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

chrome/browser/metrics/time_ticks_field_trial_win.cc

+// 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"

[jar (doing other things), 2013/04/12 02:14:54]

Is this actually a field trial?  I don't think you do this probabilistically. I
don't see references to the class etc.

...which then also makes me wonder if you should rename the file to something
like:

time_ticks_experiment_win

or

time_ticks_win_probe

or other??

[James Simonsen, 2013/04/12 02:29:46]

Good point. No it's not a field trial in the sense of that class.

I went with the first name you suggested.

+#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_non_stop_time_stamp_counter());
+  if (!cpu.has_non_stop_time_stamp_counter()) {
+    return;
+  }
+
+  SYSTEM_INFO sys_info;
+  GetSystemInfo(&sys_info);
+  DWORD num_cores = sys_info.dwNumberOfProcessors;
+  DWORD current_core = GetCurrentProcessorNumber();
+
+  base::win::OSInfo* info = base::win::OSInfo::GetInstance();
+  UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionTotal", info->version(),
+                            base::win::VERSION_WIN_LAST);
+
+  DWORD starting_core = GetCurrentProcessorNumber();
+  bool did_change_cores = false;
+
+  LARGE_INTEGER qpc_frequency;
+  QueryPerformanceFrequency(&qpc_frequency);
+
+  int min_delta = 1e9;
+  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;
+
+    if (num_cores > 1 && (i % 100) == 0) {
+      ++current_core;
+      if (current_core > num_cores) {
+        current_core = 0;
+      }
+      SetThreadAffinityMask(GetCurrentThread(), 1 << current_core);
+      if (!did_change_cores &&
+          GetCurrentProcessorNumber() != starting_core) {
+        did_change_cores = true;
+      }
+    }
+  }
+
+  if (did_change_cores) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.ChangedCores", info->version(),
+                              base::win::VERSION_WIN_LAST);
+  }
+
+  if (min_delta >= 0) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.NonDecreasing", info->version(),
+                              base::win::VERSION_WIN_LAST);
+  } else {
+    return;
+  }
+
+  int min_delta_ns = static_cast<int>(
+      min_delta * (1e9 / qpc_frequency.QuadPart));
+  UMA_HISTOGRAM_CUSTOM_COUNTS("WinTimeTicks.MinResolutionNanoseconds",
+                              min_delta_ns, 1, 1000000, 50);
+
+  bool success = min_delta_ns <= 1000;
+  if (success) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionSuccessful",
+                              info->version(), base::win::VERSION_WIN_LAST);
+  }
+}
+
+}  // namespace chrome_browser_net
+
+#endif  // defined(OS_WIN)