Github user aweisberg commented on a diff in the pull request:
https://github.com/apache/cassandra/pull/283#discussion_r225336309
--- Diff:
src/java/org/apache/cassandra/locator/DynamicEndpointSnitchEMA.java ---
@@ -0,0 +1,181 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.locator;
+
+import java.net.UnknownHostException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+
+import org.apache.cassandra.gms.Gossiper;
+import org.apache.cassandra.metrics.ExponentialMovingAverage;
+import org.apache.cassandra.net.LatencyMeasurementType;
+
+
+/**
+ * A dynamic snitching implementation that uses Exponentially Decaying
Histograms to prefer or
+ * de-prefer hosts
+ *
+ * This was the default implementation prior to Cassandra 4.0 and is being
left as the default
+ * in 4.0
+ */
+public class DynamicEndpointSnitchEMA extends DynamicEndpointSnitch
+{
+ // A ~10 sample EMA
+ private static final double EMA_ALPHA = 0.10;
+
+ private final ConcurrentHashMap<InetAddressAndPort, AnnotatedEMA>
samples = new ConcurrentHashMap<>();
+
+ /**
+ * Adds two boolean markers to the ExponentialMovingAverage for
telling if the data has been
+ * updated or requested recently.
+ *
+ * recentlyMeasured is updated through {@link
AnnotatedEMA#update(long, boolean)}
+ * recentlyRequested is updated through {@link
DynamicEndpointSnitch#markRequested}
+ *
+ * Both markers are periodically reset via {@link
DynamicEndpointSnitch#latencyProbeNeeded(long)}
+ */
+ private static class AnnotatedEMA extends ExponentialMovingAverage
+ {
+ volatile boolean recentlyRequested = false;
+ volatile boolean recentlyMeasured = false;
+
+ AnnotatedEMA(double alpha, double initialValue)
+ {
+ super(alpha, initialValue);
+ }
+
+ public void update(long value, boolean isRealRead)
+ {
+ recentlyMeasured = recentlyMeasured || isRealRead;
--- End diff --
This can be |=. This isn't checking of recently measured is already set
before writing which means it's always going to pull the cache line in for
write.
isRealRead sounds a little awkward and not proscriptive. What we are really
trying to say is that it should count as recently measured. I think it might be
clearer if it simply didn't override update.
I am also a developing fan of enums instead of booleans since true/false is
just not that informative.
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