A Guide to @Contended & False Sharing

jmh/src/main/java/com/baeldung/FalseSharing.java

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+package com.baeldung;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+
+@State(Scope.Benchmark)
+public class FalseSharing {
+
+    private java.util.concurrent.atomic.LongAdder builtin = new java.util.concurrent.atomic.LongAdder();
+    private LongAdder custom = new LongAdder();
+
+    @Benchmark
+    public void builtin() {
+        builtin.increment();
+    }
+
+    @Benchmark
+    public void custom() {
+        custom.increment();
+    }
+}

jmh/src/main/java/com/baeldung/LongAdder.java

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+package com.baeldung;
+
+import java.io.Serializable;
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * One or more variables that together maintain an initially zero
+ * {@code long} sum.  When updates (method {@link #add}) are contended
+ * across threads, the set of variables may grow dynamically to reduce
+ * contention. Method {@link #sum} (or, equivalently, {@link
+ * #longValue}) returns the current total combined across the
+ * variables maintaining the sum.
+ *
+ * <p>This class is usually preferable to {@link AtomicLong} when
+ * multiple threads update a common sum that is used for purposes such
+ * as collecting statistics, not for fine-grained synchronization
+ * control.  Under low update contention, the two classes have similar
+ * characteristics. But under high contention, expected throughput of
+ * this class is significantly higher, at the expense of higher space
+ * consumption.
+ *
+ * <p>LongAdders can be used with a {@link
+ * java.util.concurrent.ConcurrentHashMap} to maintain a scalable
+ * frequency map (a form of histogram or multiset). For example, to
+ * add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
+ * initializing if not already present, you can use {@code
+ * freqs.computeIfAbsent(k -> new LongAdder()).increment();}
+ *
+ * <p>This class extends {@link Number}, but does <em>not</em> define
+ * methods such as {@code equals}, {@code hashCode} and {@code
+ * compareTo} because instances are expected to be mutated, and so are
+ * not useful as collection keys.
+ *
+ * @since 1.8
+ * @author Doug Lea
+ */
+public class LongAdder extends Striped64 implements Serializable {
+    private static final long serialVersionUID = 7249069246863182397L;
+
+    /**
+     * Creates a new adder with initial sum of zero.
+     */
+    public LongAdder() {
+    }
+
+    /**
+     * Adds the given value.
+     *
+     * @param x the value to add
+     */
+    public void add(long x) {
+        Cell[] as; long b, v; int m; Cell a;
+        if ((as = cells) != null || !casBase(b = base, b + x)) {
+            boolean uncontended = true;
+            if (as == null || (m = as.length - 1) < 0 ||
+              (a = as[getProbe() & m]) == null ||
+              !(uncontended = a.cas(v = a.value, v + x)))
+                longAccumulate(x, null, uncontended);
+        }
+    }
+
+    /**
+     * Equivalent to {@code add(1)}.
+     */
+    public void increment() {
+        add(1L);
+    }
+
+    /**
+     * Equivalent to {@code add(-1)}.
+     */
+    public void decrement() {
+        add(-1L);
+    }
+
+    /**
+     * Returns the current sum.  The returned value is <em>NOT</em> an
+     * atomic snapshot; invocation in the absence of concurrent
+     * updates returns an accurate result, but concurrent updates that
+     * occur while the sum is being calculated might not be
+     * incorporated.
+     *
+     * @return the sum
+     */
+    public long sum() {
+        Cell[] as = cells; Cell a;
+        long sum = base;
+        if (as != null) {
+            for (int i = 0; i < as.length; ++i) {
+                if ((a = as[i]) != null)
+                    sum += a.value;
+            }
+        }
+        return sum;
+    }
+
+    /**
+     * Resets variables maintaining the sum to zero.  This method may
+     * be a useful alternative to creating a new adder, but is only
+     * effective if there are no concurrent updates.  Because this
+     * method is intrinsically racy, it should only be used when it is
+     * known that no threads are concurrently updating.
+     */
+    public void reset() {
+        Cell[] as = cells; Cell a;
+        base = 0L;
+        if (as != null) {
+            for (int i = 0; i < as.length; ++i) {
+                if ((a = as[i]) != null)
+                    a.value = 0L;
+            }
+        }
+    }
+
+    /**
+     * Equivalent in effect to {@link #sum} followed by {@link
+     * #reset}. This method may apply for example during quiescent
+     * points between multithreaded computations.  If there are
+     * updates concurrent with this method, the returned value is
+     * <em>not</em> guaranteed to be the final value occurring before
+     * the reset.
+     *
+     * @return the sum
+     */
+    public long sumThenReset() {
+        Cell[] as = cells; Cell a;
+        long sum = base;
+        base = 0L;
+        if (as != null) {
+            for (int i = 0; i < as.length; ++i) {
+                if ((a = as[i]) != null) {
+                    sum += a.value;
+                    a.value = 0L;
+                }
+            }
+        }
+        return sum;
+    }
+
+    /**
+     * Returns the String representation of the {@link #sum}.
+     * @return the String representation of the {@link #sum}
+     */
+    public String toString() {
+        return Long.toString(sum());
+    }
+
+    /**
+     * Equivalent to {@link #sum}.
+     *
+     * @return the sum
+     */
+    public long longValue() {
+        return sum();
+    }
+
+    /**
+     * Returns the {@link #sum} as an {@code int} after a narrowing
+     * primitive conversion.
+     */
+    public int intValue() {
+        return (int)sum();
+    }
+
+    /**
+     * Returns the {@link #sum} as a {@code float}
+     * after a widening primitive conversion.
+     */
+    public float floatValue() {
+        return (float)sum();
+    }
+
+    /**
+     * Returns the {@link #sum} as a {@code double} after a widening
+     * primitive conversion.
+     */
+    public double doubleValue() {
+        return (double)sum();
+    }
+
+    /**
+     * Serialization proxy, used to avoid reference to the non-public
+     * Striped64 superclass in serialized forms.
+     * @serial include
+     */
+    private static class SerializationProxy implements Serializable {
+        private static final long serialVersionUID = 7249069246863182397L;
+
+        /**
+         * The current value returned by sum().
+         * @serial
+         */
+        private final long value;
+
+        SerializationProxy(LongAdder a) {
+            value = a.sum();
+        }
+
+        /**
+         * Return a {@code LongAdder} object with initial state
+         * held by this proxy.
+         *
+         * @return a {@code LongAdder} object with initial state
+         * held by this proxy.
+         */
+        private Object readResolve() {
+            LongAdder a = new LongAdder();
+            a.base = value;
+            return a;
+        }
+    }
+
+    /**
+     * Returns a
+     * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
+     * SerializationProxy</a>
+     * representing the state of this instance.
+     *
+     * @return a {@link SerializationProxy}
+     * representing the state of this instance
+     */
+    private Object writeReplace() {
+        return new SerializationProxy(this);
+    }
+
+    /**
+     * @param s the stream
+     * @throws java.io.InvalidObjectException always
+     */
+    private void readObject(java.io.ObjectInputStream s)
+      throws java.io.InvalidObjectException {
+        throw new java.io.InvalidObjectException("Proxy required");
+    }
+
+}