Go日志，打印源码文件名和行号造成的性能开销

Llongfile    // full file name and line number: /a/b/c/d.go:23
Lshortfile   // final file name element and line number: d.go:23. overrides Llongfile

// package log
func (l *Logger) Output(calldepth int, s string) error

// package runtime
func Caller(skip int) (pc uintptr, file string, line int, ok bool)

//BenchmarkRuntimeCaller-4    	 2417739	       488 ns/op	     216 B/op	       2 allocs/op
func BenchmarkRuntimeCaller(b *testing.B) {
	for n := 0; n < b.N; n++ {
		runtime.Caller(0)
	}
}

//BenchmarkRuntimeCaller2-4   	 1213971	       983 ns/op	     216 B/op	       2 allocs/op
func BenchmarkRuntimeCaller2(b *testing.B) {
	for n := 0; n < b.N; n++ {
		runtime.Caller(2)
	}
}

//BenchmarkLog-4              	  754929	      1672 ns/op	       0 B/op	       0 allocs/op
func BenchmarkLog(b *testing.B) {
	fp, _ := os.Create("/dev/null")
	log.SetOutput(fp)
	log.SetFlags(0)
	b.ResetTimer()
	for n := 0; n < b.N; n++ {
		log.Printf("a")
	}
}

//BenchmarkLogWith-4          	  344067	      3403 ns/op	     216 B/op	       2 allocs/op
func BenchmarkLogWith(b *testing.B) {
	fp, _ := os.Create("/dev/null")
	log.SetOutput(fp)
	log.SetFlags(log.Lshortfile)
	b.ResetTimer()
	for n := 0; n < b.N; n++ {
		log.Printf("a")
	}
}