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 field-trial.
 #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"
@@ -126,12 +127,47 @@
     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);
+      // Use 'PreReadExperiment' rather than 'PreRead' for now.
+      // key.ReadValueDW(L"PreRead", &pre_read);
+
+      // The Syzygy project is a competing optimization technique. Part of the
+      // evaluation consists of an A/B experiment on Canary. 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, thus we store the result of our coin-toss in the registry. We
+      // do this using a separate registry key so that we can revert to the
+      // old behaviour after the experiment.
+      static const wchar_t kPreReadExperimentKey[] = L"PreReadExperiment";
+      // Try to look up the earlier result.
+      if (key.ReadValueDW(kPreReadExperimentKey, &pre_read) != S_OK) {
+        // If this fails, toss a coin and save the result. The coin-toss is only
+        // used if it has been successfully written. We assert that the coin-
+        // toss produces a value of 0 or 1. While this amounts to storing a
+        // random number in the registry, it's range is too small to be used
+        // for any kind of user tracking.
+        DWORD coin_toss = base::RandInt(0, 1);
+        DCHECK(coin_toss == 0 || coin_toss == 1);
+        if (key.WriteValue(kPreReadExperimentKey, coin_toss) == S_OK)
+          pre_read = coin_toss;
+      }
+
+      // 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",

[Sigurður Ásgeirsson, 2011/08/04 15:18:38]

as we discussed, this only runs if the regkey exists, but we don't actually need
the coin toss to persist. So, better to move the toss out of this block and toss
every time.

[chrisha, 2011/08/04 15:26:50]

Done.

+                  pre_read ? "1" : "0");
+
       key.Close();
     }
     if (pre_read) {