Memory benchmark
- Go course
- Intermediate
- Lesson 49/80
- 4 min
Learn to measure memory usage with b.ReportAllocs.
Memory benchmarks
b.ReportAllocs() or the -benchmem flag shows bytes allocated per operation (B/op) and the allocation count (allocs/op). The allocation count is often more stable than time and is a direct indicator of GC pressure.
func BenchmarkBuilder(b *testing.B) {
b.ReportAllocs()
for i := 0; i < b.N; i++ {
var sb strings.Builder
sb.Grow(64)
sb.WriteString("merhaba")
sink = sb.String()
}
}An allocation test
You can lock in that a hot path doesn't allocate with a test; if someone accidentally adds an allocation, the test breaks:
func TestAnahtarAyirmasiz(t *testing.T) {
n := testing.AllocsPerRun(100, func() { sink = anahtar(42) })
if n != 0 {
t.Fatalf("anahtar %v ayırma yaptı, 0 bekleniyordu", n)
}
}Practice Allocation-free key
Let anahtar(n int) int return n*31 + 7. Measure its allocations with testing.AllocsPerRun(100, ...) and print whether it is 0.
Starter code
package main
import (
"fmt"
"testing"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"testing"
)
var sink int
func anahtar(n int) int { return n*31 + 7 }
func main() {
n := testing.AllocsPerRun(100, func() { sink = anahtar(42) })
fmt.Println(n == 0)
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Outside a benchmark, what quickly gives a function's allocation count?
- go vet
- testing.AllocsPerRun Correct answer
- runtime.NumGoroutine
- time.Since
Explanation: AllocsPerRun runs the function several times and gives the average allocation count; tests use it to guarantee "zero allocations".
2 Report allocations
func BenchmarkX(b *testing.B) {
b.____()
for i := 0; i < b.N; i++ { sink = x() }
}func BenchmarkX(b *testing.B) {
b.ReportAllocs()
for i := 0; i < b.N; i++ { sink = x() }
}Explanation: ReportAllocs prints B/op and allocs/op for this benchmark even without -benchmem.
3 Measurement tool
| Term | Meaning |
|---|---|
-benchmem |
Memory for all benchmarks |
b.ReportAllocs() |
Memory for one benchmark |
testing.AllocsPerRun |
Allocation count in a test |
allocs/op |
GC pressure indicator |