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 +127,34 @@
     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.
+      const wchar_t kPreReadExperimentKey[] = L"PreReadExperiment";

[Sigurður Ásgeirsson, 2011/08/04 13:35:32]

nit: static

[chrisha, 2011/08/04 14:48:40]

Done.

+      // Try to look up the earlier result.
+      if (key.ReadValueDW(kPreReadExperimentKey, &pre_read) != 0) {
+        // If this fails, toss a coin and save the result. The coin-toss is only
+        // used if it has been successfully written.
+        DWORD coin_toss = base::RandInt(0, 1);
+        if (key.WriteValue(kPreReadExperimentKey, coin_toss) == 0)

[jar (doing other things), 2011/08/03 21:24:34]

It is undesirable to save a random number into the registry, as this tags a
browser with a unique ID.  Privacy folks would not like it, despite the fact
that is is not being used as a unique ID.

It would be preferable to save a boolean result of the compare with a threshold.
 IF you plan on changing the threshold, you could save that as well, and then
you could for example re-toss a coin when a different threshold is observed.

[Sigurður Ásgeirsson, 2011/08/04 13:35:32]

The range of the coin_toss here is 0 to 1. It would be useful to assert on this
as documentation, e.g. DCHECK(coin_toss == 0 || coin_toss == 1);

[chrisha, 2011/08/04 14:48:40]

Done.

[jar (doing other things), 2011/08/04 18:34:28]

My apologies for my mis-read.  Thanks for the assert.

On 2011/08/04 13:35:32, Ruðrugis wrote:

+          pre_read = coin_toss;
+      }
+
       key.Close();
     }
     if (pre_read) {