Showing the difference between writeStr and io.Writer.Write

new.txt

BenchmarkWrite1B-4     	100000000	        11.4 ns/op	       2 B/op	       0 allocs/op
BenchmarkWrite1B-4     	100000000	        10.5 ns/op	       2 B/op	       0 allocs/op
BenchmarkWrite1B-4     	100000000	        10.7 ns/op	       2 B/op	       0 allocs/op
BenchmarkWrite1B-4     	100000000	        11.8 ns/op	       2 B/op	       0 allocs/op
BenchmarkWrite1B-4     	100000000	        11.5 ns/op	       2 B/op	       0 allocs/op
BenchmarkWrite10B-4    	100000000	        27.5 ns/op	      24 B/op	       0 allocs/op
BenchmarkWrite10B-4    	100000000	        14.0 ns/op	      24 B/op	       0 allocs/op
BenchmarkWrite10B-4    	100000000	        13.5 ns/op	      24 B/op	       0 allocs/op
BenchmarkWrite10B-4    	100000000	        13.8 ns/op	      24 B/op	       0 allocs/op
BenchmarkWrite10B-4    	100000000	        13.5 ns/op	      24 B/op	       0 allocs/op
BenchmarkWrite1KB-4    	 5000000	      5008 ns/op	    3435 B/op	       0 allocs/op
BenchmarkWrite1KB-4    	  500000	      2172 ns/op	    2147 B/op	       0 allocs/op
BenchmarkWrite1KB-4    	 1000000	      1328 ns/op	    2147 B/op	       0 allocs/op
BenchmarkWrite1KB-4    	 3000000	      4793 ns/op	    2863 B/op	       0 allocs/op
BenchmarkWrite1KB-4    	  300000	      3706 ns/op	    3579 B/op	       0 allocs/op
BenchmarkWrite10KB-4   	  200000	     31738 ns/op	   26842 B/op	       0 allocs/op
BenchmarkWrite10KB-4   	   50000	     23848 ns/op	   26840 B/op	       0 allocs/op
BenchmarkWrite10KB-4   	  200000	     30181 ns/op	   26842 B/op	       0 allocs/op
BenchmarkWrite10KB-4   	  100000	     20815 ns/op	   26841 B/op	       0 allocs/op
BenchmarkWrite10KB-4   	  200000	     18399 ns/op	   26842 B/op	       0 allocs/op

old.txt

BenchmarkWrite1B-4           	30000000	        42.3 ns/op	      10 B/op	       1 allocs/op
BenchmarkWrite1B-4           	50000000	        40.3 ns/op	      10 B/op	       1 allocs/op
BenchmarkWrite1B-4           	50000000	        40.2 ns/op	      10 B/op	       1 allocs/op
BenchmarkWrite1B-4           	30000000	        40.2 ns/op	      10 B/op	       1 allocs/op
BenchmarkWrite1B-4           	30000000	        39.6 ns/op	      10 B/op	       1 allocs/op
BenchmarkWrite10B-4          	30000000	        59.2 ns/op	      36 B/op	       1 allocs/op
BenchmarkWrite10B-4          	30000000	        51.5 ns/op	      36 B/op	       1 allocs/op
BenchmarkWrite10B-4          	30000000	        51.6 ns/op	      36 B/op	       1 allocs/op
BenchmarkWrite10B-4          	30000000	        51.8 ns/op	      36 B/op	       1 allocs/op
BenchmarkWrite10B-4          	30000000	        51.8 ns/op	      36 B/op	       1 allocs/op
BenchmarkWrite1KB-4          	 1000000	      1404 ns/op	    3171 B/op	       1 allocs/op
BenchmarkWrite1KB-4          	 2000000	      1173 ns/op	    3171 B/op	       1 allocs/op
BenchmarkWrite1KB-4          	 2000000	      1164 ns/op	    3171 B/op	       1 allocs/op
BenchmarkWrite1KB-4          	 1000000	      1064 ns/op	    3171 B/op	       1 allocs/op
BenchmarkWrite1KB-4          	 2000000	       680 ns/op	    3171 B/op	       1 allocs/op
BenchmarkWrite10KB-4         	  200000	     14869 ns/op	   37082 B/op	       1 allocs/op
BenchmarkWrite10KB-4         	  200000	     21493 ns/op	   37082 B/op	       1 allocs/op
BenchmarkWrite10KB-4         	   50000	     24814 ns/op	   37080 B/op	       1 allocs/op
BenchmarkWrite10KB-4         	  200000	     20319 ns/op	   37082 B/op	       1 allocs/op
BenchmarkWrite10KB-4         	  200000	     23330 ns/op	   37082 B/op	       1 allocs/op

results.md

$ benchstat old.txt new.txt 
name         old time/op    new time/op    delta
Write1B-4      40.5ns ± 4%    11.2ns ± 6%   -72.41%  (p=0.008 n=5+5)
Write10B-4     51.7ns ± 0%    13.7ns ± 2%   -73.49%  (p=0.029 n=4+4)
Write1KB-4     1.10µs ±38%    3.40µs ±61%  +210.06%  (p=0.016 n=5+5)
Write10KB-4    21.0µs ±29%    25.0µs ±27%      ~     (p=0.421 n=5+5)

name         old alloc/op   new alloc/op   delta
Write1B-4       10.0B ± 0%      2.0B ± 0%   -80.00%  (p=0.008 n=5+5)
Write10B-4      36.0B ± 0%     24.0B ± 0%   -33.33%  (p=0.008 n=5+5)
Write1KB-4     3.17kB ± 0%    2.83kB ±26%      ~     (p=0.563 n=5+5)
Write10KB-4    37.1kB ± 0%    26.8kB ± 0%   -27.62%  (p=0.000 n=4+5)

name         old allocs/op  new allocs/op  delta
Write1B-4        1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.008 n=5+5)
Write10B-4       1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.008 n=5+5)
Write1KB-4       1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.008 n=5+5)
Write10KB-4      1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.008 n=5+5)

writestr_test.go

package main

import (
	"bytes"
	"io"
	"strings"
	"testing"
)

func writeStr(w io.Writer, s string) (err error) {
	_, err = w.Write([]byte(s))
	return
}

func BenchmarkWriteStr1B(b *testing.B) {
	benchmarkWriteStr(b, 1)
}

func BenchmarkWriteStr10B(b *testing.B) {
	benchmarkWriteStr(b, 10)
}

func BenchmarkWriteStr1KB(b *testing.B) {
	benchmarkWriteStr(b, 1024)
}

func BenchmarkWriteStr10KB(b *testing.B) {
	benchmarkWriteStr(b, 10*1024)
}

func benchmarkWriteStr(b *testing.B, n int) {
	str := strings.Repeat("a", n)
	buf := new(bytes.Buffer)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		if err := writeStr(buf, str); err != nil {
			b.Fatalf("#%d err: %v", i, err)
		}
	}
	b.ReportAllocs()
}

func BenchmarkWriterDotWrite1B(b *testing.B) {
	benchmarkWriterDotWrite(b, 1)
}

func BenchmarkWriterDotWrite10B(b *testing.B) {
	benchmarkWriterDotWrite(b, 10)
}

func BenchmarkWriterDotWrite1KB(b *testing.B) {
	benchmarkWriterDotWrite(b, 1024)
}

func BenchmarkWriterDotWrite10KB(b *testing.B) {
	benchmarkWriterDotWrite(b, 10*1024)
}

func benchmarkWriterDotWrite(b *testing.B, n int) {
	bs := bytes.Repeat([]byte("a"), n)
	buf := new(bytes.Buffer)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		if _, err := buf.Write(bs); err != nil {
			b.Fatalf("#%d err: %v", i, err)
		}
	}
	b.ReportAllocs()
}