16 million goroutines - "GC assist wait"

Question

I'm running a go program that computes the mandelbrot set. A gouroutine is started for every pixel to compute convergence. The program runs fine for pixelLengthx = 1000, pixelLengthy = 1000. If I run the same code for pixelLengthx = 4000, pixelLengthy = 4000, the program starts printing this after a few dozen seconds:

goroutine 650935 [GC assist wait]:
main.converges(0xa2, 0xb6e, 0xc04200c680)
.../fractals/fractals.go:41 +0x17e
created by main.main
.../fractals/fractals.go:52 +0x2af

The program does not terminate and just continues printing.

package main

import (
    "image"
    "image/color"
    "image/draw"
    "image/png"
    "log"
    "math/cmplx"
    "os"
    "sync"
)

var sidex float64 = 4.0
var sidey float64 = 4.0
var pixelLengthx int = 4000
var pixelLengthy int = 4000
var numSteps int = 100

func converges(wg *sync.WaitGroup, i, j int, m *image.RGBA) {
    wht := color.RGBA{255, 50, 128, 255}

    plx := float64(pixelLengthx)
    ply := float64(pixelLengthy)
    fi := float64(i)
    fj := float64(j)

    c := complex((fi-plx/2)/plx*sidex, (fj-ply/2)/ply*sidey)
    zn := complex(0, 0)
    for k := 0; k < numSteps; k++ {
        zn = cmplx.Pow(zn, 2) + c
    }

    if cmplx.Abs(zn) > 0.1 {
        m.Set(i, j, wht)
    }

    wg.Done()
}

func main() {
    err := Main()
    if err != nil {
        log.Fatal(err)
    }
}

func Main() error {
    m := image.NewRGBA(image.Rect(0, 0, pixelLengthx, pixelLengthy))
    blk := color.RGBA{0, 0, 0, 255}
    draw.Draw(m, m.Bounds(), &image.Uniform{blk}, image.ZP, draw.Src)

    numGoroutines := pixelLengthx * pixelLengthy
    wg := &sync.WaitGroup{}
    wg.Add(numGoroutines)

    for x := 0; x < pixelLengthx; x++ {
        for y := 0; y < pixelLengthy; y++ {
            go converges(wg, x, y, m)
        }
    }

    wg.Wait()

    f, err := os.Create("img.png")
    if err != nil {
        return err
    }
    defer f.Close()

    err = png.Encode(f, m)
    if err != nil {
        return err
    }

    return nil
}

What is going on here? Why does the program even print something?

I'm using go version go1.8 windows/amd64.

Memory usage during program execution.

Answer 1

goroutine is lightweight but you have too much overconfident. You should make worker like below, I think.

package main

import (
    "image"
    "image/color"
    "image/draw"
    "image/png"
    "log"
    "math/cmplx"
    "os"
    "sync"
)

var sidex float64 = 4.0
var sidey float64 = 4.0
var pixelLengthx int = 4000
var pixelLengthy int = 4000
var numSteps int = 100

func main() {
    err := Main()
    if err != nil {
        log.Fatal(err)
    }
}

type req struct {
    x int
    y int
    m *image.RGBA
}

func converges(wg *sync.WaitGroup, q chan *req) {
    defer wg.Done()

    wht := color.RGBA{255, 50, 128, 255}
    plx := float64(pixelLengthx)
    ply := float64(pixelLengthy)

    for r := range q {

        fi := float64(r.x)
        fj := float64(r.x)

        c := complex((fi-plx/2)/plx*sidex, (fj-ply/2)/ply*sidey)
        zn := complex(0, 0)
        for k := 0; k < numSteps; k++ {
            zn = cmplx.Pow(zn, 2) + c
        }

        if cmplx.Abs(zn) > 0.1 {
            r.m.Set(r.x, r.y, wht)
        }
    }
}

const numWorker = 10

func Main() error {
    q := make(chan *req, numWorker)
    var wg sync.WaitGroup
    wg.Add(numWorker)
    for i := 0; i < numWorker; i++ {
        go converges(&wg, q)
    }

    m := image.NewRGBA(image.Rect(0, 0, pixelLengthx, pixelLengthy))
    blk := color.RGBA{0, 0, 0, 255}
    draw.Draw(m, m.Bounds(), &image.Uniform{blk}, image.ZP, draw.Src)

    for x := 0; x < pixelLengthx; x++ {
        for y := 0; y < pixelLengthy; y++ {
            q <- &req{x: x, y: y, m: m}
        }
    }
    close(q)

    wg.Wait()

    f, err := os.Create("img.png")
    if err != nil {
        return err
    }
    defer f.Close()

    err = png.Encode(f, m)
    if err != nil {
        return err
    }

    return nil
}