentation(Page* p) {
     FreeList::SizeStats sizes;
     free_list_.CountFreeListItems(p, &sizes);
 
     intptr_t ratio;
     intptr_t ratio_threshold;
     if (identity() == CODE_SPACE) {
       ratio = (sizes.medium_size_ * 10 + sizes.large_size_ * 2) * 100 /
           Page::kObjectAreaSize;
       ratio_threshold = 10;
     } else {
@@ skipping 14 matching lines @@
                  Page::kObjectAreaSize,
              static_cast<int>(sizes.large_size_),
              static_cast<double>(sizes.large_size_ * 100) /
                  Page::kObjectAreaSize,
              static_cast<int>(sizes.huge_size_),
              static_cast<double>(sizes.huge_size_ * 100) /
                  Page::kObjectAreaSize,
              (ratio > ratio_threshold) ? "[fragmented]" : "");
     }
 
-    return (ratio > ratio_threshold) ||
-        (FLAG_always_compact && sizes.Total() != Page::kObjectAreaSize);
+    if (FLAG_always_compact && sizes.Total() != Page::kObjectAreaSize) {
+      return 1;
+    }
+    if (ratio <= ratio_threshold) return 0;  // Not fragmented.
+
+    return static_cast<int>(ratio - ratio_threshold);
   }
 
   void EvictEvacuationCandidatesFromFreeLists();
 
   bool CanExpand();
 
+  // Returns the number of total pages in this space.
+  int CountTotalPages();
+
  protected:
   // Maximum capacity of this space.
   intptr_t max_capacity_;
 
   // Accounting information for this space.
   AllocationStats accounting_stats_;
 
   // The dummy page that anchors the double linked list of pages.
   Page anchor_;
 
@@ skipping 17 matching lines @@
   // it cannot allocate requested number of pages from OS.
   bool Expand();
 
   // Generic fast case allocation function that tries linear allocation at the
   // address denoted by top in allocation_info_.
   inline HeapObject* AllocateLinearly(int size_in_bytes);
 
   // Slow path of AllocateRaw.  This function is space-dependent.
   MUST_USE_RESULT virtual HeapObject* SlowAllocateRaw(int size_in_bytes);
 
-#ifdef DEBUG
-  // Returns the number of total pages in this space.
-  int CountTotalPages();
-#endif
-
   friend class PageIterator;
 };
 
 
 class NumberAndSizeInfo BASE_EMBEDDED {
  public:
   NumberAndSizeInfo() : number_(0), bytes_(0) {}
 
   int number() const { return number_; }
   void increment_number(int num) { number_ += num; }
@@ skipping 956 matching lines @@
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_