https://github.com/golang/go/issues/36298

regressiontest2.go

package main

import (
	"fmt"
	"net"
	"sync/atomic"
	"time"
)

var (
	duration    = int64(0)
	count       = int64(0)
	connections = int64(0)

	responseDelay = time.Millisecond * 200 // Wait 200ms before sending the response.
)

func handler(c net.Conn) {
	buff := make([]byte, 4)
	for {
		if _, err := c.Read(buff); err != nil {
			panic(err)
		}
		if _, err := c.Write(buff); err != nil {
			panic(err)
		}
	}
}

func listener(ln net.Listener) {
	for {
		c, err := ln.Accept()
		if err != nil {
			panic(err)
		}
		go handler(c)
	}
}

func worker() {
	c, err := net.Dial("tcp", "127.0.0.1:5555")
	if err != nil {
		panic(err)
	}

	atomic.AddInt64(&connections, 1)

	buff := []byte{1, 2, 3, 4}
	for {
		start := time.Now()

		if _, err := c.Write(buff); err != nil {
			panic(err)
		}
		if _, err := c.Read(buff); err != nil {
			panic(err)
		}

		d := int64(time.Since(start))
		atomic.AddInt64(&duration, d)
		atomic.AddInt64(&count, 1)

		time.Sleep(responseDelay)
	}
}

func startup() {
	for i := 0; i < 9000; i++ {
		go worker()

		time.Sleep(time.Millisecond * 4)
	}
}

func main() {
	ln, err := net.Listen("tcp", "127.0.0.1:5555")
	if err != nil {
		panic(err)
	}
	go listener(ln)

	go startup()

	for {
		time.Sleep(time.Second)

		n := atomic.LoadInt64(&connections)
		d := atomic.SwapInt64(&duration, 0)
		c := atomic.SwapInt64(&count, 0)

		if c > 0 {
			fmt.Printf("connections: % 4d  latency: % 12v  (requests: %d)\n", n, time.Duration(d/c), c)
		}
	}
}