b.ResetTimer

Learn b.ResetTimer to exclude setup time.

b.ResetTimer

If there is expensive setup before the measurement (building large data, reading a file), its time leaks into the result. b.ResetTimer() resets the elapsed time and allocations so far:

func BenchmarkSirala(b *testing.B) {
    veri := rastgeleDilim(100_000)  // shouldn't be measured
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        kopya := slices.Clone(veri)
        slices.Sort(kopya)
    }
}

Why did we sort a copy?

If the slice is sorted in the first iteration, later iterations work on already-sorted data and look much faster. Every iteration of a benchmark must do the same work, so a copy is sorted each time. The copying cost also enters the result; for a fair comparison this is usually accepted, or measured with a separate benchmark and subtracted.

Exercise Measuring after setup

Inside testing.Benchmark, first prepare a 1000-element slice, call b.ResetTimer(), then compute the slice's sum in the loop and assign it to sink. Print whether sink equals 499500.

Show example solution

package main

import (
    "fmt"
    "testing"
)

var sink int

func main() {
    testing.Benchmark(func(b *testing.B) {
        veri := make([]int, 1000)
        for i := range veri {
            veri[i] = i
        }
        b.ResetTimer()
        for i := 0; i < b.N; i++ {
            t := 0
            for _, v := range veri {
                t += v
            }
            sink = t
        }
    })
    fmt.Println(sink == 499500)
}

Test yourself

What is used for work that must happen every iteration inside the loop but shouldn't be measured?
- b.Skip()
- time.Sleep
- b.StopTimer() and b.StartTimer()
- b.ResetTimer() on every iteration
Show answer

b.StopTimer() and b.StartTimer() — StopTimer/StartTimer pause the measurement temporarily; but called too often, their own cost distorts the result.

Don't measure setup

veri := hazirla()
b.____()
for i := 0; i < b.N; i++ {
    isle(veri)
}

Show answer

veri := hazirla()
b.ResetTimer()
for i := 0; i < b.N; i++ {
    isle(veri)
}

Timer method

ResetTimer

Reset so far
StopTimer

Pause measuring
StartTimer

Resume measuring
b.Loop()

Go 1.24 automatic

Next lesson: Memory benchmark

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