stream: switch _writableState.buffer to queue

In cases where many small writes are made to a stream
lacking _writev, the array data structure backing the
WriteReq buffer would greatly increase GC pressure.

Specifically, in the fs.WriteStream case, the
clearBuffer routine would only clear a single WriteReq
from the buffer before exiting, but would cause the
entire backing array to be GC'd. Switching to [].shift
lessened pressure, but still the bulk of the time was
spent in memcpy.

This replaces that structure with a linked list-backed
queue so that adding and removing from the queue is O(1).
In the _writev case, collecting the buffer requires an
O(N) loop over the buffer, but that was already being
performed to collect callbacks, so slowdown should be
neglible.

PR-URL: https://github.com/joyent/node/pull/8826
Reviewed-by: Timothy J Fontaine <tjfontaine@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>

lib/_stream_writable.js

@@ -28,6 +28,7 @@ Writable.WritableState = WritableState;
 
 var util = require('util');
 var Stream = require('stream');
+var debug = util.debuglog('stream');
 
 util.inherits(Writable, Stream);
 
@@ -35,6 +36,7 @@ function WriteReq(chunk, encoding, cb) {
   this.chunk = chunk;
   this.encoding = encoding;
   this.callback = cb;
+  this.next = null;
 }
 
 function WritableState(options, stream) {
@@ -109,7 +111,8 @@ function WritableState(options, stream) {
   // the amount that is being written when _write is called.
   this.writelen = 0;
 
-  this.buffer = [];
+  this.bufferedRequest = null;
+  this.lastBufferedRequest = null;
 
   // number of pending user-supplied write callbacks
   // this must be 0 before 'finish' can be emitted
@@ -123,6 +126,23 @@ function WritableState(options, stream) {
   this.errorEmitted = false;
 }
 
+WritableState.prototype.getBuffer = function writableStateGetBuffer() {
+  var current = this.bufferedRequest;
+  var out = [];
+  while (current) {
+    out.push(current);
+    current = current.next;
+  }
+  return out;
+};
+
+Object.defineProperty(WritableState.prototype, 'buffer', {
+  get: util.deprecate(function() {
+    return this.getBuffer();
+  }, '_writableState.buffer is deprecated. Use ' +
+      '_writableState.getBuffer() instead.')
+});
+
 function Writable(options) {
   // Writable ctor is applied to Duplexes, though they're not
   // instanceof Writable, they're instanceof Readable.
@@ -216,7 +236,7 @@ Writable.prototype.uncork = function() {
         !state.corked &&
         !state.finished &&
         !state.bufferProcessing &&
-        state.buffer.length)
+        state.bufferedRequest)
       clearBuffer(this, state);
   }
 };
@@ -255,8 +275,15 @@ function writeOrBuffer(stream, state, chunk, encoding, cb) {
   if (!ret)
     state.needDrain = true;
 
-  if (state.writing || state.corked)
-    state.buffer.push(new WriteReq(chunk, encoding, cb));
+  if (state.writing || state.corked) {
+    var last = state.lastBufferedRequest;
+    state.lastBufferedRequest = new WriteReq(chunk, encoding, cb);
+    if (last) {
+      last.next = state.lastBufferedRequest;
+    } else {
+      state.bufferedRequest = state.lastBufferedRequest;
+    }
+  }
   else
     doWrite(stream, state, false, len, chunk, encoding, cb);
 
@@ -313,7 +340,7 @@ function onwrite(stream, er) {
     if (!finished &&
         !state.corked &&
         !state.bufferProcessing &&
-        state.buffer.length) {
+        state.bufferedRequest) {
       clearBuffer(stream, state);
     }
 
@@ -349,17 +376,23 @@ function onwriteDrain(stream, state) {
 // if there's something in the buffer waiting, then process it
 function clearBuffer(stream, state) {
   state.bufferProcessing = true;
+  var entry = state.bufferedRequest;
 
-  if (stream._writev && state.buffer.length > 1) {
+  if (stream._writev && entry && entry.next) {
     // Fast case, write everything using _writev()
+    var buffer = [];
     var cbs = [];
-    for (var c = 0; c < state.buffer.length; c++)
-      cbs.push(state.buffer[c].callback);
+    while (entry) {
+      cbs.push(entry.callback);
+      buffer.push(entry);
+      entry = entry.next;
+    }
 
     // count the one we are adding, as well.
     // TODO(isaacs) clean this up
     state.pendingcb++;
-    doWrite(stream, state, true, state.length, state.buffer, '', function(err) {
+    state.lastBufferedRequest = null;
+    doWrite(stream, state, true, state.length, buffer, '', function(err) {
       for (var i = 0; i < cbs.length; i++) {
         state.pendingcb--;
         cbs[i](err);
@@ -367,34 +400,29 @@ function clearBuffer(stream, state) {
     });
 
     // Clear buffer
-    state.buffer = [];
   } else {
     // Slow case, write chunks one-by-one
-    for (var c = 0; c < state.buffer.length; c++) {
-      var entry = state.buffer[c];
+    while (entry) {
       var chunk = entry.chunk;
       var encoding = entry.encoding;
       var cb = entry.callback;
       var len = state.objectMode ? 1 : chunk.length;
 
       doWrite(stream, state, false, len, chunk, encoding, cb);
-
+      entry = entry.next;
       // if we didn't call the onwrite immediately, then
       // it means that we need to wait until it does.
       // also, that means that the chunk and cb are currently
       // being processed, so move the buffer counter past them.
       if (state.writing) {
-        c++;
         break;
       }
     }
 
-    if (c < state.buffer.length)
-      state.buffer = state.buffer.slice(c);
-    else
-      state.buffer.length = 0;
+    if (entry === null)
+      state.lastBufferedRequest = null;
   }
-
+  state.bufferedRequest = entry;
   state.bufferProcessing = false;
 }
 
@@ -435,7 +463,7 @@ Writable.prototype.end = function(chunk, encoding, cb) {
 function needFinish(stream, state) {
   return (state.ending &&
           state.length === 0 &&
-          state.buffer.length === 0 &&
+          state.bufferedRequest === null &&
           !state.finished &&
           !state.writing);
 }

lib/net.js

@@ -732,7 +732,7 @@ Socket.prototype.__defineGetter__('bytesWritten', function() {
       data = this._pendingData,
       encoding = this._pendingEncoding;
 
-  state.buffer.forEach(function(el) {
+  state.getBuffer().forEach(function(el) {
     if (util.isBuffer(el.chunk))
       bytes += el.chunk.length;
     else

test/simple/test-stream2-transform.js

@@ -81,7 +81,7 @@ test('writable side consumption', function(t) {
   t.equal(tx._readableState.length, 10);
   t.equal(transformed, 10);
   t.equal(tx._transformState.writechunk.length, 5);
-  t.same(tx._writableState.buffer.map(function(c) {
+  t.same(tx._writableState.getBuffer().map(function(c) {
     return c.chunk.length;
   }), [6, 7, 8, 9, 10]);