Repository: go
Issue: net/textproto: the DotReader/DotWriter is extremely slow
Labels: Performance,NeedsInvestigation

I didn't expect the bottle neck of my program to came from the standard lib, but this seems to be an exception.

The DotReader/DotWriter from the net/textproto library uses bufio.ReadByte/WriteByte on each input/output byte, which is over 300 times slower than bufio.Read/Write functions for the same amount of data. This is causing significant impact on data throughput for applications using these interfaces.

```go
package my_test

import (
	"bufio"
	"io"
	"net/textproto"
	"testing"
)

type FillReader byte

func (r FillReader) Read(b []byte) (n int, err error) {
	n = len(b)
	if n > 0 {
		b[0] = byte(r)
		for i := 1; i < n; i *= 2 {
			copy(b[i:], b[:i])
		}
	}
	return n, nil
}

func BenchmarkIONull(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		io.Copy(io.Discard, io.LimitReader(FillReader('.'), size))
	}
}

func BenchmarkIOBufioWrite(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r := io.LimitReader(FillReader('.'), size)
		io.Copy(io.Discard, r)
	}
}

func BenchmarkIOBufioWriteByte(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		w := bufio.NewWriter(io.Discard)
		for j := int64(0); j < size; j++ {
			w.WriteByte(0)
		}
	}
}

func BenchmarkIODotWriter(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r := io.LimitReader(FillReader('.'), size)
		w := textproto.NewWriter(bufio.NewWriter(io.Discard)).DotWriter()
		io.Copy(w, r)
	}
}

func BenchmarkIOBufioRead(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r := bufio.NewReader(io.LimitReader(FillReader('.'), size))
		io.Copy(io.Discard, r)
	}
}

func BenchmarkIOBufioReadByte(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r := bufio.NewReader(io.LimitReader(FillReader('.'), size))
		for j := int64(0); j < size; j++ {
			r.ReadByte()
		}
	}
}

func BenchmarkIODotReader(b *testing.B) {
	size := int64(1024 * 1024 * 4)
	b.SetBytes(size)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		r := textproto.NewReader(bufio.NewReader(io.LimitReader(FillReader('.'), size))).DotReader()
		io.Copy(io.Discard, r)
	}
}
```

```
❯ go test -cpuprofile cpu.prof -memprofile mem.prof -benchmem -bench IO
goos: windows
goarch: amd64
pkg: mytest
cpu: Intel(R) Core(TM) i7-8086K CPU @ 4.00GHz
BenchmarkIONull-12                         13660             91626 ns/op        45776.23 MB/s         24 B/op          1 allocs/op
BenchmarkIOBufioWrite-12                   19095             55199 ns/op        75984.94 MB/s         24 B/op          1 allocs/op
BenchmarkIOBufioWriteByte-12                  99          10311620 ns/op         406.76 MB/s        4178 B/op          1 allocs/op
BenchmarkIODotWriter-12                       67          16815748 ns/op         249.43 MB/s       36984 B/op          6 allocs/op
BenchmarkIOBufioRead-12                    20808             58743 ns/op        71401.44 MB/s       4223 B/op          3 allocs/op
BenchmarkIOBufioReadByte-12                   99          10507806 ns/op         399.16 MB/s        4202 B/op          2 allocs/op
BenchmarkIODotReader-12                       60          19456187 ns/op         215.58 MB/s        4305 B/op          5 allocs/op
PASS
ok      mytest  11.788s
```