More accurate default receive buffer size predictor implementation

src/main/java/org/jboss/netty/channel/socket/nio/DefaultReceiveBufferSizePredictor.java

@@ -22,14 +22,17 @@
  */
 package org.jboss.netty.channel.socket.nio;
 
+import java.util.ArrayList;
+import java.util.List;
+
 /**
  * The default {@link ReceiveBufferSizePredictor} implementation.
  * <p>
- * It doubles the expected number of readable bytes if the previous read
- * filled the allocated buffer.  It halves the expected number of readable
- * bytes if the read operation was not able to fill a quarter of the allocated
- * buffer two times consecutively.  Otherwise, it keeps returning the previous
- * prediction.
+ * It gradually increases the expected number of readable bytes if the
+ * previous read filled the allocated buffer.  It gradually decreases the
+ * expected number of readable bytes if the read operation was not able to
+ * fill a certain amount of the allocated buffer two times consecutively.
+ * Otherwise, it keeps returning the previous prediction.
  *
  * @author The Netty Project (netty-dev@lists.jboss.org)
  * @author Trustin Lee (tlee@redhat.com)
@@ -39,19 +42,78 @@ package org.jboss.netty.channel.socket.nio;
  */
 public class DefaultReceiveBufferSizePredictor implements
         ReceiveBufferSizePredictor {
-    private static final int DEFAULT_MINIMUM = 256;
+
+    private static final int INDEX_INCREMENT = 4;
+    private static final int INDEX_DECREMENT = 1;
+
+    private static final int[] SIZE_TABLE;
+
+    static {
+        List<Integer> sizeTable = new ArrayList<Integer>();
+        for (int i = 1; i <= 8; i ++) {
+            sizeTable.add(i);
+        }
+
+        for (int i = 4; i < 32; i ++) {
+            long v = 1L << i;
+            long inc = v >>> 4;
+            v -= inc << 3;
+
+            for (int j = 0; j < 8; j ++) {
+                v += inc;
+                if (v > Integer.MAX_VALUE) {
+                    sizeTable.add(Integer.MAX_VALUE);
+                } else {
+                    sizeTable.add((int) v);
+                }
+            }
+        }
+
+        SIZE_TABLE = new int[sizeTable.size()];
+        for (int i = 0; i < SIZE_TABLE.length; i ++) {
+            SIZE_TABLE[i] = sizeTable.get(i);
+        }
+    }
+
+    private static int getSizeTableIndex(final int size) {
+        if (size <= 16) {
+            return size - 1;
+        }
+
+        int bits = 0;
+        int v = size;
+        do {
+            v >>>= 1;
+            bits ++;
+        } while (v != 0);
+
+        final int baseIdx = bits << 3;
+        final int startIdx = baseIdx - 18;
+        final int endIdx = baseIdx - 25;
+
+        for (int i = startIdx; i >= endIdx; i --) {
+            if (size >= SIZE_TABLE[i]) {
+                return i;
+            }
+        }
+
+        throw new Error("shouldn't reach here; please file a bug report.");
+    }
+
+    private static final int DEFAULT_MINIMUM = 64;
     private static final int DEFAULT_INITIAL = 1024;
     private static final int DEFAULT_MAXIMUM = 65536;
 
-    private final int minimum;
-    private final int maximum;
-    private int nextReceiveBufferSize = 1024;
-    private boolean shouldHalveNow;
+    private final int minIndex;
+    private final int maxIndex;
+    private int index;
+    private int nextReceiveBufferSize;
+    private boolean decreaseNow;
 
     /**
      * Creates a new predictor with the default parameters.  With the default
      * parameters, the expected buffer size starts from {@code 1024}, doesn't
-     * go down below {@code 256}, and doesn't go up above {@code 1048576}.
+     * go down below {@code 64}, and doesn't go up above {@code 65536}.
      */
     public DefaultReceiveBufferSizePredictor() {
         this(DEFAULT_MINIMUM, DEFAULT_INITIAL, DEFAULT_MAXIMUM);
@@ -74,9 +136,23 @@ public class DefaultReceiveBufferSizePredictor implements
         if (maximum < initial) {
             throw new IllegalArgumentException("maximum: " + maximum);
         }
-        this.minimum = minimum;
-        nextReceiveBufferSize = initial;
-        this.maximum = maximum;
+
+        int minIndex = getSizeTableIndex(minimum);
+        if (SIZE_TABLE[minIndex] < minimum) {
+            this.minIndex = minIndex + 1;
+        } else {
+            this.minIndex = minIndex;
+        }
+
+        int maxIndex = getSizeTableIndex(maximum);
+        if (SIZE_TABLE[maxIndex] > maximum) {
+            this.maxIndex = maxIndex - 1;
+        } else {
+            this.maxIndex = maxIndex;
+        }
+
+        index = getSizeTableIndex(initial);
+        nextReceiveBufferSize = SIZE_TABLE[index];
     }
 
     public int nextReceiveBufferSize() {
@@ -84,16 +160,18 @@ public class DefaultReceiveBufferSizePredictor implements
     }
 
     public void previousReceiveBufferSize(int previousReceiveBufferSize) {
-        if (previousReceiveBufferSize < nextReceiveBufferSize >>> 1) {
-            if (shouldHalveNow) {
-                nextReceiveBufferSize = Math.max(minimum, nextReceiveBufferSize >>> 1);
-                shouldHalveNow = false;
+        if (previousReceiveBufferSize <= SIZE_TABLE[Math.max(0, index - 2)]) {
+            if (decreaseNow) {
+                index = Math.max(index - INDEX_DECREMENT, minIndex);
+                nextReceiveBufferSize = SIZE_TABLE[index];
+                decreaseNow = false;
             } else {
-                shouldHalveNow = true;
+                decreaseNow = true;
             }
         } else if (previousReceiveBufferSize >= nextReceiveBufferSize) {
-            nextReceiveBufferSize = Math.min(maximum, nextReceiveBufferSize << 1);
-            shouldHalveNow = false;
+            index = Math.min(index + INDEX_INCREMENT, maxIndex);
+            nextReceiveBufferSize = SIZE_TABLE[index];
+            decreaseNow = false;
         }
     }
 }