Extend --trace-gc to output amount of memory reserved for V8 heap.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index ff978cf3b7f5f198165fcd01e5c82ca6b1ed3059..3b77714affc7a1ab07c4b6c77e74f613dedac62c 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -6513,7 +6513,8 @@ static intptr_t CountTotalHolesSize() {
 
 GCTracer::GCTracer(Heap* heap)
     : start_time_(0.0),
-      start_size_(0),
+      start_object_size_(0),
+      start_memory_size_(0),
       gc_count_(0),
       full_gc_count_(0),
       allocated_since_last_gc_(0),
@@ -6522,7 +6523,8 @@ GCTracer::GCTracer(Heap* heap)
       heap_(heap) {
   if (!FLAG_trace_gc && !FLAG_print_cumulative_gc_stat) return;
   start_time_ = OS::TimeCurrentMillis();
-  start_size_ = heap_->SizeOfObjects();
+  start_object_size_ = heap_->SizeOfObjects();
+  start_memory_size_ = heap_->isolate()->memory_allocator()->Size();
 
   for (int i = 0; i < Scope::kNumberOfScopes; i++) {
     scopes_[i] = 0;
@@ -6572,10 +6574,15 @@ GCTracer::~GCTracer() {
   if (!FLAG_trace_gc_nvp) {
     int external_time = static_cast<int>(scopes_[Scope::EXTERNAL]);
 
-    PrintF("%s %.1f -> %.1f MB, ",
+    double end_memory_size_mb =
+        static_cast<double>(heap_->isolate()->memory_allocator()->Size()) / MB;
+
+    PrintF("%s %.1f (%.1f) -> %.1f (%.1f) MB, ",
            CollectorString(),
-           static_cast<double>(start_size_) / MB,
-           SizeOfHeapObjects());
+           static_cast<double>(start_object_size_) / MB,
+           static_cast<double>(start_memory_size_) / MB,
+           SizeOfHeapObjects(),
+           end_memory_size_mb);
 
     if (external_time > 0) PrintF("%d / ", external_time);
     PrintF("%d ms", time);
@@ -6629,7 +6636,7 @@ GCTracer::~GCTracer() {
     PrintF("misc_compaction=%d ",
            static_cast<int>(scopes_[Scope::MC_UPDATE_MISC_POINTERS]));
 
-    PrintF("total_size_before=%" V8_PTR_PREFIX "d ", start_size_);
+    PrintF("total_size_before=%" V8_PTR_PREFIX "d ", start_object_size_);
     PrintF("total_size_after=%" V8_PTR_PREFIX "d ", heap_->SizeOfObjects());
     PrintF("holes_size_before=%" V8_PTR_PREFIX "d ",
            in_free_list_or_wasted_before_gc_);