Bug 810454: Adds A/B test for preloading cookie db r=ehsan

netwerk/cookie/nsCookieService.cpp

@@ -48,6 +48,8 @@
 #include "nsNetCID.h"
 #include "mozilla/storage.h"
 #include "mozilla/AutoRestore.h"
+#include "mozilla/FileUtils.h"
+#include "mozilla/Telemetry.h"
 #include "nsIAppsService.h"
 #include "mozIApplication.h"
 
@@ -779,12 +781,23 @@ nsCookieService::TryInitDB(bool aRecreateDB)
     NS_ENSURE_SUCCESS(rv, RESULT_FAILURE);
   }
 
+  Telemetry::ID histID;
+  TimeStamp start = TimeStamp::Now();
+  // rand() is being used here as a poor-man's solution for a/b testing
+  if (rand() % 2) {
+    histID = Telemetry::MOZ_SQLITE_COOKIES_OPEN_READAHEAD_MS;
+    ReadAheadFile(mDefaultDBState->cookieFile);
+  } else {
+    histID = Telemetry::MOZ_SQLITE_COOKIES_OPEN_MS;
+  }
+
   // open a connection to the cookie database, and only cache our connection
   // and statements upon success. The connection is opened unshared to eliminate
   // cache contention between the main and background threads.
   rv = mStorageService->OpenUnsharedDatabase(mDefaultDBState->cookieFile,
     getter_AddRefs(mDefaultDBState->dbConn));
   NS_ENSURE_SUCCESS(rv, RESULT_RETRY);
+  Telemetry::AccumulateDelta_impl<Telemetry::Millisecond>::compute(histID, start);
 
   // Set up our listeners.
   mDefaultDBState->insertListener = new InsertCookieDBListener(mDefaultDBState);

toolkit/components/telemetry/Histograms.json

@@ -1665,6 +1665,20 @@
     "extended_statistics_ok": true,
     "description": "Time spent on SQLite read() (ms)"
   },
+  "MOZ_SQLITE_COOKIES_OPEN_MS": {
+    "kind": "exponential",
+    "high": "3000",
+    "n_buckets": 10,
+    "extended_statistics_ok": true,
+    "description": "Time spent on cookie DB open (ms)"
+  },
+  "MOZ_SQLITE_COOKIES_OPEN_READAHEAD_MS": {
+    "kind": "exponential",
+    "high": "3000",
+    "n_buckets": 10,
+    "extended_statistics_ok": true,
+    "description": "Time spent on cookie DB open with readahead (ms)"
+  },
   "MOZ_SQLITE_COOKIES_READ_MS": {
     "kind": "exponential",
     "high": "3000",