Fix slowTickIfNecessary with infrequently used EWMA (#3929)

Author: aweisberg

metrics-core/src/main/java/com/codahale/metrics/EWMA.java

@@ -81,6 +81,14 @@ public void update(long n) {
         uncounted.add(n);
     }
 
+    /**
+     * Set the rate to the smallest possible positive value. Used to avoid calling tick a large number of times.
+     */
+    public void reset() {
+        uncounted.reset();
+        rate = Double.MIN_NORMAL;
+    }
+
     /**
      * Mark the passage of time and decay the current rate accordingly.
      */

metrics-core/src/main/java/com/codahale/metrics/ExponentialMovingAverages.java

@@ -11,8 +11,28 @@
  */
 public class ExponentialMovingAverages implements MovingAverages {
 
+    /**
+     * If ticking would reduce even Long.MAX_VALUE in the 15 minute EWMA below this target then don't bother
+     * ticking in a loop and instead reset all the EWMAs.
+     */
+    private static final double maxTickZeroTarget = 0.0001;
+    private static final int maxTicks;
     private static final long TICK_INTERVAL = TimeUnit.SECONDS.toNanos(5);
 
+    static
+    {
+        int m3Ticks = 1;
+        final EWMA m3 = EWMA.fifteenMinuteEWMA();
+        m3.update(Long.MAX_VALUE);
+        do
+        {
+            m3.tick();
+            m3Ticks++;
+        }
+        while (m3.getRate(TimeUnit.SECONDS) > maxTickZeroTarget);
+        maxTicks = m3Ticks;
+    }
+
     private final EWMA m1Rate = EWMA.oneMinuteEWMA();
     private final EWMA m5Rate = EWMA.fiveMinuteEWMA();
     private final EWMA m15Rate = EWMA.fifteenMinuteEWMA();
@@ -51,10 +71,19 @@ public void tickIfNecessary() {
             final long newIntervalStartTick = newTick - age % TICK_INTERVAL;
             if (lastTick.compareAndSet(oldTick, newIntervalStartTick)) {
                 final long requiredTicks = age / TICK_INTERVAL;
-                for (long i = 0; i < requiredTicks; i++) {
-                    m1Rate.tick();
-                    m5Rate.tick();
-                    m15Rate.tick();
+                if (requiredTicks >= maxTicks) {
+                    m1Rate.reset();
+                    m5Rate.reset();
+                    m15Rate.reset();
+                }
+                else
+                {
+                    for (long i = 0; i < requiredTicks; i++)
+                    {
+                        m1Rate.tick();
+                        m5Rate.tick();
+                        m15Rate.tick();
+                    }
                 }
             }
         }

metrics-core/src/test/java/com/codahale/metrics/ExponentialMovingAveragesTest.java

@@ -0,0 +1,26 @@
+package com.codahale.metrics;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class ExponentialMovingAveragesTest
+{
+    @Test
+    public void testMaxTicks()
+    {
+        final Clock clock = mock(Clock.class);
+        when(clock.getTick()).thenReturn(0L, Long.MAX_VALUE);
+        final ExponentialMovingAverages ema = new ExponentialMovingAverages(clock);
+        ema.update(Long.MAX_VALUE);
+        ema.tickIfNecessary();
+        final long secondNanos = TimeUnit.SECONDS.toNanos(1);
+        assertEquals(ema.getM1Rate(), Double.MIN_NORMAL * secondNanos, 0.0);
+        assertEquals(ema.getM5Rate(), Double.MIN_NORMAL * secondNanos, 0.0);
+        assertEquals(ema.getM15Rate(), Double.MIN_NORMAL * secondNanos, 0.0);
+    }
+}