FLAG_max_new_space_size is in MB.

src/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 "v8.h"
 
 #include "accessors.h"
 #include "api.h"
 #include "bootstrapper.h"
 #include "codegen.h"
@@ skipping 4962 matching lines @@
   // output a flag to the snapshot.  However at this point the serializer and
   // deserializer are deliberately a little unsynchronized (see above) so the
   // checking of the sync flag in the snapshot would fail.
 }
 
 
 // TODO(1236194): Since the heap size is configurable on the command line
 // and through the API, we should gracefully handle the case that the heap
 // size is not big enough to fit all the initial objects.
 bool Heap::ConfigureHeap(int max_semispace_size,
-                         intptr_t max_old_gen_size,
+                         intptr_t max_old_space_size,
                          intptr_t max_executable_size,
                          intptr_t code_range_size) {
   if (HasBeenSetUp()) return false;
 
   // If max space size flags are specified overwrite the configuration.
   if (FLAG_max_new_space_size > 0) {
-    max_semispace_size = FLAG_max_new_space_size * kLumpOfMemory;
+    max_semispace_size = (FLAG_max_new_space_size / 2) * kLumpOfMemory;
   }
   if (FLAG_max_old_space_size > 0) {
-    max_old_gen_size = FLAG_max_old_space_size * kLumpOfMemory;
+    max_old_space_size = FLAG_max_old_space_size * kLumpOfMemory;
   }
   if (FLAG_max_executable_size > 0) {
     max_executable_size = FLAG_max_executable_size * kLumpOfMemory;
   }
 
   if (FLAG_stress_compaction) {
     // This will cause more frequent GCs when stressing.
     max_semispace_size_ = Page::kPageSize;
   }
 
@@ skipping 20 matching lines @@
         PrintPID("Max semispace size cannot be more than %dkbytes\n",
                  reserved_semispace_size_ >> 10);
       }
     }
   } else {
     // If we are not using snapshots we reserve space for the actual
     // max semispace size.
     reserved_semispace_size_ = max_semispace_size_;
   }
 
-  if (max_old_gen_size > 0) max_old_generation_size_ = max_old_gen_size;
+  if (max_old_space_size > 0) max_old_generation_size_ = max_old_space_size;
   if (max_executable_size > 0) {
     max_executable_size_ = RoundUp(max_executable_size, Page::kPageSize);
   }
 
   // The max executable size must be less than or equal to the max old
   // generation size.
   if (max_executable_size_ > max_old_generation_size_) {
     max_executable_size_ = max_old_generation_size_;
   }
 
@@ skipping 1433 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal