Scale backing store cache size

chrome/browser/renderer_host/backing_store.cc

Index: chrome/browser/renderer_host/backing_store.cc
diff --git a/chrome/browser/renderer_host/backing_store.cc b/chrome/browser/renderer_host/backing_store.cc
index 8f8a1c98f925ad728b3a48362239911983bf870e..b354e5944822ca803a586c1cb126b1e131ff28fe 100644
--- a/chrome/browser/renderer_host/backing_store.cc
+++ b/chrome/browser/renderer_host/backing_store.cc
@@ -4,7 +4,9 @@
 
 #include "chrome/browser/renderer_host/backing_store.h"
 
+#include "base/sys_info.h"
 #include "chrome/browser/renderer_host/render_widget_host.h"
+#include "chrome/common/chrome_constants.h"
 
 namespace {
 
@@ -12,11 +14,20 @@ typedef OwningMRUCache<RenderWidgetHost*, BackingStore*> BackingStoreCache;
 static BackingStoreCache* cache = NULL;
 
 // Returns the size of the backing store cache.
-// TODO(iyengar) Make this dynamic, i.e. based on the available resources
-// on the machine.
 static int GetBackingStoreCacheSize() {
-  const int kMaxSize = 5;
-  return kMaxSize;
+  // This uses a similar approach to GetMaxRendererProcessCount.  The goal
+  // is to reduce memory pressure and swapping on low-resource machines.
+  static const int kMaxDibCountByRamTier[] = {
+    2,  // less than 256MB
+    3,  // 256MB
+    4,  // 512MB
+    5   // 768MB and above
+  };
+
+  static int max_size = kMaxDibCountByRamTier[
+          std::min(base::SysInfo::AmountOfPhysicalMemoryMB() / 256,
+                   static_cast<int>(arraysize(kMaxDibCountByRamTier)) - 1)];
+  return max_size;
 }
 
 // Creates the backing store for the host based on the dimensions passed in.