Implement new bounded buffer for constant query collection

community/data-collector/src/main/java/org/neo4j/internal/collector/ConcurrentLinkedQueueRecentBuffer.java

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+/*
+ * Copyright (c) 2002-2019 "Neo4j,"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.internal.collector;
+
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Consumer;
+
+/**
+ * Implementation of {@link RecentBuffer} using {@link ConcurrentLinkedQueue}.
+ */
+public class ConcurrentLinkedQueueRecentBuffer<T> implements RecentBuffer<T>
+{
+    private final ConcurrentLinkedQueue<T> queue;
+    private final int maxSize;
+    private final AtomicInteger size;
+
+    public ConcurrentLinkedQueueRecentBuffer( int bitSize )
+    {
+        maxSize = 1 << bitSize;
+        queue = new ConcurrentLinkedQueue<>();
+        size = new AtomicInteger( 0 );
+    }
+
+    /* ---- many producers ---- */
+
+    @Override
+    public void produce( T t )
+    {
+        queue.add( t );
+        int newSize = size.incrementAndGet();
+        if ( newSize > maxSize )
+        {
+            queue.poll();
+            size.decrementAndGet();
+        }
+    }
+
+    /* ---- single consumer ---- */
+
+    @Override
+    public void clear()
+    {
+        queue.clear();
+        // might go out of sync with queue here, but should be minor slippage.
+        // Will not accumulate leaks either, but reset on every clear.
+        size.set( 0 );
+    }
+
+    @Override
+    public void foreach( Consumer<T> consumer )
+    {
+        queue.forEach( consumer );
+    }
+}

community/data-collector/src/main/java/org/neo4j/internal/collector/RecentBuffer.java

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+/*
+ * Copyright (c) 2002-2019 "Neo4j,"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.internal.collector;
+
+import java.util.function.Consumer;
+
+/**
+ * Bounded buffer which holds the last n elements. When the buffer is full, each
+ * produce will replace the elements in the buffer that was added first.
+ *
+ * This collection thread-safely allows
+ *  - multiple threads concurrently calling `produce`
+ *  - serialized calling of `clear` and `foreach`
+ *
+ * @param <T> type of elements in this buffer.
+ */
+public interface RecentBuffer<T>
+{
+    /**
+     * Produce element into the buffer.
+     *
+     * @param t element to produce
+     */
+    void produce( T t );
+
+    /**
+     * Clear all elements from the buffer.
+     */
+    void clear();
+
+    /**
+     * Iterate over all elements in the buffer. No elements are removed from the buffer.
+     *
+     * @param consumer consumer to apply on each element
+     */
+    void foreach( Consumer<T> consumer );
+}

community/data-collector/src/main/java/org/neo4j/internal/collector/RingRecentBuffer.java

@@ -0,0 +1,100 @@
+/*
+ * Copyright (c) 2002-2019 "Neo4j,"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.internal.collector;
+
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Consumer;
+
+/**
+ * Implementation of {@link RecentBuffer} using ring buffer.
+ */
+public class RingRecentBuffer<T> implements RecentBuffer<T>
+{
+    private final int size;
+    private final int mask;
+    private final VolatileRef<T>[] data;
+
+    private final AtomicLong produceCount;
+    private final AtomicLong consumeCount;
+
+    public RingRecentBuffer( int bitSize )
+    {
+        size = 1 << bitSize;
+        mask = size - 1;
+
+        //noinspection unchecked
+        data = new VolatileRef[size];
+        for ( int i = 0; i < size; i++ )
+        {
+            data[i] = new VolatileRef<>();
+        }
+
+        produceCount = new AtomicLong( 0 );
+        consumeCount = new AtomicLong( 0 );
+    }
+
+    /* ---- many producers ---- */
+
+    @Override
+    public void produce( T t )
+    {
+        long produceNumber = produceCount.getAndIncrement();
+        int offset = (int) (produceNumber & mask);
+        VolatileRef<T> volatileRef = data[offset];
+        volatileRef.ref = t;
+        volatileRef.produceNumber = produceNumber;
+    }
+
+    /* ---- single consumer ---- */
+
+    @Override
+    public void clear()
+    {
+        for ( VolatileRef<T> volatileRef : data )
+        {
+            volatileRef.ref = null;
+        }
+        long snapshotProduce = produceCount.get();
+        consumeCount.set( snapshotProduce );
+    }
+
+    @Override
+    public void foreach( Consumer<T> consumer )
+    {
+        long snapshotProduce = produceCount.get();
+        long snapshotConsume = Math.max( consumeCount.get(), snapshotProduce - size );
+        for ( long i = snapshotConsume; i < snapshotProduce; i++ )
+        {
+            int offset = (int) (i & mask);
+            VolatileRef<T> volatileRef = data[offset];
+            if ( volatileRef.produceNumber < i )
+            {
+                return;
+            }
+            consumer.accept( volatileRef.ref );
+        }
+    }
+
+    private static class VolatileRef<T>
+    {
+        private volatile T ref;
+        private volatile long produceNumber;
+    }
+}