Use conservative estimate for GC speed instead of bailing out when computing

src/heap/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/bits.h"
 #include "src/base/once.h"
@@ skipping 4667 matching lines @@
   }
   if (mark_compact_collector()->sweeping_in_progress()) {
     mark_compact_collector()->EnsureSweepingCompleted();
   }
   DCHECK(IsHeapIterable());
 }
 
 
 static double ComputeMutatorUtilization(double mutator_speed, double gc_speed) {
   const double kMinMutatorUtilization = 0.0;
-  if (mutator_speed == 0 || gc_speed == 0) return kMinMutatorUtilization;
+  const double kConservativeGcSpeedInBytesPerMillisecond = 200000;
+  if (mutator_speed == 0) return kMinMutatorUtilization;
+  if (gc_speed == 0) gc_speed = kConservativeGcSpeedInBytesPerMillisecond;
   // Derivation:
   // mutator_utilization = mutator_time / (mutator_time + gc_time)
   // mutator_time = 1 / mutator_speed
   // gc_time = 1 / gc_speed
   // mutator_utilization = (1 / mutator_speed) /
   //                       (1 / mutator_speed + 1 / gc_speed)
   // mutator_utilization = gc_speed / (mutator_speed + gc_speed)
   return gc_speed / (mutator_speed + gc_speed);
 }
 
@@ skipping 2202 matching lines @@
     *object_type = "CODE_TYPE";                                                \
     *object_sub_type = "CODE_AGE/" #name;                                      \
     return true;
     CODE_AGE_LIST_COMPLETE(COMPARE_AND_RETURN_NAME)
 #undef COMPARE_AND_RETURN_NAME
   }
   return false;
 }
 }  // namespace internal
 }  // namespace v8