add benchmark

examples/goroutine/go.mod

@@ -0,0 +1,3 @@
+module github.com/felixge/go-profiler-notes/examples/goroutine
+
+go 1.16

examples/goroutine/main_test.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"testing"
+	"time"
+)
+
+func BenchmarkProfilerGoroutines(b *testing.B) {
+	max := 1024 * 1024
+	buf := make([]runtime.StackRecord, max*2)
+	for g := 1; g <= max; g = g * 2 {
+		g := g
+		name := fmt.Sprintf("%d goroutines", g)
+
+		b.Run(name, func(b *testing.B) {
+			initalRoutines := runtime.NumGoroutine()
+
+			readyCh := make(chan struct{})
+			stopCh := make(chan struct{})
+			for i := 0; i < g; i++ {
+				go atStackDepth(8, func() {
+					defer func() { stopCh <- struct{}{} }()
+					readyCh <- struct{}{}
+				})
+				<-readyCh
+			}
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				n, ok := runtime.GoroutineProfile(buf)
+				if !ok {
+					b.Logf("GoroutineProfile not ok")
+				} else if gotRoutines := n - initalRoutines; gotRoutines != g {
+					b.Logf("want %d goroutines, but got %d on iteration %d", g, gotRoutines, i)
+				}
+			}
+			b.StopTimer()
+			for i := 0; i < g; i++ {
+				<-stopCh
+			}
+			start := time.Now()
+			for i := 0; ; i++ {
+				if runtime.NumGoroutine() == initalRoutines {
+					break
+				}
+				time.Sleep(20 * time.Millisecond)
+				if time.Since(start) > 10*time.Second {
+					b.Fatalf("%d goroutines still running, want %d", runtime.NumGoroutine(), initalRoutines)
+				}
+			}
+		})
+	}
+}
+
+func atStackDepth(depth int, fn func()) {
+	pcs := make([]uintptr, depth+10)
+	n := runtime.Callers(1, pcs)
+	if n > depth {
+		panic("depth exceeded")
+	} else if n < depth {
+		atStackDepth(depth, fn)
+		return
+	}
+
+	fn()
+}