Reduce GC from external allocation.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
===================================================================
--- runtime/vm/heap.cc	(revision 34384)
+++ runtime/vm/heap.cc	(working copy)
@@ -34,6 +34,8 @@
 DEFINE_FLAG(int, old_gen_heap_size, Heap::kHeapSizeInMB,
             "old gen heap size in MB,"
             "e.g: --old_gen_heap_size=1024 allocates a 1024MB old gen heap");
+DEFINE_FLAG(int, new_gen_ext_limit, 64,
+            "maximum total external size (MB) in new gen before triggering GC");
 
 Heap::Heap() : read_only_(false), gc_in_progress_(false) {
   for (int sel = 0;
@@ -94,6 +96,11 @@
 void Heap::AllocateExternal(intptr_t size, Space space) {
   if (space == kNew) {
     new_space_->AllocateExternal(size);
+    if (new_space_->ExternalInWords() > FLAG_new_gen_ext_limit * MBInWords) {

[Anders Johnsen, 2014/03/26 07:09:04]

DBC:

Just to be sure I get it right: This means that the maximum allocated memory is:
new-gen-size/2 + new-gen-ext-size + old-gen-size?

[koda, 2014/03/26 16:16:28]

Not exactly. The total amount of memory that this heap allocates from the OS is
around new_gen_heap_size + old_gen_heap_size.

The maximum amount of memory that that can actually be used by Dart heap objects
at any moment is slightly less than new_gen_heap_size/2 + old_gen_heap_size.

The total memory footprint that the VM is "aware" of, is new_gen_ext_limit +
new_gen_heap_size + old_gen_heap_size. But there can of course be an arbitrary
amount of user allocations that the user does not tell us about (either because
they are not related to Dart objects, or because they just prefer not to).

[Anders Johnsen, 2014/03/26 18:05:27]

I see. Thanks for explaining!

[Ivan Posva, 2014/03/27 21:18:16]

(FLAG_new_gen_ext_limit * MBInWords)

[koda, 2014/03/27 22:11:13]

Done.

+      // Attempt to free some external allocation by a scavenge. (If the total
+      // remains above the limit, next external alloc will trigger another.)
+      CollectGarbage(kNew);
+    }
   } else {
     ASSERT(space == kOld);
     old_space_->AllocateExternal(size);