Adapt fragmentation heuristics for over reserved pages.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 67f6e8e0a7556c89a1f5162ac441ec3da5b4a1c4..97352e69beeef5eb2e6a12dc1f30536cdaef40ae 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -500,12 +500,10 @@ void MarkCompactCollector::CollectEvacuationCandidates(PagedSpace* space) {
          space->identity() == OLD_DATA_SPACE ||
          space->identity() == CODE_SPACE);
 
+  static const int kMaxMaxEvacuationCandidates = 1000;
   int number_of_pages = space->CountTotalPages();
-
-  const int kMaxMaxEvacuationCandidates = 1000;
-  int max_evacuation_candidates = Min(
-    kMaxMaxEvacuationCandidates,
-    static_cast<int>(sqrt(static_cast<double>(number_of_pages / 2)) + 1));
+  int max_evacuation_candidates =
+      static_cast<int>(sqrt(static_cast<double>(number_of_pages / 2)) + 1);
 
   if (FLAG_stress_compaction || FLAG_always_compact) {
     max_evacuation_candidates = kMaxMaxEvacuationCandidates;
@@ -535,17 +533,28 @@ void MarkCompactCollector::CollectEvacuationCandidates(PagedSpace* space) {
   intptr_t over_reserved = reserved - space->SizeOfObjects();
   static const intptr_t kFreenessThreshold = 50;
 
-  if (over_reserved >= 2 * space->AreaSize() &&
-      reduce_memory_footprint_) {
-    mode = REDUCE_MEMORY_FOOTPRINT;
+  if (over_reserved >= 2 * space->AreaSize()) {
 
-    // We expect that empty pages are easier to compact so slightly bump the
-    // limit.
-    max_evacuation_candidates += 2;
+    // If reduction of memory footprint was requested, we are aggressive
+    // about choosing pages to free.  We expect that half-empty pages
+    // are easier to compact so slightly bump the limit.
+    if (reduce_memory_footprint_) {
+      mode = REDUCE_MEMORY_FOOTPRINT;
+      max_evacuation_candidates += 2;
+    }
 
-    if (FLAG_trace_fragmentation) {
-      PrintF("Estimated over reserved memory: %.1f MB (setting threshold %d)\n",
+    // If over-usage is very high (more than a third of the space), we
+    // try to free all mostly empty pages.  We expect that almost empty
+    // pages are even easier to compact so bump the limit even more.
+    if (over_reserved > reserved / 3) {
+      mode = REDUCE_MEMORY_FOOTPRINT;
+      max_evacuation_candidates *= 2;
+    }
+
+    if (FLAG_trace_fragmentation && mode == REDUCE_MEMORY_FOOTPRINT) {
+      PrintF("Estimated over reserved memory: %.1f / %.1f MB (threshold %d)\n",
              static_cast<double>(over_reserved) / MB,
+             static_cast<double>(reserved) / MB,
              static_cast<int>(kFreenessThreshold));
     }
   }
@@ -554,6 +563,9 @@ void MarkCompactCollector::CollectEvacuationCandidates(PagedSpace* space) {
 
   Candidate candidates[kMaxMaxEvacuationCandidates];
 
+  max_evacuation_candidates =
+      Min(kMaxMaxEvacuationCandidates, max_evacuation_candidates);
+
   int count = 0;
   int fragmentation = 0;
   Candidate* least = NULL;