Add a FieldTrial and histograms for measuring impact of http prioritization.

Currently this FieldTrial is measuring the transaction latencies of requests, separated into frame/subframe requests vs subresource requests.  We run a 2% holdback experiment where the people in the holdback don't have http prioritization enabled.  Add class static method to ResourceDispatcherHost to disable http prioritization.
BUG=10486

Review URL: http://codereview.chromium.org/67119

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@14040 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 42 insertions, 21 deletions

diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index 9e9d656..83422e6 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -478,6 +478,15 @@ int BrowserMain(const MainFunctionParams& parameters) {
     net::HttpNetworkSession::set_max_sockets_per_group(4);
   }
 
+  scoped_refptr<FieldTrial> http_prioritization_trial =
+      new FieldTrial("HttpPrioritization", 100);
+  // Put 10% of people in the fallback experiment with the http prioritization
+  // code disabled.
+  const int holdback_group =
+      http_prioritization_trial->AppendGroup("_no_http_prioritization", 10);
+  if (http_prioritization_trial->group() == holdback_group) {
+    ResourceDispatcherHost::DisableHttpPrioritization();
+  }
 
 #if defined(OS_WIN)
   // Init common control sex.
diff --git a/chrome/browser/renderer_host/resource_dispatcher_host.cc b/chrome/browser/renderer_host/resource_dispatcher_host.cc
index db852b4..a4cd3ba 100644
--- a/chrome/browser/renderer_host/resource_dispatcher_host.cc
+++ b/chrome/browser/renderer_host/resource_dispatcher_host.cc
@@ -68,17 +68,7 @@ using base::TimeTicks;
 
 // ----------------------------------------------------------------------------
 
-// The interval for calls to ResourceDispatcherHost::UpdateLoadStates
-static const int kUpdateLoadStatesIntervalMsec = 100;
-
-// Maximum number of pending data messages sent to the renderer at any
-// given time for a given request.
-static const int kMaxPendingDataMessages = 20;
-
-// Maximum byte "cost" of all the outstanding requests for a renderer.
-// See delcaration of |max_outstanding_requests_cost_per_process_| for details.
-// This bound is 25MB, which allows for around 6000 outstanding requests.
-static const int kMaxOutstandingRequestsCostPerProcess = 26214400;
+bool ResourceDispatcherHost::g_is_http_prioritization_enabled = true;
 
 // A ShutdownTask proxies a shutdown task from the UI thread to the IO thread.
 // It should be constructed on the UI thread and run in the IO thread.
@@ -97,6 +87,18 @@ class ResourceDispatcherHost::ShutdownTask : public Task {
 
 namespace {
 
+// The interval for calls to ResourceDispatcherHost::UpdateLoadStates
+const int kUpdateLoadStatesIntervalMsec = 100;
+
+// Maximum number of pending data messages sent to the renderer at any
+// given time for a given request.
+const int kMaxPendingDataMessages = 20;
+
+// Maximum byte "cost" of all the outstanding requests for a renderer.
+// See delcaration of |max_outstanding_requests_cost_per_process_| for details.
+// This bound is 25MB, which allows for around 6000 outstanding requests.
+const int kMaxOutstandingRequestsCostPerProcess = 26214400;
+
 // Consults the RendererSecurity policy to determine whether the
 // ResourceDispatcherHost should service this request.  A request might be
 // disallowed if the renderer is not authorized to restrive the request URL or
@@ -340,16 +342,16 @@ void ResourceDispatcherHost::BeginRequest(
   request->set_context(context);
   request->set_origin_pid(request_data.origin_pid);
 
-  // If the request is for the top level page or a frame/iframe, then we should
-  // prioritize it higher than other resource types.  Currently, we just use
-  // priorities 1 and 0.
-  // TODO(willchan): Revisit the actual priorities when looking at considering
-  // boosting priorities for requests for the foreground tab.
-  if (request_data.resource_type == ResourceType::MAIN_FRAME ||
-      request_data.resource_type == ResourceType::SUB_FRAME) {
-    request->set_priority(1);
-  } else {
-    request->set_priority(0);
+  if (IsHttpPrioritizationEnabled()) {
+    // If the request is for the top level page or a frame/iframe, then we
+    // should prioritize it higher than other resource types.  Currently, we
+    // just use priorities 1 and 0.
+    if (request_data.resource_type == ResourceType::MAIN_FRAME ||
+        request_data.resource_type == ResourceType::SUB_FRAME) {
+      request->set_priority(1);
+    } else {
+      request->set_priority(0);
+    }
   }
 
   // Set upload data.
diff --git a/chrome/browser/renderer_host/resource_dispatcher_host.h b/chrome/browser/renderer_host/resource_dispatcher_host.h
index ba699bc..fd6e06c 100644
--- a/chrome/browser/renderer_host/resource_dispatcher_host.h
+++ b/chrome/browser/renderer_host/resource_dispatcher_host.h
@@ -370,6 +370,14 @@ class ResourceDispatcherHost : public URLRequest::Delegate {
     return false;
   }
 
+  static void DisableHttpPrioritization() {
+    g_is_http_prioritization_enabled = false;
+  }
+
+  static bool IsHttpPrioritizationEnabled() {
+    return g_is_http_prioritization_enabled;
+  }
+
  private:
   FRIEND_TEST(ResourceDispatcherHostTest, TestBlockedRequestsProcessDies);
   FRIEND_TEST(ResourceDispatcherHostTest,
@@ -556,6 +564,8 @@ class ResourceDispatcherHost : public URLRequest::Delegate {
   // to the source of the message.
   Receiver* receiver_;
 
+  static bool g_is_http_prioritization_enabled;
+
   DISALLOW_COPY_AND_ASSIGN(ResourceDispatcherHost);
 };