Initially unmap one of the semispaces to reduce memory use at boot time.

test/cctest/test-mark-compact.cc

 // Copyright 2006-2008 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 516 matching lines @@
 TEST(BootUpMemoryUse) {
   intptr_t initial_memory = MemoryInUse();
   FLAG_crankshaft = false;  // Avoid flakiness.
   // Only Linux has the proc filesystem and only if it is mapped.  If it's not
   // there we just skip the test.
   if (initial_memory >= 0) {
     InitializeVM();
     intptr_t booted_memory = MemoryInUse();
     if (sizeof(initial_memory) == 8) {
       if (v8::internal::Snapshot::IsEnabled()) {
-        CHECK_LE(booted_memory - initial_memory, 7654 * 1024);  // 7468.
+        CHECK_LE(booted_memory - initial_memory, 6654 * 1024);  // 6444.
       } else {
-        CHECK_LE(booted_memory - initial_memory, 7777 * 1024);  // 7620.
+        CHECK_LE(booted_memory - initial_memory, 6777 * 1024);  // 6596.
       }
     } else {
       if (v8::internal::Snapshot::IsEnabled()) {
-        CHECK_LE(booted_memory - initial_memory, 7500 * 1024);  // 7380.
+        CHECK_LE(booted_memory - initial_memory, 6500 * 1024);  // 6365.
       } else {
-        CHECK_LE(booted_memory - initial_memory, 7654 * 1024);  // 7448
+        CHECK_LE(booted_memory - initial_memory, 6654 * 1024);  // 6424
       }
     }
   }
 }
 
 #endif  // __linux__ and !USE_SIMULATOR