Memory benchmark

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)
    }
}

Exercise 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.

Show example 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)
}

Test yourself

Outside a benchmark, what quickly gives a function's allocation count?
- go vet
- testing.AllocsPerRun
- runtime.NumGoroutine
- time.Since
Show answer

testing.AllocsPerRun — AllocsPerRun runs the function several times and gives the average allocation count; tests use it to guarantee "zero allocations".

Report allocations

func BenchmarkX(b *testing.B) {
    b.____()
    for i := 0; i < b.N; i++ { sink = x() }
}

Show answer

func BenchmarkX(b *testing.B) {
    b.ReportAllocs()
    for i := 0; i < b.N; i++ { sink = x() }
}

Measurement tool

-benchmem

Memory for all benchmarks
b.ReportAllocs()

Memory for one benchmark
testing.AllocsPerRun

Allocation count in a test
allocs/op

GC pressure indicator

Next lesson: Sub-benchmark

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