WIP: idle scavenge

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 954919ef9be2a5085c994595d4ef35da0528097c..1180f3471027d5b912c49018088acb646ef05f82 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -98,6 +98,8 @@ class ScavengeStats {
 
   int64_t DurationMicros() const { return end_micros_ - start_micros_; }
 
+  intptr_t UsedBefore() const { return before_.used_in_words * kWordSize; }
+
  private:
   int64_t start_micros_;
   int64_t end_micros_;
@@ -214,6 +216,29 @@ class Scavenger {
     return usage;
   }
 
+  intptr_t ScavengeRateInBytesPerMicro() const {
+    intptr_t total_used = 0;
+    intptr_t total_micros = 0;
+    for (intptr_t i = 0; i < stats_history_.Size(); i++) {
+      total_used += stats_history_.Get(i).UsedBefore();
+      total_micros += stats_history_.Get(i).DurationMicros();
+    }
+    intptr_t scavenge_rate;
+    if (total_micros != 0) {
+      scavenge_rate = total_used / total_micros;
+    } else {
+      scavenge_rate = 400;  // Conservative initial guess.
+    }
+    if (scavenge_rate == 0) {
+      scavenge_rate = 1;
+    }
+    return scavenge_rate;
+  }
+
+  intptr_t EstimatedScavengeTime() const {
+    return UsedInWords() * kWordSize / ScavengeRateInBytesPerMicro();
+  }
+
   void VisitObjects(ObjectVisitor* visitor) const;
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;
 
@@ -330,7 +355,7 @@ class Scavenger {
 
   int64_t gc_time_micros_;
   intptr_t collections_;
-  static const int kStatsHistoryCapacity = 2;
+  static const int kStatsHistoryCapacity = 8;
   RingBuffer<ScavengeStats, kStatsHistoryCapacity> stats_history_;
 
   // The total size of external data associated with objects in this scavenger.