[Windows] One TimeTicks clock: efficient/reliable high-res, with low-res fallback.

base/time/time_win_unittest.cc

Index: base/time/time_win_unittest.cc
diff --git a/base/time/time_win_unittest.cc b/base/time/time_win_unittest.cc
index 058dfd79d147d6f6092bfc17680b1fc75a0583ff..4069208d75911f27caf7d8d858af246624410c86 100644
--- a/base/time/time_win_unittest.cc
+++ b/base/time/time_win_unittest.cc
@@ -114,7 +114,7 @@ TEST(TimeTicks, WinRollover) {
 TEST(TimeTicks, SubMillisecondTimers) {
   // HighResNow doesn't work on some systems.  Since the product still works
   // even if it doesn't work, it makes this entire test questionable.
-  if (!TimeTicks::IsHighResClockWorking())
+  if (!TimeTicks::IsHighResolution())
     return;
 
   const int kRetries = 1000;
@@ -183,7 +183,7 @@ TEST(TimeTicks, TimerPerformance) {
   TestCase cases[] = {
     { reinterpret_cast<TestFunc>(Time::Now), "Time::Now" },
     { TimeTicks::Now, "TimeTicks::Now" },
-    { TimeTicks::HighResNow, "TimeTicks::HighResNow" },
+    { TimeTicks::NowFromSystemTraceTime, "TimeTicks::NowFromSystemTraceTime" },
     { NULL, "" }
   };
 
@@ -207,65 +207,32 @@ TEST(TimeTicks, TimerPerformance) {
   }
 }
 
-// http://crbug.com/396384
-TEST(TimeTicks, DISABLED_Drift) {
-  // If QPC is disabled, this isn't measuring anything.
-  if (!TimeTicks::IsHighResClockWorking())
+TEST(TimeTicks, FromQPCValue) {
+  if (!TimeTicks::IsHighResolution())
     return;
 
   const int kIterations = 100;
-  int64 total_drift = 0;
-
-  for (int i = 0; i < kIterations; ++i) {
-    int64 drift_microseconds = TimeTicks::GetQPCDriftMicroseconds();
 
-    // Make sure the drift never exceeds our limit.
-    EXPECT_LT(drift_microseconds, 50000);
+  LARGE_INTEGER frequency;
+  ASSERT_TRUE(QueryPerformanceFrequency(&frequency));
+  const int64 ticks_per_second = frequency.QuadPart;
 
-    // Sleep for a few milliseconds (note that it means 1000 microseconds).
-    // If we check the drift too frequently, it's going to increase
-    // monotonically, making our measurement less realistic.
-    base::PlatformThread::Sleep(
-        base::TimeDelta::FromMilliseconds((i % 2 == 0) ? 1 : 2));
+  TimeTicks start_time = TimeTicks::Now();
+  LARGE_INTEGER start_ticks;
+  ASSERT_TRUE(QueryPerformanceCounter(&start_ticks));
+  LARGE_INTEGER ticks = start_ticks;
+  int64 ticks_increment = 10;
 
-    total_drift += drift_microseconds;
+  for (int i = 0; i < kIterations; ++i) {
+    ticks.QuadPart += ticks_increment;
+    ticks_increment = ticks_increment * 6 / 5;
+
+    const int64 ticks_advanced = ticks.QuadPart - start_ticks.QuadPart;
+    const TimeTicks expected_value = start_time +
+        TimeDelta::FromMicroseconds(
+            ticks_advanced * Time::kMicrosecondsPerSecond / ticks_per_second);
+    const TimeTicks value = TimeTicks::FromQPCValue(ticks.QuadPart);
+    EXPECT_GT(1.0, (value - expected_value).magnitude().InMillisecondsF())
+        << "iteration: " << i;
   }
-
-  // Sanity check. We expect some time drift to occur, especially across
-  // the number of iterations we do.
-  EXPECT_LT(0, total_drift);
-
-  printf("average time drift in microseconds: %lld\n",
-         total_drift / kIterations);
-}
-
-int64 QPCValueToMicrosecondsSafely(LONGLONG qpc_value,
-                                   int64 ticks_per_second) {
-  int64 whole_seconds = qpc_value / ticks_per_second;
-  int64 leftover_ticks = qpc_value % ticks_per_second;
-  int64 microseconds = (whole_seconds * Time::kMicrosecondsPerSecond) +
-                       ((leftover_ticks * Time::kMicrosecondsPerSecond) /
-                        ticks_per_second);
-  return microseconds;
-}
-
-TEST(TimeTicks, FromQPCValue) {

[brianderson, 2015/01/08 21:10:36]

Please keep this test. It verifies the (qpc_value < Time::kQPCOverflowThreshold)
optimization in QPCValueToTimeDelta is correct.

[miu, 2015/01/08 22:16:01]

It's still there (the TimeTicks.Drift test is what I deleted).  I rewrote the
test because the test code doing the math was copy-paste from the impl (i.e.,
nothing was really being tested).

Actually, now that I've looked at the code I wrote again, I felt I could do
better.  PTAL.

-  if (!TimeTicks::IsHighResClockWorking())
-    return;
-  LARGE_INTEGER frequency;
-  QueryPerformanceFrequency(&frequency);
-  int64 ticks_per_second = frequency.QuadPart;
-  LONGLONG qpc_value = Time::kQPCOverflowThreshold;
-  TimeTicks expected_value = TimeTicks::FromInternalValue(
-    QPCValueToMicrosecondsSafely(qpc_value + 1, ticks_per_second));
-  EXPECT_EQ(expected_value,
-            TimeTicks::FromQPCValue(qpc_value + 1));
-  expected_value = TimeTicks::FromInternalValue(
-    QPCValueToMicrosecondsSafely(qpc_value, ticks_per_second));
-  EXPECT_EQ(expected_value,
-            TimeTicks::FromQPCValue(qpc_value));
-  expected_value = TimeTicks::FromInternalValue(
-    QPCValueToMicrosecondsSafely(qpc_value - 1, ticks_per_second));
-  EXPECT_EQ(expected_value,
-            TimeTicks::FromQPCValue(qpc_value - 1));
 }