Abundant concurrency in Go

Today, I merged a Pull Request that’s so obviously the right model for my Go rendering engine that I had to sit down and figure out why I did it any other way in the first place.

It turns out I have a JavaScripter’s mindset on concurrency.

In JavaScript, you have two levels of concurrency to choose from. On a single CPU you have the single-threaded, non-blocking type, where an event loop lets you simulate I/O concurrency. This doesn’t help with hashing passwords or processing long lists, but you can read a file without ignoring HTTP requests. Across CPUs you have workers (web workers in the browser, cluster workers in node). Each worker forks its own process with all that that entails for startup time, memory use, and inter-process communication.

This generates two habits: First, you always write async code because you’re sharing a single thread with the rest of the process. Iterating over a large array can cause performance issues in a JS app. Second, you consider concurrency in terms of single digits to map processes to CPUs. If you spend too much time on a task, you block other functions from the event loop; if you spin up too many processes, you create contention and waste resources.

So, when I added concurrency to pbr’s API, I forced the user to spin up a handful of goroutines that they’d monitor over channels. It was made “easier” via this complex Monitor type that created goroutines for you:

// AddSampler creates a new worker with that sampler.
// (user's responsibility)
func (m *Monitor) AddSampler(s *Sampler) {
	m.active++
	go func() {
		for {
			frame := s.SampleFrame()
			m.samples.Lock()
			m.samples.count += frame
			total := m.samples.count
			m.samples.Unlock()
			m.Progress <- total
			select {
			case <-m.cancel:
				m.active--
				m.Results <- s.Pixels()
				return
			default:
			}
		}
	}()
}

Pushing the concurrent requirements up to the user resulted in this monstrosity which will be familiar to anyone who’s used node’s cluster API.

As I sketched out a 100-line, 2-channel “hello, world” example, I realized my mistake. The pbr renderer could start as many goroutines as it needs to quickly render an image without ever exposing them to the user. In Go, I can build internal concurrency while exposing a simple, sequential API.

In JavaScript, it would be unthinkable to spawn several async routines that each for loop through two billion pixels at once, but that’s exactly what pbr does now:

// Sample samples every pixel in the Camera's frame at least once.
// (package's responsibility)
// (yeah I know this is a little messy, I'll clean it up later, the point is the user doesn't deal with the mess)
func (s *Sampler) Sample() {
	length := index(len(s.samples))
	workers := index(runtime.NumCPU())
	ch := make(chan sampleStat, workers)

	for i := index(0); i < workers; i++ {
		go func(i index, adapt, max int, mean float64) {
			var stat sampleStat
			rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
			for p := i * Stride; p < length; p += Stride * workers {
				samples := adaptive(s.samples[p+Noise], adapt, max, mean)
				stat.noise += s.samplePixel(p, rnd, samples)
				stat.count += samples
			}
			ch <- stat
		}(i, s.Adapt, s.Adapt*3, s.meanNoise+Bias)
	}

	var sample sampleStat
	for i := index(0); i < workers; i++ {
		stat := <-ch
		sample.count += stat.count
		sample.noise += stat.noise
	}
	s.count += sample.count
	s.meanNoise = sample.noise / float64(sample.count)
}

The new method approaches concurrency like a Go developer, with an abundance mindset.

Now you can render pbr’s Hello, world scene with 15 lines and zero channels. Underneath, sampler.Sample() creates goroutines for every block of N pixels to saturate your CPU. But why would you care? You just want a pretty picture.

func main() {
	scene := pbr.EmptyScene()
	camera := pbr.NewCamera(960, 540)
	sampler := pbr.NewSampler(camera, scene)
	renderer := pbr.NewRenderer(sampler)

	scene.SetSky(pbr.Vector3{256, 256, 256}, pbr.Vector3{})
	scene.Add(pbr.UnitSphere(pbr.Plastic(1, 0, 0, 1)))

	for sampler.PerPixel() < 200 {
		sampler.Sample()
		fmt.Printf("\r%.1f samples / pixel", sampler.PerPixel())
	}
	pbr.WritePNG("hello.png", renderer.Rgb())
}