Use estimated vsync period on Android

ui/android/java/src/org/chromium/ui/VSyncMonitor.java

Index: ui/android/java/src/org/chromium/ui/VSyncMonitor.java
diff --git a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
index af8f87b5ffd898c832a1fa2f0db7cd590e50ca89..962f3eda14df67ee182fb5101d601293efa8f312 100644
--- a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
+++ b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
@@ -24,6 +24,8 @@ public class VSyncMonitor {
     private static final long NANOSECONDS_PER_MILLISECOND = 1000000;
     private static final long NANOSECONDS_PER_MICROSECOND = 1000;
 
+    private static final long MAX_VALID_VSYNC_PERIOD_NANOSECONDS = NANOSECONDS_PER_SECOND / 30;
+
     private boolean mInsideVSync = false;
 
     /**
@@ -41,7 +43,7 @@ public class VSyncMonitor {
     private Listener mListener;
 
     // Display refresh rate as reported by the system.
-    private final long mRefreshPeriodNano;
+    private long mRefreshPeriodNano;
 
     private boolean mHaveRequestInFlight;
 
@@ -85,10 +87,21 @@ public class VSyncMonitor {
         if (enableJBVSync && Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
             // Use Choreographer on JB+ to get notified of vsync.
             mChoreographer = Choreographer.getInstance();
+            final boolean useEstimatedRefreshPeriod =
+                mRefreshPeriodNano > MAX_VALID_VSYNC_PERIOD_NANOSECONDS;
             mVSyncFrameCallback = new Choreographer.FrameCallback() {
                 @Override
                 public void doFrame(long frameTimeNanos) {
                     TraceEvent.begin("VSync");
+                    if (useEstimatedRefreshPeriod) {
+                        // Display.getRefreshRate() is unreliable on some platforms.
+                        // Adjust refresh period- initial value is based on Display.getRefreshRate()
+                        // after that it asymptotically approaches the real value.
+                        long lastRefreshDurationNano = frameTimeNanos - mGoodStartingPointNano;

[no sievers, 2014/10/01 17:48:09]

+1 to what Sami said. Only doing the adjustments when we are somewhat certain
that we are dealing with a buggy Display.getRefreshRate() would be better.

+                        if (lastRefreshDurationNano <= MAX_VALID_VSYNC_PERIOD_NANOSECONDS) {
+                            mRefreshPeriodNano = (mRefreshPeriodNano + lastRefreshDurationNano) / 2;

[no sievers, 2014/10/01 17:48:09]

Do you maybe want to use a bigger weight than 0.5 for the moving average
mRefreshPeriodNano?

[jmanko, 2014/10/02 12:02:29]

Sure, I can change that. Would 0.75 be enough?

[no sievers, 2014/10/02 19:51:42]

If you make sure that we don't have outliers (see discussion about other comment
and only measuring when we update this if mGoodStartingPointNano is really from
the previous vsync) then using the average will actually get us to the best
value possible over time.

How about just calculating the moving average over the last N values?
You only need to track the sum of the last N samples and the values of the last
N (= let's say 30 or so) samples. Then curr_avg = (sum - oldest_sample +
curr_sample) / N.

[jmanko, 2014/10/03 09:10:54]

That's already been done, the period is only updated on consecutive vsyncs.
See mConsecutiveVSync usage.
Is this still valid now, with outliers eliminated? Like you wrote above, using
all
the samples would eventually produce better result than last N average

+                        }
+                    }
                     mGoodStartingPointNano = frameTimeNanos;
                     onVSyncCallback(frameTimeNanos, getCurrentNanoTime());
                     TraceEvent.end("VSync");