Make a pre-read A/B field-trial

chrome/app/client_util.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 <windows.h>
 #include <shlwapi.h>
 
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/environment.h"
 #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"
 #include "chrome/common/chrome_result_codes.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/installer/util/browser_distribution.h"
 #include "chrome/installer/util/channel_info.h"
 #include "chrome/installer/util/install_util.h"
 #include "chrome/installer/util/google_update_constants.h"
 #include "chrome/installer/util/google_update_settings.h"
@@ skipping 91 matching lines @@
     // for other sections. We can skip over these pages to avoid a soft page
     // fault which may not occur during code execution. However skipping 4K at
     // a time still has better performance over 32K and 16K according to data.
     // TODO(ananta): Investigate this and tune.
     const size_t kStepSize = 4 * 1024;
 
     DWORD pre_read_size = 0;
     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()) {
       key.ReadValueDW(L"PreReadSize", &pre_read_size);
       key.ReadValueDW(L"PreReadStepSize", &pre_read_step_size);
       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);
       TRACE_EVENT_END_ETW("PreReadImage", 0, "");
     }
   }
 #endif  // NDEBUG
 #endif  // WIN_DISABLE_PREREAD
 
   return ::LoadLibraryExW(dir->c_str(), NULL,
@@ skipping 162 matching lines @@
   }
 };
 
 MainDllLoader* MakeMainDllLoader() {
 #if defined(GOOGLE_CHROME_BUILD)
   return new ChromeDllLoader();
 #else
   return new ChromiumDllLoader();
 #endif
 }