Go program that shows every N connections are dramatically slower

slow-connections.go

package main

import (
	"flag"
	"net"
	"time"

	"log"
)

var (
	connections = flag.Int("c", 100, "connections to send")
)

func main() {
	flag.Parse()
	listener := spawnListener()

	if err := sendTraffic(listener.Addr().String()); err != nil {
		log.Fatal(err)
	}
}

func spawnListener() net.Listener {
	listener, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		log.Fatal(err)
	}
	go func() {
		for {
			_, err := listener.Accept()
			if err != nil {
				return
			}
		}
	}()
	return listener
}

func sendTraffic(addr string) error {
	log.Println("starting connections", *connections)
	for i := 0; i < *connections; i++ {
		err := call(i, addr)
		if err != nil {
			return err
		}
	}
	return nil
}

func call(c int, addr string) error {
	t0 := time.Now()
	_, err := net.Dial("tcp", addr)
	if err != nil {
		return err
	}
	log.Printf("connection sent n=%v latency=%v", c, time.Since(t0))
	return err
}