os.Getenv多少钱？

I have a piece of code that's called every few seconds and makes use of an environment variable:

for {
    myVar := os.Getenv("MY_VAR")
    //Do something
    time.Sleep(3 * time.Second)
}

But how costly are repeat calls to os.Getenv?

The value of the environment variable will not change during runtime, so I could set it as a package level variable:

package blah

var myVar = os.Getenv("MY_VAR")

But this does hurt testability of the code.

Should I set it as a package level variable? Or is os.Getenv benign enough?

EDIT: I've benchmarked the call to os.Getenv but is it reliable?

package main_test

import (
    "os"
    "testing"
)

var result string

func BenchmarkEnv(b *testing.B) {
    var r string
    for n := 0; n < b.N; n++ {
        r = os.Getenv("PATH")
    }
    result = r
}

goos: darwin
goarch: amd64
BenchmarkEnv-8      20000000            78.7 ns/op
PASS

You can benchmark os.Getenv and see how fast it is.

By looking at its implementation here, it costs:

1. A read-lock;
2. Lookup in global map;
3. Linear search of char '='.