Fix for issue 25954.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 2f894b53a2bfc9ed7c7710aed1d831533190203f..bc90261b8bdcf3595dd6247408a61445d495c1b6 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -1157,16 +1157,19 @@ void PageSpaceController::EvaluateGarbageCollection(
     SpaceUsage before, SpaceUsage after, int64_t start, int64_t end) {
   ASSERT(end >= start);
   history_.AddGarbageCollectionTime(start, end);
-  int gc_time_fraction = history_.GarbageCollectionTimeFraction();
+  const int gc_time_fraction = history_.GarbageCollectionTimeFraction();
   heap_->RecordData(PageSpace::kGCTimeFraction, gc_time_fraction);
 
   // Assume garbage increases linearly with allocation:
   // G = kA, and estimate k from the previous cycle.
-  intptr_t allocated_since_previous_gc =
+  const intptr_t allocated_since_previous_gc =
       before.used_in_words - last_usage_.used_in_words;
-  intptr_t garbage = before.used_in_words - after.used_in_words;
-  double k = garbage / static_cast<double>(allocated_since_previous_gc);
-  heap_->RecordData(PageSpace::kGarbageRatio, static_cast<int>(k * 100));
+  ASSERT(allocated_since_previous_gc > 0);
+  const intptr_t garbage = before.used_in_words - after.used_in_words;
+  ASSERT(garbage >= 0);
+  const double k = garbage / static_cast<double>(allocated_since_previous_gc);
+  const int garbage_ratio = static_cast<int>(k * 100);
+  heap_->RecordData(PageSpace::kGarbageRatio, garbage_ratio);
 
   // Define GC to be 'worthwhile' iff at least fraction t of heap is garbage.
   double t = 1.0 - desired_utilization_;
@@ -1175,21 +1178,45 @@ void PageSpaceController::EvaluateGarbageCollection(
     t += (gc_time_fraction - garbage_collection_time_ratio_) / 100.0;
   }
 
-  // Find minimum 'grow_heap_' such that after increasing capacity by
-  // 'grow_heap_' pages and filling them, we expect a GC to be worthwhile.
-  for (grow_heap_ = 0; grow_heap_ < heap_growth_max_; ++grow_heap_) {
-    intptr_t limit =
-        after.capacity_in_words + (grow_heap_ * PageSpace::kPageSizeInWords);
-    intptr_t allocated_before_next_gc = limit - after.used_in_words;
-    double estimated_garbage = k * allocated_before_next_gc;
-    if (t <= estimated_garbage / limit) {
-      break;
+  if (garbage_ratio == 0) {

[zra, 2016/08/11 18:29:15]

Why 0? Why not e.g. <= 5? Maybe add TODO to investigate.

[siva, 2016/08/11 23:05:52]

I did not want to change the growth pattern when garbage is present even if
small as it does not establish a pattern of growth.

+    // No garbage in the previous cycle so it would be hard to compute a
+    // grow_heap_ size based on estimated garbage so we use growth ratio
+    // instead.
+    const intptr_t grow_ratio = (
+        static_cast<intptr_t>(after.capacity_in_words / desired_utilization_) -
+         - after.capacity_in_words) / PageSpace::kPageSizeInWords;

[zra, 2016/08/11 18:29:16]

There are two '-'. One at the end of the previous line and one at the beginning
of this line.

[siva, 2016/08/11 23:05:51]

Good catch.

+    grow_heap_ = Utils::Maximum(static_cast<intptr_t>(heap_growth_max_),
+                                grow_ratio);
+  } else {
+    // Find minimum 'grow_heap_' such that after increasing capacity by
+    // 'grow_heap_' pages and filling them, we expect a GC to be worthwhile.
+    intptr_t max = heap_growth_max_;
+    intptr_t min = 0;
+    intptr_t adjustment = 0;
+    grow_heap_ = 0;

[zra, 2016/08/11 18:29:15]

Maybe use a local, and then write the field at the end.

[siva, 2016/08/11 23:05:51]

Done.

+    while (min < max) {
+      grow_heap_ = (max + min) / 2;
+      const intptr_t limit =
+          after.capacity_in_words + (grow_heap_ * PageSpace::kPageSizeInWords);
+      const intptr_t allocated_before_next_gc = limit - after.used_in_words;
+      const double estimated_garbage = k * allocated_before_next_gc;
+      if (t <= estimated_garbage / limit) {
+        max = grow_heap_ - 1;
+        adjustment = -1;
+      } else {
+        min = grow_heap_ + 1;
+        adjustment = 1;
+      }
     }
+    grow_heap_ += adjustment;

[zra, 2016/08/11 18:29:16]

Why is this adjustment needed? If the search results in grow_heap_ = 0, couldn't
this adjustment make it negative?

[siva, 2016/08/11 23:05:52]

I added the adjustment to keep it in sync with the old computation, we always
set it to a positive value so it won't become negative, I have added an ASSERT
to ensure grow_heap_ is always positive or 0.

   }
   heap_->RecordData(PageSpace::kPageGrowth, grow_heap_);
+  if (garbage_ratio == 0) {
+    OS::PrintErr("grow_heap_ = %" Pd "\n", grow_heap_);

[zra, 2016/08/11 18:29:16]

Leftover debug print?

[siva, 2016/08/11 23:05:52]

Removed.

+  }
 
   // Limit shrinkage: allow growth by at least half the pages freed by GC.
-  intptr_t freed_pages =
+  const intptr_t freed_pages =
       (before.capacity_in_words - after.capacity_in_words) /
       PageSpace::kPageSizeInWords;
   grow_heap_ = Utils::Maximum(grow_heap_, freed_pages / 2);