HTTP/2 Priority Tree Benchmark

Motivation:
There is no benchmark to measure the priority tree implementation performance.

Modifications:
Introduce a new benchmark which will populate the priority tree, and then shuffle parent/child links around.

Result:
A simple benchmark to get a baseline for the HTTP/2 codec's priority tree implementation.

microbench/src/main/java/io/netty/microbench/http2/Http2PriorityTreeBenchmark.java

@@ -0,0 +1,109 @@
+/*
+ * Copyright 2015 The Netty Project
+ *
+ * The Netty Project 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 io.netty.microbench.http2;
+
+import io.netty.handler.codec.http2.DefaultHttp2Connection;
+import io.netty.handler.codec.http2.Http2Connection;
+import io.netty.handler.codec.http2.Http2Exception;
+import io.netty.handler.codec.http2.Http2Stream;
+import io.netty.microbench.util.AbstractMicrobenchmark;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+import org.openjdk.jmh.annotations.Threads;
+
+/**
+ * This benchmark is designed to exercise re-prioritization events and measure the ops/sec.
+ */
+@Threads(1)
+@State(Scope.Benchmark)
+public class Http2PriorityTreeBenchmark extends AbstractMicrobenchmark {
+    private static final short WEIGHT = 1;
+
+    public Http2Connection connection;
+
+    @Param({ "1000", "10000" })
+    public int numStreams;
+
+    @Param({ "10", "100", "1000" })
+    public int d_ary;
+
+    @Param({ "10", "100", "1000" })
+    public int exclusiveOnCount;
+
+    @Setup(Level.Trial)
+    public void setup() throws Http2Exception {
+        connection = new DefaultHttp2Connection(false);
+        for (int i = 0; i < numStreams; ++i) {
+            connection.local().createStream(toStreamId(i));
+        }
+    }
+
+    @TearDown(Level.Iteration)
+    public void teardown() throws Http2Exception {
+        final int connectionId = connection.connectionStream().id();
+        for (int i = 0; i < numStreams; ++i) {
+            connection.stream(toStreamId(i)).setPriority(connectionId, WEIGHT, false);
+        }
+    }
+
+    /**
+     * A priority tree will be build using the {@link #d_ary} variable to determine the number of children for each
+     * node. After the priority tree is built the nodes closest to the root will be pushed down to be dependent on leaf
+     * nodes. This is to simulate an "expensive" tree operation.
+     */
+    @Benchmark
+    public void prioritizeStreams() throws Http2Exception {
+        int streamId = 0;
+        int parentId = 0;
+        boolean exclusive = false;
+        for (int i = 0; i < numStreams; ++i) {
+            // Treat all streams as they exist in a logical array in the range [0, numStreams].
+            // From this we can find the current parent stream via a i / d_ary operation.
+            parentId = toStreamId(i / d_ary);
+            streamId = toStreamId(i);
+            if (parentId == streamId) {
+                exclusive = i % exclusiveOnCount == 0;
+                continue;
+            }
+            Http2Stream stream = connection.stream(streamId);
+            stream.setPriority(parentId, WEIGHT, exclusive);
+            exclusive = i % exclusiveOnCount == 0;
+        }
+
+        // Now change the parent assignments by pushing the root nodes out to the leafs.
+        for (int i = 0; i < numStreams; ++i) {
+            parentId = toStreamId((numStreams - i) / d_ary);
+            streamId = toStreamId(i);
+            if (parentId == streamId) {
+                exclusive = i % exclusiveOnCount == 0;
+                continue;
+            }
+            Http2Stream stream = connection.stream(streamId);
+            stream.setPriority(parentId, WEIGHT, exclusive);
+            exclusive = i % exclusiveOnCount == 0;
+        }
+    }
+
+    private static int toStreamId(int i) {
+        return 2 * i + 1;
+    }
+}