golang：cmd.Exec（）：如何读取应用程序的非缓冲标准输出？

So I have the simple app which starts other apps and reads their output.

package main

import (
    "bufio"
    "io"
    "log"
    "os/exec"
    "time"
)

func main() {
    cmd := exec.Command("perl", "-e", "my $x = 0; while (1) { print ++$x.qx'date'; sleep 1; }")
    stdout, _ := cmd.StdoutPipe()
    stderr, _ := cmd.StderrPipe()

    in := bufio.NewReaderSize(io.MultiReader(stdout, stderr), 100)
    cmd.Start()
    defer cmd.Wait()
    for {
        log.Printf("....")
        time.Sleep(1 * time.Second)
        l, _ := in.ReadString('
')
        log.Printf(string(l))
    }
}

The point of the real app is to read output of running process and parse it.. however it doesn't works well with real apps which don't explicitly sync/flush their stdout (it takes ~60 lines of iperf output before starting printing).

What is the most efficient way to read output byte-by-byte?