GTTF: Avoid assertion failures caused by clock drift....

base/time_win.cc

Index: base/time_win.cc
===================================================================
--- base/time_win.cc	(revision 54549)
+++ base/time_win.cc	(working copy)
@@ -331,22 +331,20 @@
   }
 
   TimeDelta Now() {
-    // Our maximum tolerance for QPC drifting.
-    const int kMaxTimeDriftMicroseconds = 60150;
+    if (IsUsingHighResClock())
+      return TimeDelta::FromMicroseconds(UnreliableNow());
 
-    if (IsUsingHighResClock()) {
-      int64 now = UnreliableNow();
-
-      // Verify that QPC does not seem to drift.
-      DCHECK_LT(abs((now - ReliableNow()) - skew_), kMaxTimeDriftMicroseconds);
-
-      return TimeDelta::FromMicroseconds(now);
-    }
-
     // Just fallback to the slower clock.
     return RolloverProtectedNow();
   }
 
+  int64 GetQPCDriftMicroseconds() {
+    if (!IsUsingHighResClock())
+      return 0;
+
+    return abs((UnreliableNow() - ReliableNow()) - skew_);

[brettw, 2010/08/03 18:17:35]

Is this right? It seems like the sign of UnreliableNow() - ReliableNow() could
be either positive or negative. So I don't really know what it means when you
subtract skew_ from that. Do you need another abs in there?

[cpu_(ooo_6.6-7.5), 2010/08/04 17:44:42]

Seems ok to me. You *could* remove the outer abs() and then just say that the
drift can be positive or negative.

Let me elaborate with a table

First case one clock faster than the other in a widening case

time   U       R      Drift
0      10      11     (computed skew = -1)
10     20      22     -2 - (-1) = -1     
 
Second case is the same but inverted

time   U       R      Drift
0      11      10     (computed skew = 1)
10     22      20     2 - 1 =  1     

If so then you put the abs() in the test itself.

+  }
+
  private:
   // Synchronize the QPC clock with GetSystemTimeAsFileTime.
   void InitializeClock() {
@@ -398,3 +396,8 @@
 TimeTicks TimeTicks::HighResNow() {
   return TimeTicks() + Singleton<HighResNowSingleton>::get()->Now();
 }
+
+// static
+int64 TimeTicks::GetQPCDriftMicroseconds() {
+  return Singleton<HighResNowSingleton>::get()->GetQPCDriftMicroseconds();
+}