pmalhaire

Posted on Feb 8, 2019 • Updated on Apr 20, 2019

Concatenate strings in golang a quick benchmark : + or fmt.Sprintf ?

#go #c #benchmark

Concatenate strings in `golang` a quick benchmark

Introduction

When I begin to enter golang code bases with developer of various seniority level. I was surprised to see many ways to do join strings. Making onetwo out of one and two is used a lot in any programming language.

I decided to do a quick benchmark. Comming from the C/C++ world the result in golang surprized me (in a good way).

The origin

Let's do it in a C maner using Sprintf.

I use pointers here to simulate the fact that we are in a function.

#include <stdlib.h>
#include <stdio.h>

int main(){
    char str[] = "my_string";
    char *s = malloc(sizeof(str) + sizeof(str));
    sprintf(s, "my_string%s", str);
    printf("%s\n", s);
}

Let's go golang

Let's do like in C. This was (at least when I begin with golang) a instinctive way for me.

package main

import "fmt"

func main() {
    str := "my_string"
    fmt.Println(fmt.Sprintf("my_string%s", str))
}

Let's try an other way `strings.Join`

Discussing with others I realize that strings.Join is quite popular.

package main

import (
    "fmt"
    "strings"
)

func main() {
    str := "my_string"
    fmt.Println(strings.Join([]string{"my_string", str}, ""))
}

Let's try C++ way using `strings.Builder`

Having experienced string builder in C++ and Java I thought I could give it a try.

package main

import (
    "fmt"
    "strings"
)

func main() {
    str := "my_string"
    var b strings.Builder
    b.WriteString("my_string")
    b.WriteString(str)
}

Let's try Cpp way using `+`

Plus is kind of obvious but does not feel smart.

package main

import "fmt"

func main() {
    str := "my_string"
    fmt.Println("my_string" + str)
}

Bench tells us

Golang has tremendous test utilities. We can do a benchmark really easily.

$go test -bench=. > bench.result
goos: linux
goarch: amd64

BenchmarkSprintf-4          10000000           156 ns/op
BenchmarkLongSprintf-4      10000000           186 ns/op
BenchmarkConstSprintf-4     10000000           158 ns/op

BenchmarkJoin-4             20000000            68.8 ns/op
BenchmarkLongJoin-4         20000000            86.9 ns/op
BenchmarkConstJoin-4        20000000            66.1 ns/op

BenchmarkBuilder-4          20000000           104 ns/op
BenchmarkLongBuilder-4      20000000           101 ns/op
BenchmarkConstBuilder-4     20000000           102 ns/op

BenchmarkPlus-4             50000000            25.9 ns/op
BenchmarkLongPlus-4         20000000            74.4 ns/op
BenchmarkConstPlus-4        2000000000           0.39 ns/op

Conclusion

Surprisingly (at least for me) a simple + is the fastest.printf. Here Go make the simple way the most powerful which rocks.

Behind the curtain

While doing this post I noticed the binary size for golang and C are quite different :

golang : 1,9M
c : 8,3K

It's not surprising as C does not include any runtime, but it made me curious.

To get an overview of what's make golang and C programs different I looked at the generated syscalls using strace. See this interesting post on strace.

Note : As said in the comments it's not related to string concatenation, but deserves a future post.

C version

#include <stdlib.h>
#include <stdio.h>

int main(){
    char str[] = "my_string";
    char *s = malloc(sizeof(str) + sizeof(str));
    sprintf(s, "my_string%s", str);
    printf("%s\n", s);
}

Let's have a look at strace.

gcc -o sprintf sprintf.c
strace -c ./sprintf
my_stringmy_string
% time     seconds  usecs/call     calls    errors syscall
----------- ----------- ----------- --------- --------- ----------------
 24.84    0.000305          23        13           mmap
 23.05    0.000283          28        10           mprotect
 10.18    0.000125          25         5           openat
  8.63    0.000106         106         1           munmap
  8.14    0.000100          17         6         6 access
  4.48    0.000055          14         4           read
  4.15    0.000051           9         6           fstat
  3.83    0.000047          47         1           write
  3.34    0.000041          41         1           arch_prctl
  2.44    0.000030           6         5           close
  2.12    0.000026           9         3           brk
  1.47    0.000018          18         1           execve
  1.14    0.000014           7         2           rt_sigaction
  0.65    0.000008           8         1           prlimit64
  0.57    0.000007           7         1           set_tid_address
  0.49    0.000006           6         1           rt_sigprocmask
  0.49    0.000006           6         1           set_robust_list
----------- ----------- ----------- --------- --------- ----------------
100.00    0.001228                    62         6 total

Golang version

package main

import "fmt"

func main() {
    str := "my_string"
    fmt.Println("my_string" + str)
}

Let's have a look at strace.

$ go build plus.go
$ strace -c ./plus
my_stringmy_string
% time     seconds  usecs/call     calls    errors syscall
----------- ----------- ----------- --------- --------- ----------------
 69.80    0.001900          17       114           rt_sigaction
  8.74    0.000238          79         3           clone
  5.95    0.000162          20         8           rt_sigprocmask
  4.70    0.000128          32         4           futex
  4.41    0.000120         120         1           readlinkat
  2.24    0.000061           8         8           mmap
  1.84    0.000050          50         1           write
  1.69    0.000046          15         3           fcntl
  0.62    0.000017          17         1           gettid
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2           sigaltstack
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         1           sched_getaffinity
----------- ----------- ----------- --------- --------- ----------------
100.00    0.002722                   148           total

What `strace` tells us

Go does a bit more system calls than c does, which seems legit given the fact that it has a gc.

To get to understand more I invite you to disassemble the result binaries.

As a matter of fact we must also have a look at the default compilation parameters for C and golang to have a proper comparison.

Code reference

Here is the code used to bench different options.

package main_test

import (
    "fmt"
    "strings"
    "testing"
)

var str, longStr string = "my_string", `qwertyuiopqwertyuiopqwertyuio
qwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiop`

const cStr = "my_string"

func BenchmarkPlus(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = "my_string" + str
    }
}

func BenchmarkLongPlus(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = "my_string" + longStr
    }
}

func BenchmarkConstPlus(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = "my_string" + cStr
    }
}

func BenchmarkJoin(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = strings.Join([]string{"my_string%s", str}, "")
    }
}

func BenchmarkLongJoin(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = strings.Join([]string{"my_string%s", longStr}, "")
    }
}

func BenchmarkConstJoin(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = strings.Join([]string{"my_string%s", cStr}, "")
    }
}
func BenchmarkSprintf(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = fmt.Sprintf("my_string%s", str)
    }
}

func BenchmarkLongSprintf(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = fmt.Sprintf("my_string%s", longStr)
    }
}

func BenchmarkConstSprintf(b *testing.B) {
    for n := 0; n < b.N; n++ {
        _ = fmt.Sprintf("my_string%s", cStr)
    }
}

func BenchmarkBuilder(b *testing.B) {
    for n := 0; n < b.N; n++ {
        var b strings.Builder
        b.WriteString("my_string")
        b.WriteString(longStr)
    }
}

func BenchmarkLongBuilder(b *testing.B) {
    for n := 0; n < b.N; n++ {
        var b strings.Builder
        b.WriteString("my_string")
        b.WriteString(longStr)
    }
}

func BenchmarkConstBuilder(b *testing.B) {
    for n := 0; n < b.N; n++ {
        var b strings.Builder
        b.WriteString("my_string")
        b.WriteString(longStr)
    }
}

Top comments (11)

Matthias Fasching • Nov 30 '21 • Edited

Your benchmark is incorrect, the only thing you are measuring is the optimizations of the golang compiler / runtime in simple cases. Here is a slightly improved benchmark that shows how hugely different results you get if you disable golang optimizations (or make it hard for go to optimize your code):

package test

import (
    "fmt"
    "strings"
    "testing"
)

var str, longStr string = "my_string", `qwertyuiopqwertyuiopqwertyuio
qwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiopqwertyuiop`

const cStr = "my_string"

var result string

func BenchmarkPlus(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s += str
    }
    result = s
}

func BenchmarkLongPlus(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s += longStr
    }
    result = s
}

func BenchmarkConstPlus(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s += cStr
    }
    result = s
}

func BenchmarkJoin(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = strings.Join([]string{s, str}, "")
    }
    result = s
}

func BenchmarkLongJoin(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = strings.Join([]string{s, longStr}, "")
    }
    result = s
}

func BenchmarkConstJoin(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = strings.Join([]string{s, cStr}, "")
    }
    result = s
}
func BenchmarkSprintf(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = fmt.Sprintf("%s %s", s, str)
    }
    result = s
}

func BenchmarkLongSprintf(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = fmt.Sprintf("%s %s", s, longStr)
    }
    result = s
}

func BenchmarkConstSprintf(b *testing.B) {
    var s string
    for n := 0; n < b.N; n++ {
        s = fmt.Sprintf("%s %s", s, cStr)
    }
    result = s
}

func BenchmarkBuilder(b *testing.B) {
    var sb strings.Builder
    for n := 0; n < b.N; n++ {
        sb.WriteString(str)
    }
    result = sb.String()
}

func BenchmarkLongBuilder(b *testing.B) {
    var sb strings.Builder
    for n := 0; n < b.N; n++ {
        sb.WriteString(longStr)
    }
    result = sb.String()
}

func BenchmarkConstBuilder(b *testing.B) {
    var sb strings.Builder
    for n := 0; n < b.N; n++ {
        sb.WriteString(cStr)
    }
    result = sb.String()
}

If executed with go test -gcflags=-N -bench=. returns the following results:

BenchmarkPlus-4                   114433            119694 ns/op
BenchmarkLongPlus-4                10000            118300 ns/op
BenchmarkConstPlus-4              122350            128256 ns/op
BenchmarkJoin-4                    91172            122361 ns/op
BenchmarkLongJoin-4                10000            142659 ns/op
BenchmarkConstJoin-4               86749            114199 ns/op
BenchmarkSprintf-4                 57369            152416 ns/op
BenchmarkLongSprintf-4             10000            268300 ns/op
BenchmarkConstSprintf-4            47094            139441 ns/op
BenchmarkBuilder-4              29206484                77.87 ns/op
BenchmarkLongBuilder-4           8734220              1438 ns/op
BenchmarkConstBuilder-4         37201794                41.32 ns/op
PASS

As you can see a Builder is often more than 1000x faster than other approaches. fmt.Sprintf and strings.Join have about the same speed as +, but this changes as soon as you do multiple concatenations in a single call:

s := "string1" + "string2" + "string3"
s := strings.Join([]string{"string1", "string2", "string3"})

here strings.Join will be measurable faster than +.

Adrian B.G. • Feb 8 '19

I tried to do this micro optimization a few months ago and kinda failed.

The recommended way is to use strings.Builder. As I did not knew the string size a simple + worked better in benchmarks (at least for strings less than ~20 characters.

I ended up approximating the result (and pre allocate memory with a buffer) and got the best result, but most of the times + is the best choice.

Matthias Fasching • Nov 30 '21 • Edited

+ will always be the slowest way to concatenate strings.

In simple cases (concatenate only exactly 2 strings) every other method: builder, join, and sprintf will be ~ the same speed as +.

The benchmark here is just incorrect. Because the resulting string in the Plus tests isn't assigned to anything the compiler just makes it a NOP before executing the tests.

Run the benchmarks again and disable optimizations (go test -gcflags=-N -bench=.) and you will see that all methods have ~ the same execution time. In cases where you concatenate more than 2 strings + will always be the slowest (and most memory hungry) method.

pmalhaire • Feb 9 '19

Thanks for your comment I'll update my post accordingly, note that the c version preallocates the buffer.

Tito George • Aug 2 '22

In my case join wins by a long margin, cant figure out whats wrong.

var word = []string{"9b4e6f7f-1c37-4730-9417-e23747572608", "9b4e6f7f-1c37-4730-9417-e23747572608", "9b4e6f7f-1c37-4730-9417-e23747572608", "9b4e6f7f-1c37-4730-9417-e23747572608", "9b4e6f7f-1c37-4730-9417-e23747572608", "9b4e6f7f-1c37-4730-9417-e23747572608"}

func BenchmarkStringsSprint(b *testing.B) {
    b.ReportAllocs()
    values := ""
    for i := 0; i < b.N; i++ {
        for _, s := range word {
            values = fmt.Sprintf("%s %s", values, s)
        }
    }
}

func BenchmarkStringsJoin(b *testing.B) {
    b.ReportAllocs()
    for i := 0; i < b.N; i++ {
        values := strings.Join(word, " ")
        _ = values
    }
}

func BenchmarkBuilder(b *testing.B) {
    b.ReportAllocs()
    for n := 0; n < b.N; n++ {
        var b strings.Builder
        for _, s := range word {
            b.WriteString(s)
            b.WriteByte(' ')
        }
        _ = b.String()
    }
}

Join has less allocs as well

BenchmarkStringsSprint
BenchmarkStringsSprint-16                      10000       2180843 ns/op    13404016 B/op         32 allocs/op
BenchmarkStringsJoin
BenchmarkStringsJoin-16                     10398318           139.0 ns/op       224 B/op          1 allocs/op
BenchmarkBuilder
BenchmarkBuilder-16                          4303065           247.6 ns/op       720 B/op          4 allocs/op

Jay R. Wren • Feb 8 '19

On strace: string concatenation isn't a system call. The strace for these programs should be the same as the strace for hello world.

pmalhaire • Feb 8 '19

I didn't mean to say that. What made you think this way ? Maybe I Can make my post more clear with your help.

Jay R. Wren • Feb 8 '19

The article is about string concatenation. Why look at strace at all?

pmalhaire • Feb 8 '19

It's to explain why C is more efficient than Go, which is no explicitly explained.

Jay R. Wren • Feb 8 '19

I don't agree that a syscall count has anything to do with a languages efficiency compared to another language.

pmalhaire • Feb 8 '19

I'll make it more clear.

DEV Community

Concatenate strings in golang a quick benchmark : + or fmt.Sprintf ?

Concatenate strings in `golang` a quick benchmark

Introduction

The origin

Let's go golang

Let's try an other way `strings.Join`

Let's try C++ way using `strings.Builder`

Let's try Cpp way using `+`

Bench tells us

Conclusion

Behind the curtain

C version

Golang version

What `strace` tells us

Code reference

Top comments (11)

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Concatenate strings in golang a quick benchmark

Introduction

The origin

Let's go golang

Let's try an other way strings.Join

Let's try C++ way using strings.Builder

Let's try Cpp way using +

Bench tells us

Conclusion

Behind the curtain

C version

Golang version

What strace tells us

Code reference

Read next

How I write HTTP services in Go

Build a Discord Bot with Go - Step-by-Step Tutorial via Webhooks

Spin a Framework-Free HTTP Router Server in Go 1.22 With Ease

More About File Operations in C Including File Descriptors

Concatenate strings in `golang` a quick benchmark

Let's try an other way `strings.Join`

Let's try C++ way using `strings.Builder`

Let's try Cpp way using `+`

What `strace` tells us