Create a field trial to test if we can detect good QPC implementations.

chrome/browser/metrics/time_ticks_field_trial_win.cc

Index: chrome/browser/metrics/time_ticks_field_trial_win.cc
diff --git a/chrome/browser/metrics/time_ticks_field_trial_win.cc b/chrome/browser/metrics/time_ticks_field_trial_win.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8e911a30be364597a41f02e6f0a7ec8af12a9745
--- /dev/null
+++ b/chrome/browser/metrics/time_ticks_field_trial_win.cc
@@ -0,0 +1,74 @@
+// Copyright (c) 2013 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 "chrome/browser/metrics/time_ticks_field_trial_win.h"
+
+#if defined(OS_WIN)
+
+#include "base/cpu.h"
+#include "base/metrics/field_trial.h"
+#include "base/metrics/histogram.h"
+#include "base/win/windows_version.h"
+
+#include <windows.h>
+
+namespace chrome_browser_metrics {
+
+namespace {
+
+const int kNumIterations = 1000;
+
+}  // anonymous namespace
+
+void CollectTimeTicksStats() {
+  // This bit is supposed to indicate that rdtsc is safe across cores. If so, we
+  // can use QPC as long as it uses rdtsc.
+  // TODO(simonjam): We should look for other signals that QPC might be safe and
+  // test them out here.
+  base::CPU cpu;
+  UMA_HISTOGRAM_BOOLEAN("WinTimeTicks.NonStopTsc", cpu.has_nonstoptsc());
+  if (!cpu.has_nonstoptsc()) {
+    return;
+  }
+
+  base::win::OSInfo* info = base::win::OSInfo::GetInstance();
+  UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionTotal", info->version(),
+                             base::win::VERSION_WIN_LAST);
+
+  LARGE_INTEGER qpc_frequency;
+  QueryPerformanceFrequency(&qpc_frequency);
+
+  int min_delta = 0;
+  LARGE_INTEGER qpc_last;
+  QueryPerformanceCounter(&qpc_last);
+  for (int i = 0; i < kNumIterations; ++i) {
+    LARGE_INTEGER qpc_now;
+    QueryPerformanceCounter(&qpc_now);
+    int delta = static_cast<int>(qpc_now.QuadPart - qpc_last.QuadPart);
+    if (delta != 0) {
+      min_delta = std::min(min_delta, delta);
+    }
+    qpc_last = qpc_now;
+  }
+
+  UMA_HISTOGRAM_BOOLEAN("WinTimeTicks.NonDecreasing", min_delta > 0);
+  if (min_delta < 0) {
+    return;

[jar (doing other things), 2013/04/04 00:26:13]

I'm surprised you don't want a histogram of how often we failed by version,
similar to line 67.

[James Simonsen, 2013/04/04 02:26:51]

Good point. Done.

+  }
+
+  int min_delta_us = static_cast<int>(
+      min_delta * (1e6 / qpc_frequency.QuadPart));
+  UMA_HISTOGRAM_CUSTOM_COUNTS("WinTimeTicks.MinResolutionMicroseconds",
+                              min_delta_us, 1, 1000, 50);
+
+  bool success = min_delta_us <= 100;  // Our goal is 0.1ms resolution.
+  if (success) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionSuccessful",
+                              info->version(), base::win::VERSION_WIN_LAST);
+  }
+}
+
+}  // namespace chrome_browser_net
+
+#endif  // defined(OS_WIN)