spark-14290 avoid significant memory copy in netty's transferTo

Author: liyezhang556520

common/network-common/src/main/java/org/apache/spark/network/protocol/MessageWithHeader.java

@@ -18,6 +18,7 @@
 package org.apache.spark.network.protocol;
 
 import java.io.IOException;
+import java.nio.ByteBuffer;
 import java.nio.channels.WritableByteChannel;
 import javax.annotation.Nullable;
 
@@ -43,6 +44,14 @@ class MessageWithHeader extends AbstractReferenceCounted implements FileRegion {
   private final long bodyLength;
   private long totalBytesTransferred;
 
+  /**
+   * When the write buffer size is larger than this limit, I/O will be done in chunks of this size.
+   * The size should not be too large as it will waste underlying memory copy. e.g. If network
+   * avaliable buffer is smaller than this limit, the data cannot be sent within one single write
+   * operation while it still will make memory copy with this size.
+   */
+  private static final int NIO_BUFFER_LIMIT = 512 * 1024;
+
   /**
    * Construct a new MessageWithHeader.
    *
@@ -128,8 +137,27 @@ protected void deallocate() {
   }
 
   private int copyByteBuf(ByteBuf buf, WritableByteChannel target) throws IOException {
-    int written = target.write(buf.nioBuffer());
+    ByteBuffer buffer = buf.nioBuffer();
+    int written = (buffer.remaining() <= NIO_BUFFER_LIMIT) ?
+      target.write(buffer) : writeNioBuffer(target, buffer);
     buf.skipBytes(written);
     return written;
   }
+
+  private int writeNioBuffer(
+      WritableByteChannel writeCh,
+      ByteBuffer buf) throws IOException {
+    int originalLimit = buf.limit();
+    int ret = 0;
+
+    try {
+      int ioSize = Math.min(buf.remaining(), NIO_BUFFER_LIMIT);
+      buf.limit(buf.position() + ioSize);
+      ret = writeCh.write(buf);
+    } finally {
+      buf.limit(originalLimit);
+    }
+
+    return ret;
+  }
 }