Disable Huffman encoding for small headers (netty#9260)

Motivation:

Huffman coding saves only a little space, but has a huge CPU cost

Modification:

Disable huff coding for headers smaller than 512 bytes.  Also, add a
configurable limit to the encoder.

Result:

Faster HPACK

BEFORE:
```
Benchmark                     (duplicates)  (limitToAscii)  (sensitive)  (size)  Mode  Cnt       Score       Error  Units
HpackEncoderBenchmark.encode          true            true         true   SMALL  avgt   10    2572.595 ±    16.184  ns/op
HpackEncoderBenchmark.encode          true            true         true  MEDIUM  avgt   10   19580.815 ±   397.780  ns/op
HpackEncoderBenchmark.encode          true            true         true   LARGE  avgt   10  379456.381 ±  2059.919  ns/op
HpackEncoderBenchmark.encode          true            true        false   SMALL  avgt   10     730.579 ±     8.116  ns/op
HpackEncoderBenchmark.encode          true            true        false  MEDIUM  avgt   10    2087.590 ±    84.644  ns/op
HpackEncoderBenchmark.encode          true            true        false   LARGE  avgt   10   11725.228 ±    89.298  ns/op
HpackEncoderBenchmark.encode          true           false         true   SMALL  avgt   10     555.971 ±     5.120  ns/op
HpackEncoderBenchmark.encode          true           false         true  MEDIUM  avgt   10    2831.874 ±    41.801  ns/op
HpackEncoderBenchmark.encode          true           false         true   LARGE  avgt   10   36054.025 ±   179.504  ns/op
HpackEncoderBenchmark.encode          true           false        false   SMALL  avgt   10     340.337 ±     3.313  ns/op
HpackEncoderBenchmark.encode          true           false        false  MEDIUM  avgt   10    1006.817 ±     8.942  ns/op
HpackEncoderBenchmark.encode          true           false        false   LARGE  avgt   10    8784.168 ±   164.014  ns/op
HpackEncoderBenchmark.encode         false            true         true   SMALL  avgt   10    2561.934 ±    27.056  ns/op
HpackEncoderBenchmark.encode         false            true         true  MEDIUM  avgt   10   22061.105 ±   154.533  ns/op
HpackEncoderBenchmark.encode         false            true         true   LARGE  avgt   10  435744.897 ±  8853.388  ns/op
HpackEncoderBenchmark.encode         false            true        false   SMALL  avgt   10    2737.683 ±    47.142  ns/op
HpackEncoderBenchmark.encode         false            true        false  MEDIUM  avgt   10   22385.146 ±    98.430  ns/op
HpackEncoderBenchmark.encode         false            true        false   LARGE  avgt   10  408159.698 ± 12044.931  ns/op
HpackEncoderBenchmark.encode         false           false         true   SMALL  avgt   10     544.213 ±     3.279  ns/op
HpackEncoderBenchmark.encode         false           false         true  MEDIUM  avgt   10    2908.978 ±    31.026  ns/op
HpackEncoderBenchmark.encode         false           false         true   LARGE  avgt   10   36471.262 ±  1044.010  ns/op
HpackEncoderBenchmark.encode         false           false        false   SMALL  avgt   10     609.305 ±     4.371  ns/op
HpackEncoderBenchmark.encode         false           false        false  MEDIUM  avgt   10    3223.946 ±    23.505  ns/op
HpackEncoderBenchmark.encode         false           false        false   LARGE  avgt   10   39975.152 ±   655.196  ns/op
```

AFTER:
```
NEW AFTER

Benchmark                     (duplicates)  (limitToAscii)  (sensitive)  (size)  Mode  Cnt     Score     Error  Units
HpackEncoderBenchmark.encode          true            true         true   SMALL  avgt    5   379.473 ± 133.815  ns/op
HpackEncoderBenchmark.encode          true            true         true  MEDIUM  avgt    5  1118.772 ±  89.258  ns/op
HpackEncoderBenchmark.encode          true            true         true   LARGE  avgt    5  5366.828 ±  89.746  ns/op
HpackEncoderBenchmark.encode          true            true        false   SMALL  avgt    5   284.401 ±   2.088  ns/op
HpackEncoderBenchmark.encode          true            true        false  MEDIUM  avgt    5   922.805 ±  10.796  ns/op
HpackEncoderBenchmark.encode          true            true        false   LARGE  avgt    5  8727.831 ± 462.138  ns/op
HpackEncoderBenchmark.encode          true           false         true   SMALL  avgt    5   337.093 ±  22.585  ns/op
HpackEncoderBenchmark.encode          true           false         true  MEDIUM  avgt    5   693.689 ±  16.351  ns/op
HpackEncoderBenchmark.encode          true           false         true   LARGE  avgt    5  5616.786 ±  98.647  ns/op
HpackEncoderBenchmark.encode          true           false        false   SMALL  avgt    5   286.708 ±  13.765  ns/op
HpackEncoderBenchmark.encode          true           false        false  MEDIUM  avgt    5   906.279 ±  32.338  ns/op
HpackEncoderBenchmark.encode          true           false        false   LARGE  avgt    5  8304.736 ± 128.584  ns/op
HpackEncoderBenchmark.encode         false            true         true   SMALL  avgt    5   351.381 ±  15.547  ns/op
HpackEncoderBenchmark.encode         false            true         true  MEDIUM  avgt    5  1188.166 ±   7.023  ns/op
HpackEncoderBenchmark.encode         false            true         true   LARGE  avgt    5  6876.009 ±  48.117  ns/op
HpackEncoderBenchmark.encode         false            true        false   SMALL  avgt    5   434.759 ±   8.619  ns/op
HpackEncoderBenchmark.encode         false            true        false  MEDIUM  avgt    5   954.588 ±  58.514  ns/op
HpackEncoderBenchmark.encode         false            true        false   LARGE  avgt    5  8534.017 ± 552.597  ns/op
HpackEncoderBenchmark.encode         false           false         true   SMALL  avgt    5   223.713 ±   4.823  ns/op
HpackEncoderBenchmark.encode         false           false         true  MEDIUM  avgt    5  1181.538 ±  11.851  ns/op
HpackEncoderBenchmark.encode         false           false         true   LARGE  avgt    5  6670.830 ± 267.927  ns/op
HpackEncoderBenchmark.encode         false           false        false   SMALL  avgt    5   424.609 ±  27.477  ns/op
HpackEncoderBenchmark.encode         false           false        false  MEDIUM  avgt    5  1003.578 ±  53.991  ns/op
HpackEncoderBenchmark.encode         false           false        false   LARGE  avgt    5  8428.932 ± 102.838  ns/op
```

Author: carl-mastrangelo

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2HeadersEncoder.java

@@ -43,7 +43,13 @@ public DefaultHttp2HeadersEncoder(SensitivityDetector sensitivityDetector, boole
 
     public DefaultHttp2HeadersEncoder(SensitivityDetector sensitivityDetector, boolean ignoreMaxHeaderListSize,
                                       int dynamicTableArraySizeHint) {
-        this(sensitivityDetector, new HpackEncoder(ignoreMaxHeaderListSize, dynamicTableArraySizeHint));
+        this(sensitivityDetector, ignoreMaxHeaderListSize, dynamicTableArraySizeHint, HpackEncoder.HUFF_CODE_THRESHOLD);
+    }
+
+    public DefaultHttp2HeadersEncoder(SensitivityDetector sensitivityDetector, boolean ignoreMaxHeaderListSize,
+                                      int dynamicTableArraySizeHint, int huffCodeThreshold) {
+        this(sensitivityDetector,
+                new HpackEncoder(ignoreMaxHeaderListSize, dynamicTableArraySizeHint, huffCodeThreshold));
     }
 
     /**

codec-http2/src/main/java/io/netty/handler/codec/http2/HpackEncoder.java

@@ -54,13 +54,15 @@
 import static java.lang.Math.min;
 
 final class HpackEncoder {
+    static final int HUFF_CODE_THRESHOLD = 512;
     // a linked hash map of header fields
     private final HeaderEntry[] headerFields;
     private final HeaderEntry head = new HeaderEntry(-1, AsciiString.EMPTY_STRING,
             AsciiString.EMPTY_STRING, Integer.MAX_VALUE, null);
     private final HpackHuffmanEncoder hpackHuffmanEncoder = new HpackHuffmanEncoder();
     private final byte hashMask;
     private final boolean ignoreMaxHeaderListSize;
+    private final int huffCodeThreshold;
     private long size;
     private long maxHeaderTableSize;
     private long maxHeaderListSize;
@@ -76,13 +78,13 @@ final class HpackEncoder {
      * Creates a new encoder.
      */
     HpackEncoder(boolean ignoreMaxHeaderListSize) {
-        this(ignoreMaxHeaderListSize, 16);
+        this(ignoreMaxHeaderListSize, 16, HUFF_CODE_THRESHOLD);
     }
 
     /**
      * Creates a new encoder.
      */
-    HpackEncoder(boolean ignoreMaxHeaderListSize, int arraySizeHint) {
+    HpackEncoder(boolean ignoreMaxHeaderListSize, int arraySizeHint, int huffCodeThreshold) {
         this.ignoreMaxHeaderListSize = ignoreMaxHeaderListSize;
         maxHeaderTableSize = DEFAULT_HEADER_TABLE_SIZE;
         maxHeaderListSize = MAX_HEADER_LIST_SIZE;
@@ -91,6 +93,7 @@ final class HpackEncoder {
         headerFields = new HeaderEntry[findNextPositivePowerOfTwo(max(2, min(arraySizeHint, 128)))];
         hashMask = (byte) (headerFields.length - 1);
         head.before = head.after = head;
+        this.huffCodeThreshold = huffCodeThreshold;
     }
 
     /**
@@ -250,8 +253,9 @@ private static void encodeInteger(ByteBuf out, int mask, int n, long i) {
      * Encode string literal according to Section 5.2.
      */
     private void encodeStringLiteral(ByteBuf out, CharSequence string) {
-        int huffmanLength = hpackHuffmanEncoder.getEncodedLength(string);
-        if (huffmanLength < string.length()) {
+        int huffmanLength;
+        if (string.length() >= huffCodeThreshold
+                && (huffmanLength = hpackHuffmanEncoder.getEncodedLength(string)) < string.length()) {
             encodeInteger(out, 0x80, 7, huffmanLength);
             hpackHuffmanEncoder.encode(out, string);
         } else {

codec-http2/src/test/java/io/netty/handler/codec/http2/DefaultHttp2FrameWriterTest.java

@@ -15,6 +15,7 @@
 package io.netty.handler.codec.http2;
 
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
 import io.netty.buffer.UnpooledByteBufAllocator;
 import io.netty.channel.Channel;
@@ -65,17 +66,18 @@ public class DefaultHttp2FrameWriterTest {
     @Before
     public void setUp() throws Exception {
         MockitoAnnotations.initMocks(this);
+        http2HeadersEncoder = new DefaultHttp2HeadersEncoder(
+                Http2HeadersEncoder.NEVER_SENSITIVE, new HpackEncoder(false, 16, 0));
 
-        frameWriter = new DefaultHttp2FrameWriter();
+        frameWriter = new DefaultHttp2FrameWriter(new DefaultHttp2HeadersEncoder(
+                Http2HeadersEncoder.NEVER_SENSITIVE, new HpackEncoder(false, 16, 0)));
 
         outbound = Unpooled.buffer();
 
         expectedOutbound = Unpooled.EMPTY_BUFFER;
 
         promise = new DefaultChannelPromise(channel, ImmediateEventExecutor.INSTANCE);
 
-        http2HeadersEncoder = new DefaultHttp2HeadersEncoder();
-
         Answer<Object> answer = new Answer<Object>() {
             @Override
             public Object answer(InvocationOnMock var1) throws Throwable {

codec-http2/src/test/java/io/netty/handler/codec/http2/Http2TestUtil.java

@@ -119,7 +119,7 @@ public static HpackEncoder newTestEncoder() {
 
     public static HpackEncoder newTestEncoder(boolean ignoreMaxHeaderListSize,
                                               long maxHeaderListSize, long maxHeaderTableSize) throws Http2Exception {
-        HpackEncoder hpackEncoder = new HpackEncoder();
+        HpackEncoder hpackEncoder = new HpackEncoder(false, 16, 0);
         ByteBuf buf = Unpooled.buffer();
         try {
             hpackEncoder.setMaxHeaderTableSize(buf, maxHeaderTableSize);