Golang: Performance test of string variable copy

strcopy_test.go

package main

import "testing"
import "strings"
import "fmt"

type StructWithString struct {
	Str string
}

func Benchmark_StringCopy(b *testing.B) {
	strLen := new(int)
	scanTest := func(b *testing.B) {
		a := strings.Repeat("a", *strLen)
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			_ = strings.Index(a, "!")
		}
	}
	copyStringTest := func(b *testing.B) {
		x := strings.Repeat("a", *strLen)
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			_ = x
		}
	}
	copyStructTest := func(b *testing.B) {
		x := StructWithString{strings.Repeat("a", *strLen)}
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			_ = x
		}
	}

	for i := 1; i < 10; i++ {
		*strLen = i * 1024 * 1024
		b.Run(fmt.Sprintf("%dMB_ScanTest", i), scanTest)
		b.Run(fmt.Sprintf("%dMB_CopyString", i), copyStringTest)
		b.Run(fmt.Sprintf("%dMB_CopyStruct", i), copyStructTest)
	}
}

It seems Go does not copy content (characters inside) of string variables when making a copy of it -- copying is far faster than scanning it (Go should copying the address of the string content, not the content them selves). So, we don't need to worry about performance of copying string variables.

Test result on my laptop:

suguru@x1 ~/go/src/github.com/sgryjp/strcopy$ go test -bench .
goos: windows
goarch: amd64
pkg: github.com/sgryjp/strcopy
Benchmark_StringCopy/1MB_ScanTest-4                50000             31296 ns/op
Benchmark_StringCopy/1MB_CopyString-4           2000000000               0.34 ns/op
Benchmark_StringCopy/1MB_CopyStruct-4           2000000000               0.33 ns/op
Benchmark_StringCopy/2MB_ScanTest-4                20000             62769 ns/op
Benchmark_StringCopy/2MB_CopyString-4           2000000000               0.33 ns/op
Benchmark_StringCopy/2MB_CopyStruct-4           2000000000               0.33 ns/op
Benchmark_StringCopy/3MB_ScanTest-4                10000            117098 ns/op
Benchmark_StringCopy/3MB_CopyString-4           2000000000               0.34 ns/op
Benchmark_StringCopy/3MB_CopyStruct-4           2000000000               0.35 ns/op
Benchmark_StringCopy/4MB_ScanTest-4                10000            187655 ns/op
Benchmark_StringCopy/4MB_CopyString-4           2000000000               0.33 ns/op
Benchmark_StringCopy/4MB_CopyStruct-4           2000000000               0.33 ns/op
Benchmark_StringCopy/5MB_ScanTest-4                 5000            302590 ns/op
Benchmark_StringCopy/5MB_CopyString-4           2000000000               0.33 ns/op
Benchmark_StringCopy/5MB_CopyStruct-4           2000000000               0.33 ns/op
Benchmark_StringCopy/6MB_ScanTest-4                 3000            336664 ns/op
Benchmark_StringCopy/6MB_CopyString-4           2000000000               0.33 ns/op
Benchmark_StringCopy/6MB_CopyStruct-4           2000000000               0.32 ns/op
Benchmark_StringCopy/7MB_ScanTest-4                 3000            402793 ns/op
Benchmark_StringCopy/7MB_CopyString-4           2000000000               0.32 ns/op
Benchmark_StringCopy/7MB_CopyStruct-4           2000000000               0.34 ns/op
Benchmark_StringCopy/8MB_ScanTest-4                 3000            474773 ns/op
Benchmark_StringCopy/8MB_CopyString-4           2000000000               0.34 ns/op
Benchmark_StringCopy/8MB_CopyStruct-4           2000000000               0.33 ns/op
Benchmark_StringCopy/9MB_ScanTest-4                 2000            556626 ns/op
Benchmark_StringCopy/9MB_CopyString-4           2000000000               0.34 ns/op
Benchmark_StringCopy/9MB_CopyStruct-4           2000000000               0.34 ns/op
PASS
ok      github.com/sgryjp/strcopy       28.279s