stream: switch _writableState.buffer to queue

[chrisdickinson]

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.

Example code (courtesy @hayes, who originally discovered this slowdown while dealing with log output to bunyan):

var fs = require('fs')
var stream = fs.createWriteStream('/dev/null', {flags: 'w+'})
var buf = new Buffer('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\n', 'utf8')
var start = Date.now()
var written = 0
function log() {
  for(var i = 0; i < 1000; i++) {
    ++written
    stream.write(buf, function() {
      --written
    })
  }
  if(Date.now() - start < 1000) {
    return setImmediate(log) 
  }
  setInterval(function() {
    console.log(written, process.memoryUsage())
  }, 1000).unref()
  console.log('done writing')
}
log()

before the change:

array.shift()

(this replaces the common c=1 slice case with [].shift()):

as queue:

I'm working on getting a better "after" flamegraph at the moment.

As a measurable effect: the current version clears ~300 or so WriteReqs/second on my machine; the queue version clears ~50k WriteReqs/second.

cc @wraithan

R=@trevnorris @misterdjules

[tjfontaine]

This is internal/private/undocumented state, I'm not sure we need to do this deprecation warning as we've never really explained to people how to interact with this.

[chrisdickinson]

Cool -- I was mostly concerned with supporting projects like vstream that reach into ReadableState for metrics. I figured that if there's one I know about, there's probably more that I don't know about :)

[tjfontaine]

This looks great, I would love to also get a proposal on what it means for streams consumers to inspect the undocumented readable/writable state, and work up an API around that.

[trevnorris]

I like how you did this, and I'm cool with this being merged.

[chrisdickinson]

Merged in 9158666.