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

src/spaces.cc

Index: src/spaces.cc
===================================================================
--- src/spaces.cc	(revision 10490)
+++ src/spaces.cc	(working copy)
@@ -922,16 +922,15 @@
          2 * heap()->ReservedSemiSpaceSize());
   ASSERT(IsAddressAligned(chunk_base_, 2 * reserved_semispace_capacity, 0));
 
-  if (!to_space_.SetUp(chunk_base_,
-                       initial_semispace_capacity,
-                       maximum_semispace_capacity)) {
+  to_space_.SetUp(chunk_base_,
+                  initial_semispace_capacity,
+                  maximum_semispace_capacity);
+  from_space_.SetUp(chunk_base_ + reserved_semispace_capacity,
+                    initial_semispace_capacity,
+                    maximum_semispace_capacity);
+  if (!to_space_.Commit()) {
     return false;
   }
-  if (!from_space_.SetUp(chunk_base_ + reserved_semispace_capacity,
-                         initial_semispace_capacity,
-                         maximum_semispace_capacity)) {
-    return false;
-  }
 
   start_ = chunk_base_;
   address_mask_ = ~(2 * reserved_semispace_capacity - 1);
@@ -1162,7 +1161,7 @@
 // -----------------------------------------------------------------------------
 // SemiSpace implementation
 
-bool SemiSpace::SetUp(Address start,
+void SemiSpace::SetUp(Address start,
                       int initial_capacity,
                       int maximum_capacity) {
   // Creates a space in the young generation. The constructor does not
@@ -1181,8 +1180,6 @@
   object_mask_ = address_mask_ | kHeapObjectTagMask;
   object_expected_ = reinterpret_cast<uintptr_t>(start) | kHeapObjectTag;
   age_mark_ = start_;
-
-  return Commit();
 }
 
 
@@ -1232,6 +1229,10 @@
 
 
 bool SemiSpace::GrowTo(int new_capacity) {
+  if (!is_committed()) {
+    bool ok = Commit();
+    if (!ok) return false;

[Vyacheslav Egorov (Chromium), 2012/01/24 15:57:37]

if (!Commit()) return false;

+  }
   ASSERT((new_capacity & Page::kPageAlignmentMask) == 0);
   ASSERT(new_capacity <= maximum_capacity_);
   ASSERT(new_capacity > capacity_);