Add histograms to ResourceFetcher to evaluate memory cache hit rate

Source/core/fetch/ResourceFetcher.cpp

Index: Source/core/fetch/ResourceFetcher.cpp
diff --git a/Source/core/fetch/ResourceFetcher.cpp b/Source/core/fetch/ResourceFetcher.cpp
index ea44911232428333aea6d34d69eedae383194cb6..66c9e0ff0f3bdd089fa1095f2e7ab6c4536c71bd 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -632,6 +632,9 @@ ResourcePtr<Resource> ResourceFetcher::requestResource(Resource::Type type, Fetc
         break;
     }
 
+    if (!resource->hasClients())

[Nate Chapin, 2014/01/29 18:19:56]

We null-check resource just below this. Either this should be after the
null-check, or we should remove the null-check. I'm pretty sure it's
unnecessary.

+        m_deadStatsRecorder.update(policy);
+
     if (!resource)
         return 0;
 
@@ -1373,4 +1376,38 @@ const ResourceLoaderOptions& ResourceFetcher::defaultResourceOptions()
     return options;
 }
 
+ResourceFetcher::DeadResourceStatsRecorder::DeadResourceStatsRecorder()
+    : m_useCount(0)
+    , m_revalidateCount(0)
+    , m_loadCount(0)
+{
+}
+
+ResourceFetcher::DeadResourceStatsRecorder::~DeadResourceStatsRecorder()
+{
+    blink::Platform::current()->histogramCustomCounts(
+        "WebCore.ResourceFetcher.HitCount", m_useCount, 0, 1000, 50);
+    blink::Platform::current()->histogramCustomCounts(
+        "WebCore.ResourceFetcher.RevalidateCount", m_revalidateCount, 0, 1000, 50);
+    blink::Platform::current()->histogramCustomCounts(
+        "WebCore.ResourceFetcher.LoadCount", m_loadCount, 0, 1000, 50);
+}
+
+void ResourceFetcher::DeadResourceStatsRecorder::update(RevalidationPolicy policy)
+{
+    switch (policy) {
+    case Reload:
+        // Fall through.

[Nate Chapin, 2014/01/29 18:19:56]

Given that Reload and Load cases are identical, this comment probably isn't
necessary.

+    case Load:
+        ++m_loadCount;
+        break;

[Nate Chapin, 2014/01/29 18:19:56]

return instead?

+    case Revalidate:
+        ++m_revalidateCount;
+        break;
+    case Use:
+        ++m_useCount;
+        break;
+    }
+}
+
 }