Provide a mechanism for the embedder to set a TotalPhysicalMemory callback function.

src/defaults.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 32 matching lines @@
 const bool kOsHasSwap = true;
 #endif
 
 
 bool ConfigureResourceConstraintsForCurrentPlatform(
     ResourceConstraints* constraints) {
   if (constraints == NULL) {
     return false;
   }
 
-  uint64_t physical_memory = i::OS::TotalPhysicalMemory();
+  uint64_t physical_memory;
+  i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  if (isolate != NULL &&
+      isolate->thread_local_top()->total_physical_memory_callback_ != NULL) {
+    physical_memory =
+        isolate->thread_local_top()->total_physical_memory_callback_();
+  } else {
+    physical_memory = i::OS::TotalPhysicalMemory();
+  }
   int lump_of_memory = (i::kPointerSize / 4) * i::MB;
 
   // The young_space_size should be a power of 2 and old_generation_size should
   // be a multiple of Page::kPageSize.
   if (physical_memory <= 512ul * i::MB) {
     constraints->set_max_young_space_size(2 * lump_of_memory);
     constraints->set_max_old_space_size(128 * lump_of_memory);
     constraints->set_max_executable_size(96 * lump_of_memory);
   } else if (physical_memory <= (kOsHasSwap ? 768ul * i::MB : 1ul * i::GB)) {
     constraints->set_max_young_space_size(8 * lump_of_memory);
@@ skipping 13 matching lines @@
 
 
 bool SetDefaultResourceConstraintsForCurrentPlatform() {
   ResourceConstraints constraints;
   if (!ConfigureResourceConstraintsForCurrentPlatform(&constraints))
     return false;
   return SetResourceConstraints(&constraints);
 }
 
 }  // namespace v8