Make a pre-read A/B field-trial

chrome/app/client_util.cc

Index: chrome/app/client_util.cc
===================================================================
--- chrome/app/client_util.cc	(revision 95298)
+++ chrome/app/client_util.cc	(working copy)
@@ -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 @@
     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 @@
       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);