go test -cpuprofile

Learn to profile during tests.

Profiling with tests

Benchmarks are the easiest place to take profiles; adding flags is enough, no extra code needed:

go test -bench=Ayristir -cpuprofile cpu.out -memprofile mem.out ./parser
go tool pprof parser.test cpu.out

The profile covers only the code the benchmark measures; it shows a repeatable scenario, not production traffic.

Other test profiles

-blockprofile records waiting points on channels and locks, and -mutexprofile records lock contention. -trace trace.out produces an event timeline. Adding -run=^$ while profiling keeps normal tests from running and the results clean.

Exercise Benchmark function

Run a benchmark from a program with testing.Benchmark: pass a func(b *testing.B) that calls strings.Repeat("go", 100). Print whether the result's N is greater than 0.

Show example solution

package main

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

func main() {
    r := testing.Benchmark(func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            strings.Repeat("go", 100)
        }
    })
    fmt.Println(r.N > 0)
}

Test yourself

What does the <code>-benchmem</code> flag add?
- A CPU profile
- Race detection
- Test coverage
- Bytes and allocations per operation
Show answer

Bytes and allocations per operation — -benchmem adds B/op and allocs/op columns to the output; it doesn't produce a profile file.

Memory profile flag

go test -bench=. ____ mem.out

Show answer

go test -bench=. -memprofile mem.out

go test flags

-cpuprofile

CPU profile
-memprofile

Memory profile
-benchmem

B/op, allocs/op
-run=^$

Skip tests

Next lesson: The trace tool

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