Go 1.12 network slowdown on Darwin

main.go

package main

import (
	"fmt"
	"io"
	"log"
	"net"
	"os"
	"sync"
	"sync/atomic"
	"time"
)

func main() {
	switch {
	case len(os.Args) > 1 && os.Args[1] == "bench":
		bench()
	case len(os.Args) > 1 && os.Args[1] == "server":
		server()
	default:
		fmt.Fprintf(os.Stderr, "usage: <command> server|bench\n")
		os.Exit(1)
	}
}

func server() {
	log.Printf("echo server started\n")
	os.RemoveAll("socket")
	ln, err := net.Listen("unix", "socket")
	if err != nil {
		log.Fatal(err)
	}
	defer ln.Close()
	for {
		conn, err := ln.Accept()
		if err != nil {
			log.Fatal(err)
		}
		go func(conn net.Conn) {
			defer conn.Close()
			var packet [256]byte
			for {
				n, err := conn.Read(packet[:])
				if err != nil {
					return
				}
				conn.Write(packet[:n])
			}
		}(conn)
	}
}

func bench() {
	c := 50
	n := 100000
	log.Printf("echo benchmark connecting %d clients\n", c)
	start := time.Now()
	var total int32
	var wg sync.WaitGroup
	wg.Add(c)
	for i := 0; i < c; i++ {
		go func() {
			defer wg.Done()
			conn, err := net.Dial("unix", "socket")
			if err != nil {
				log.Fatal(err)
			}
			defer conn.Close()
			packet := []byte("ping\n")
			for i := 0; i < n/c; i++ {
				conn.Write(packet)
				if _, err := io.ReadFull(conn, packet); err != nil {
					log.Fatal(err)
				}
				if atomic.AddInt32(&total, 1)%3131 == 0 {
					fmt.Printf("\r%.0f ops/sec ",
						float64(total)/time.Since(start).Seconds())
				}
			}
		}()
	}
	wg.Wait()
	fmt.Printf("\r%.0f ops/sec\n",
		float64(total)/time.Since(start).Seconds())
}

Still continue in 1.12.5

go 1.11     darwin  268.093 ops/sec
go 1.12.5   darwin  71.273 ops/sec