Make a pre-read A/B field-trial

chrome/browser/browser_main_win.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/browser_main.h"
 #include "chrome/browser/browser_main_win.h"
 
 #include <windows.h>
 #include <shellapi.h>
 
@@ skipping 72 matching lines @@
   }
 }
 
 void RecordBrowserStartupTime() {
   // Calculate the time that has elapsed from our own process creation.
   FILETIME creation_time = {};
   FILETIME ignore = {};
   ::GetProcessTimes(::GetCurrentProcess(), &creation_time, &ignore, &ignore,
       &ignore);
 
-  base::TimeDelta elapsed_from_startup =
-      base::Time::Now() - base::Time::FromFileTime(creation_time);
-
-  // Record the time to present in a histogram.
-  UMA_HISTOGRAM_MEDIUM_TIMES("Startup.BrowserMessageLoopStartTime",
-                             elapsed_from_startup);
+  RecordPreReadExperimentTime(
+      "Startup.BrowserMessageLoopStartTime",
+      base::Time::Now() - base::Time::FromFileTime(creation_time));
 }
 
 int AskForUninstallConfirmation() {
   int ret = content::RESULT_CODE_NORMAL_EXIT;
   views::Widget::CreateWindow(new UninstallView(ret))->Show();
   views::AcceleratorHandler accelerator_handler;
   MessageLoopForUI::current()->Run(&accelerator_handler);
   return ret;
 }
 
@@ skipping 160 matching lines @@
         uninstall_cmd.AppendSwitch(
             installer::switches::kDoNotRemoveSharedItems);
         base::LaunchProcess(uninstall_cmd, base::LaunchOptions(), NULL);
       }
       return true;
     }
   }
   return false;
 }
 
+// This code is specific to the Windows-only PreReadExperiment field-trial.
+static void AddPreReadHistogramTime(const char* name, base::TimeDelta time) {
+  const base::TimeDelta kMin(base::TimeDelta::FromMilliseconds(1));
+  const base::TimeDelta kMax(base::TimeDelta::FromHours(1));
+  static const size_t kBuckets(100);
+
+  // FactoryTimeGet will always return a pointer to the same histogram object,
+  // keyed on its name. There's no need for us to store it explicitly anywhere.
+  base::Histogram* counter = base::Histogram::FactoryTimeGet(
+      name, kMin, kMax, kBuckets, base::Histogram::kUmaTargetedHistogramFlag);
+  DCHECK(counter != NULL);

[jar (doing other things), 2011/08/04 19:11:03]

nit: I always think it is strange to DCHECK (or CHECK) a pointer, and then
dereference it, as the deref will crash both debug and release builds.

If you really like the DCHECK, use either:
DCHECK(counter);
or
DCHECK_NE(counter, NULL);

[chrisha, 2011/08/04 19:19:06]

Removed.

+
+  counter->AddTime(time);
+}
+
+// This code is specific to the Windows-only PreReadExperiment field-trial.
+void RecordPreReadExperimentTime(const char* name, base::TimeDelta time) {
+  DCHECK(name != NULL);
+
+  static const char kEnvVar[] = "CHROME_PRE_READ_EXPERIMENT";
+
+  // This gets called with different histogram names, so we don't want to use
+  // the UMA_HISTOGRAM_CUSTOM_TIMES macro--it uses a static variable, and the
+  // first call wins.
+  AddPreReadHistogramTime(name, time);
+
+  scoped_ptr<base::Environment> env(base::Environment::Create());
+  DCHECK(env.get() != NULL);
+
+  // Only record the sub-histogram result if the experiment is running
+  // (environment variable is set, and valid).
+  std::string pre_read;
+  if (env->GetVar(kEnvVar, &pre_read) && (pre_read == "0" || pre_read == "1")) {
+    std::string uma_name(name);
+    uma_name += "_PreRead";
+    uma_name += pre_read == "1" ? "Enabled" : "Disabled";
+    AddPreReadHistogramTime(uma_name.c_str(), time);
+  }
+}
+
 // BrowserMainPartsWin ---------------------------------------------------------
 
 class BrowserMainPartsWin : public BrowserMainParts {
  public:
   explicit BrowserMainPartsWin(const MainFunctionParams& parameters)
       : BrowserMainParts(parameters) {}
 
  protected:
   virtual void PreEarlyInitialization() {
     // Initialize Winsock.
@@ skipping 26 matching lines @@
       crypto::EnsureNSPRInit();
     }
   }
 };
 
 // static
 BrowserMainParts* BrowserMainParts::CreateBrowserMainParts(
     const MainFunctionParams& parameters) {
   return new BrowserMainPartsWin(parameters);
 }