From 5b9a815a3e075778fa38d6a68f9ba527da2405a0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Felix=20Geisend=C3=B6rfer?=
 <felix.geisendoerfer@datadoghq.com>
Date: Fri, 26 Mar 2021 14:15:45 +0100
Subject: [PATCH] Add benchmark

---
 examples/runtime-stack/go.mod       |  3 ++
 examples/runtime-stack/main_test.go | 67 +++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 examples/runtime-stack/go.mod
 create mode 100644 examples/runtime-stack/main_test.go

diff --git a/examples/runtime-stack/go.mod b/examples/runtime-stack/go.mod
new file mode 100644
index 0000000..a5cc82a
--- /dev/null
+++ b/examples/runtime-stack/go.mod
@@ -0,0 +1,3 @@
+module github.com/felixge/go-profiler-notes/examples/runtime-stack
+
+go 1.16
diff --git a/examples/runtime-stack/main_test.go b/examples/runtime-stack/main_test.go
new file mode 100644
index 0000000..661cffd
--- /dev/null
+++ b/examples/runtime-stack/main_test.go
@@ -0,0 +1,67 @@
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"testing"
+	"time"
+)
+
+func BenchmarkRuntimeStack(b *testing.B) {
+	buf := make([]byte, 1024*1024*64)
+	for g := 1; g <= 1024*1024; 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(16, func() {
+					defer func() { stopCh <- struct{}{} }()
+					readyCh <- struct{}{}
+				})
+				<-readyCh
+			}
+
+			gotRoutines := runtime.NumGoroutine() - initalRoutines
+			if gotRoutines != g {
+				b.Logf("want %d goroutines, but got %d", g, gotRoutines)
+			}
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				runtime.Stack(buf, true)
+			}
+			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()
+}