Go
Challenge 2 - Performance
Write a performance analysis that inspects GC and allocs.
Challenge: reduce GC pressure
A log processor creates a new []string for every line:
func alanlar(line string) []string {
return strings.Split(line, " ")
}
At hundreds of thousands of lines per second this means constant GC. Your task: write alanlarInto(dst []string, line string) []string, which writes the fields into a reusable buffer, and show that allocations in the loop drop to zero.
Plan
Substrings in Go aren't copies but new headers pointing at the same memory, so line[bas:i] doesn't allocate. The function: dst = dst[:0], walk the line character by character, append the piece at each space, append the remainder at the end. Measure: call it repeatedly on the same buf with testing.AllocsPerRun.
Exercise Zero allocations
Write alanlarInto. Process the line "GET /api 200 12ms" with buf := make([]string, 0, 8); print the field count and the testing.AllocsPerRun result on one line.
Show example solution
package main
import (
"fmt"
"testing"
)
func alanlarInto(dst []string, line string) []string {
dst = dst[:0]
bas := 0
for i := 0; i < len(line); i++ {
if line[i] == ' ' {
dst = append(dst, line[bas:i])
bas = i + 1
}
}
return append(dst, line[bas:])
}
func main() {
line := "GET /api 200 12ms"
buf := make([]string, 0, 8)
a := testing.AllocsPerRun(100, func() { buf = alanlarInto(buf, line) })
fmt.Println(len(buf), a)
}
Test yourself
-
What does the expression <code>dst[:0]</code> do?
- Returns nil
- Keeps the capacity and resets the length to zero
- Allocates a new array
- Deletes the array
Show answer
Keeps the capacity and resets the length to zero — The same underlying array is kept; append writes from the start without reallocating.
-
Reset the buffer
func alanlarInto(dst []string, line string) []string {
dst = dst____
// ...
Show answer
func alanlarInto(dst []string, line string) []string {
dst = dst[:0]
// ...
-
Expression and cost
- strings.Split
- A new slice per call
- dst[:0]
- Reuse the buffer
- line[a:b]
- Copy-free substring
- AllocsPerRun
- Allocation measurement
Next lesson: Challenge 3 - Compiler
