Make a pre-read A/B field-trial

This CL disables/enables the Windows pre-read optimization based on a client-side coin-toss. The base::FieldTrial mechanism has not been used as it is not available until *after* chrome.dll is loaded, and this needs to occur before.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/7508034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95579 0039d316-1c4b-4281-b951-d872f2087c98

chrome/app/client_util.cc

@@ -11,10 +11,11 @@
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/win/registry.h"
+#include "base/rand_util.h"  // For PreReadExperiment.
 #include "base/string_util.h"
 #include "base/utf_string_conversions.h"
 #include "base/version.h"
+#include "base/win/registry.h"
 #include "chrome/app/breakpad_win.h"
 #include "chrome/app/client_util.h"
 #include "chrome/common/chrome_constants.h"
@@ -126,6 +127,9 @@ HMODULE LoadChromeWithDirectory(std::wstring* dir) {
     DWORD pre_read_step_size = kStepSize;
     DWORD pre_read = 1;
 
+    // TODO(chrisha): This path should not be ChromeFrame specific, and it
+    //     should not be hard-coded with 'Google' in the path. Rather, it should
+    //     use the product name.
     base::win::RegKey key(HKEY_CURRENT_USER, L"Software\\Google\\ChromeFrame",
                           KEY_QUERY_VALUE);
     if (key.Valid()) {
@@ -134,6 +138,26 @@ HMODULE LoadChromeWithDirectory(std::wstring* dir) {
       key.ReadValueDW(L"PreRead", &pre_read);
       key.Close();
     }
+
+    // The Syzygy project is a competing optimization technique. Part of the
+    // evaluation consists of an A/B experiment. As a baseline, we wish to
+    // evaluate startup time with preread enabled and disabled. We can't use
+    // base::FieldTrial as this only exists *after* chrome.dll is loaded. We
+    // override the registry setting with a coin-toss for the duration of the
+    // experiment.
+    // NOTE: This experiment is intended for Canary and Dev only, and should be
+    //     removed from any branch heading out to beta and beyond!
+    pre_read = base::RandInt(0, 1);
+    DCHECK(pre_read == 0 || pre_read == 1);
+
+    // We communicate the coin-toss result via a side-channel
+    // (environment variable) to chrome.dll. This ensures that chrome.dll
+    // only reports experiment results if it has been launched by a
+    // chrome.exe that is actually running the experiment.
+    scoped_ptr<base::Environment> env(base::Environment::Create());
+    DCHECK(env.get() != NULL);
+    env->SetVar("CHROME_PRE_READ_EXPERIMENT", pre_read ? "1" : "0");
+
     if (pre_read) {
       TRACE_EVENT_BEGIN_ETW("PreReadImage", 0, "");
       file_util::PreReadImage(dir->c_str(), pre_read_size, pre_read_step_size);

chrome/browser/browser_main.cc

@@ -165,6 +165,7 @@
 #include <commctrl.h>
 #include <shellapi.h>
 
+#include "base/environment.h"  // For PreRead experiment.
 #include "base/win/scoped_com_initializer.h"
 #include "base/win/windows_version.h"
 #include "chrome/browser/browser_trial.h"
@@ -227,6 +228,48 @@ namespace net {
 class NetLog;
 }  // namespace net
 
+// 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);
+
+  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);
+
+  // 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);
+
+#if defined(OS_WIN)
+  static const char kEnvVar[] = "CHROME_PRE_READ_EXPERIMENT";
+
+  // The pre-read experiment is Windows specific.
+  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);
+  }
+#endif
+}
+
 // BrowserMainParts ------------------------------------------------------------
 
 BrowserMainParts::BrowserMainParts(const MainFunctionParams& parameters)
@@ -2026,8 +2069,8 @@ int BrowserMain(const MainFunctionParams& parameters) {
       if (pool)
         pool->Recycle();
 
-      UMA_HISTOGRAM_MEDIUM_TIMES("Startup.BrowserOpenTabs",
-                                 base::TimeTicks::Now() - browser_open_start);
+      RecordPreReadExperimentTime("Startup.BrowserOpenTabs",
+                                  base::TimeTicks::Now() - browser_open_start);
 
       // TODO(mad): Move this call in a proper place on CrOS.
       // http://crosbug.com/17687

chrome/browser/browser_main.h

@@ -205,4 +205,11 @@ void WarnAboutMinimumSystemRequirements();
 // the UMA histogram |metric_name|.
 void RecordBrowserStartupTime();
 
+// Records a Startup category value to an UMA histogram in the context of the
+// PreReadExperiment field-trial. This will also report to the appropriate
+// Startup.HistogramName_PreRead(Enabled|Disabled) histogram as appropriate.
+// This is only called from OS_WIN specific code, so the implementation is
+// in browser_main_win.cc. This is a Windows specific experiment.
+void RecordPreReadExperimentTime(const char* name, base::TimeDelta time);
+
 #endif  // CHROME_BROWSER_BROWSER_MAIN_H_

chrome/browser/browser_main_win.cc

@@ -90,12 +90,9 @@ void RecordBrowserStartupTime() {
   ::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() {

chrome/chrome_browser.gypi

@@ -283,6 +283,7 @@
         'browser/browser_about_handler.cc',
         'browser/browser_about_handler.h',
         'browser/browser_main.cc',
+        'browser/browser_main.h',
         'browser/browser_main_gtk.cc',
         'browser/browser_main_gtk.h',
         'browser/browser_main_mac.mm',