Map performance degradation of key length

main_test.go

package main

import (
	"bytes"
	"math/rand"
	"testing"
)

func genKey(n int) string {
	ss := &bytes.Buffer{}
	for i := 0; i < n; i++ {
		ss.WriteRune(rune(rand.Intn(93) + 33))
	}
	return ss.String()
}

func BenchmarkMapKeyLength_1(b *testing.B)    { benchmarkMap(b, 1) }
func BenchmarkMapKeyLength_5(b *testing.B)    { benchmarkMap(b, 5) }
func BenchmarkMapKeyLength_10(b *testing.B)   { benchmarkMap(b, 10) }
func BenchmarkMapKeyLength_20(b *testing.B)   { benchmarkMap(b, 20) }
func BenchmarkMapKeyLength_50(b *testing.B)   { benchmarkMap(b, 50) }
func BenchmarkMapKeyLength_100(b *testing.B)  { benchmarkMap(b, 100) }
func BenchmarkMapKeyLength_500(b *testing.B)  { benchmarkMap(b, 500) }
func BenchmarkMapKeyLength_1000(b *testing.B) { benchmarkMap(b, 1000) }

func benchmarkMap(b *testing.B, l int) {
	keys := make([]string, 0, b.N)
	for i := 0; i < b.N; i++ {
		keys = append(keys, genKey(l))
	}
	m := make(map[string]struct{}, b.N)
	b.ResetTimer()

	for i := 0; i < b.N; i++ {
		m[keys[i]] = struct{}{}
	}

	for i := 0; i < b.N; i++ {
		a, b := keys[rand.Intn(b.N)], keys[rand.Intn(b.N)]
		m[a], m[b] = m[b], m[a]
	}
}

result

$ go test -v -bench .
goos: darwin
goarch: amd64
pkg: github.com/mackee/sandbox/gostudy-map-benchmark
BenchmarkMapKeyLength_1-8        5959239               216 ns/op
BenchmarkMapKeyLength_5-8        3397646               709 ns/op
BenchmarkMapKeyLength_10-8       2078060               602 ns/op
BenchmarkMapKeyLength_20-8       1972189               629 ns/op
BenchmarkMapKeyLength_50-8       1816406               673 ns/op
BenchmarkMapKeyLength_100-8      1711934               745 ns/op
BenchmarkMapKeyLength_500-8      1298208              1028 ns/op
BenchmarkMapKeyLength_1000-8     1000000              1170 ns/op
PASS
ok      github.com/mackee/sandbox/gostudy-map-benchmark 93.767s