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

chrome/browser/metrics/time_ticks_experiment_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_experiment_win.h"
+
+#if defined(OS_WIN)
+
+#include "base/cpu.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_non_stop_time_stamp_counter());
+  if (!cpu.has_non_stop_time_stamp_counter()) {
+    return;
+  }
+
+  DWORD default_mask;
+  DWORD system_mask;
+  if (!GetProcessAffinityMask(GetCurrentProcess(),
+                              &default_mask, &system_mask)) {
+    return;
+  }
+
+  SYSTEM_INFO sys_info;
+  GetSystemInfo(&sys_info);
+  DWORD num_cores = sys_info.dwNumberOfProcessors;
+  DWORD current_core = 0;
+  bool failed_to_change_cores = false;
+
+  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 = 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;
+      }
+      if (!SetThreadAffinityMask(GetCurrentThread(), 1 << current_core)) {
+        failed_to_change_cores = true;
+        break;
+      }
+    }
+  }
+
+  if (num_cores > 1) {
+    SetThreadAffinityMask(GetCurrentThread(), default_mask);

[Sigurður Ásgeirsson, 2013/04/19 12:59:17]

ah - great catch :)

+    if (failed_to_change_cores) {
+      UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.FailedToChangeCores",
+                                info->version(), base::win::VERSION_WIN_LAST);
+      return;
+    }
+  }
+
+  if (min_delta < 0) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.TickedBackwards", info->version(),
+                              base::win::VERSION_WIN_LAST);
+    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 <= 10000;
+  if (success) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionSuccessful",
+                              info->version(), base::win::VERSION_WIN_LAST);
+  }
+}
+
+}  // namespace chrome_browser_net
+
+#endif  // defined(OS_WIN)