Seems the FreelistMapType has a poor delete performance while nosync

[absolute8511]

I wrote a simple benchmark to reproduce the compare result.

func BenchmarkBoltDBDeleteUsingMap(b *testing.B) {
	benchmarkBoltDBDelete(b, bolt.FreelistMapType)
}
func BenchmarkBoltDBDeleteUsingArray(b *testing.B) {
	benchmarkBoltDBDelete(b, bolt.FreelistArrayType)
}
func benchmarkBoltDBDelete(b *testing.B, listType bolt.FreelistType) {
	o := &bolt.Options{
		FreelistType: listType,
		NoSync:       true,
	}
	f := tempfile()
	bdb, err := bolt.Open(f, 0666, o)
	if err != nil {
		panic(err)
	}
	db := &DB{
		DB: bdb,
		f:  f,
		o:  o,
	}
	defer db.MustClose()
	if err := db.Update(func(tx *bolt.Tx) error {
		_, err := tx.CreateBucket([]byte("bench"))
		return err
	}); err != nil {
		b.Fatal(err)
	}

	buf := make([]byte, 4)
	for i := 0; i < b.N; i++ {
		binary.LittleEndian.PutUint32(buf, uint32(i))
		insert := func(tx *bolt.Tx) error {
			b := tx.Bucket([]byte("bench"))
			b.Put(buf, []byte("filler"))
			return nil
		}
		if err := db.Update(insert); err != nil {
			b.Error(err)
			return
		}
	}

	b.ReportAllocs()
	b.ResetTimer()
	s := time.Now()
	buf = make([]byte, 4)
	for i := 0; i < b.N; i++ {
		binary.LittleEndian.PutUint32(buf, uint32(i))
		delop := func(tx *bolt.Tx) error {
			bu := tx.Bucket([]byte("bench"))
			v := bu.Get(buf)
			if v == nil {
				b.Errorf("value empty")
			}
			bu.Delete(buf)
			return nil
		}
		if err := db.Update(delop); err != nil {
			b.Error(err)
			return
		}
	}

	cost := time.Since(s)
	b.Logf("cost %s", cost)
	b.StopTimer()
}

the benchmark showed as below:

$ go test -run xxx -bench BenchmarkBoltDBDelete -benchtime=10s
seed: 62743
quick settings: count=5, items=1000, ksize=1024, vsize=1024
goos: linux
goarch: amd64
pkg: github.com/absolute8511/bolt
BenchmarkBoltDBDeleteUsingMap-4     	  257385	    171098 ns/op	   35214 B/op	      77 allocs/op
--- BENCH: BenchmarkBoltDBDeleteUsingMap-4
    db_test.go:1904: cost 27.924µs
    db_test.go:1904: cost 2.909692ms
    db_test.go:1904: cost 466.201181ms
    db_test.go:1904: cost 44.037952974s
BenchmarkBoltDBDeleteUsingArray-4   	  265882	     58489 ns/op	   35972 B/op	      65 allocs/op
--- BENCH: BenchmarkBoltDBDeleteUsingArray-4
    db_test.go:1904: cost 25.178µs
    db_test.go:1904: cost 2.068921ms
    db_test.go:1904: cost 451.299584ms
    db_test.go:1904: cost 15.551035833s
PASS
ok  	github.com/absolute8511/bolt	95.002s

using FreelistMapType the performance is worse compared to the FreelistArrayType.(about 30% more) I am wondering is it expected?

By profile on the test, I found the getFreePageIDs for MapType is much slower than the ArrayType.