Buffer the entire http header to one packet, and buffer other data in chunks of 4-16 kb.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 55445491b8d6c28110af30ee02d3b9e8ca1ea0a7..f54aea27597c74c88f126d89d8008a62e5bbcf12 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -449,6 +449,7 @@ abstract class _HttpOutboundMessage<T> implements IOSink {
       ioSink.close();
       return stream.reduce(null, (x, y) {}).then((_) => this);
     }
+    stream = stream.transform(new _BufferTransformer());
     if (headers.chunkedTransferEncoding) {
       if (asGZip) {
         stream = stream.transform(new ZLibDeflater(gzip: true, level: 6));
@@ -475,6 +476,39 @@ class _HttpOutboundConsumer implements StreamConsumer {
   Future consume(var stream) => _consume(_ioSink, stream, _asGZip);
 }
 
+class _BufferTransformer extends StreamEventTransformer<List<int>, List<int>> {
+  const int MIN_CHUNK_SIZE = 4 * 1024;
+  const int MAX_BUFFER_SIZE = 16 * 1024;
+
+  final _BufferList _buffer = new _BufferList();
+
+  void handleData(List<int> data, EventSink<List<int>> sink) {
+    // TODO(ajohnsen): Use timeout?
+    if (data.length == 0) return;
+    if (data.length >= MIN_CHUNK_SIZE) {
+      flush(sink);
+      sink.add(data);
+    } else {
+      _buffer.add(data);
+      if (_buffer.length >= MAX_BUFFER_SIZE) {
+        flush(sink);
+      }
+    }
+  }
+
+  void handleDone(EventSink<List<int>> sink) {
+    flush(sink);
+    sink.close();
+  }
+
+  void flush(EventSink<List<int>> sink) {
+    if (_buffer.length > 0) {
+      sink.add(_buffer.readBytes());
+      _buffer.clear();
+    }
+  }
+}
+
 
 class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     implements HttpResponse {
@@ -514,19 +548,20 @@ class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
   HttpConnectionInfo get connectionInfo => _httpRequest.connectionInfo;
 
   void _writeHeader() {
-    writeSP() => _ioSink.writeBytes([_CharCode.SP]);
-    writeCRLF() => _ioSink.writeBytes([_CharCode.CR, _CharCode.LF]);
+    var buffer = new _BufferList();
+    writeSP() => buffer.add(const [_CharCode.SP]);
+    writeCRLF() => buffer.add(const [_CharCode.CR, _CharCode.LF]);
 
     // Write status line.
     if (headers.protocolVersion == "1.1") {
-      _ioSink.writeBytes(_Const.HTTP11);
+      buffer.add(_Const.HTTP11);
     } else {
-      _ioSink.writeBytes(_Const.HTTP10);
+      buffer.add(_Const.HTTP10);
     }
     writeSP();
-    _ioSink.write(statusCode.toString());
+    buffer.add(statusCode.toString().codeUnits);
     writeSP();
-    _ioSink.write(reasonPhrase);
+    buffer.add(reasonPhrase.codeUnits);
     writeCRLF();
 
     var session = _httpRequest._session;
@@ -560,8 +595,9 @@ class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     headers._finalize();
 
     // Write headers.
-    headers._write(_ioSink);
+    headers._write(buffer);
     writeCRLF();
+    _ioSink.writeBytes(buffer.readBytes());
   }
 
   String _findReasonPhrase(int statusCode) {
@@ -710,10 +746,11 @@ class _HttpClientRequest extends _HttpOutboundMessage<HttpClientRequest>
   }
 
   void _writeHeader() {
-    writeSP() => _ioSink.writeBytes([_CharCode.SP]);
-    writeCRLF() => _ioSink.writeBytes([_CharCode.CR, _CharCode.LF]);
+    var buffer = new _BufferList();

[Søren Gjesse, 2013/03/12 11:30:55]

I am not sure that using a _BufferList for the header this way is a good idea,
but it is easy to change later.

E.g. adding CR LF is now adding a full Queue entry in the _BufferList for two
bytes of output. One option for the header is to calculate its size and then
create and fill a UInt8List.

[Anders Johnsen, 2013/03/12 12:11:30]

Yes, we have to look into this once we really dive into the performance story.

+    writeSP() => buffer.add(const [_CharCode.SP]);
+    writeCRLF() => buffer.add(const [_CharCode.CR, _CharCode.LF]);
 
-    _ioSink.write(method);
+    buffer.add(method.codeUnits);
     writeSP();
     // Send the path for direct connections and the whole URL for
     // proxy connections.
@@ -727,12 +764,12 @@ class _HttpClientRequest extends _HttpOutboundMessage<HttpClientRequest>
           path = "${path}?${uri.query}";
         }
       }
-      _ioSink.write(path);
+      buffer.add(path.codeUnits);
     } else {
-      _ioSink.write(uri.toString());
+      buffer.add(uri.toString().codeUnits);
     }
     writeSP();
-    _ioSink.writeBytes(_Const.HTTP11);
+    buffer.add(_Const.HTTP11);
     writeCRLF();
 
     // Add the cookies to the headers.
@@ -750,8 +787,9 @@ class _HttpClientRequest extends _HttpOutboundMessage<HttpClientRequest>
     headers._finalize();
 
     // Write headers.
-    headers._write(_ioSink);
+    headers._write(buffer);
     writeCRLF();
+    _ioSink.writeBytes(buffer.readBytes());
   }
 }