From 224167a80e78038acbb58427dc906f50c76638b2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Felix=20Geisend=C3=B6rfer?= <felix@felixge.de>
Date: Fri, 5 Feb 2021 15:11:12 +0100
Subject: [PATCH] demonstrate bias

---
 examples/block-sample/go.mod  |  5 ++
 examples/block-sample/go.sum  |  2 +
 examples/block-sample/main.go | 86 +++++++++++++++++++++++++++++++++++
 3 files changed, 93 insertions(+)
 create mode 100644 examples/block-sample/go.mod
 create mode 100644 examples/block-sample/go.sum
 create mode 100644 examples/block-sample/main.go

diff --git a/examples/block-sample/go.mod b/examples/block-sample/go.mod
new file mode 100644
index 0000000..b1611b4
--- /dev/null
+++ b/examples/block-sample/go.mod
@@ -0,0 +1,5 @@
+module github.com/felixge/go-profiler-notes/examples/block-sample
+
+go 1.15
+
+require golang.org/x/sync v0.0.0-20201207232520-09787c993a3a
diff --git a/examples/block-sample/go.sum b/examples/block-sample/go.sum
new file mode 100644
index 0000000..5f7eb37
--- /dev/null
+++ b/examples/block-sample/go.sum
@@ -0,0 +1,2 @@
+golang.org/x/sync v0.0.0-20201207232520-09787c993a3a h1:DcqTD9SDLc+1P/r1EmRBwnVsrOwW+kk2vWf9n+1sGhs=
+golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
diff --git a/examples/block-sample/main.go b/examples/block-sample/main.go
new file mode 100644
index 0000000..01a56b3
--- /dev/null
+++ b/examples/block-sample/main.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"time"
+
+	"golang.org/x/sync/errgroup"
+)
+
+func main() {
+	if err := run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func run() error {
+	runtime.SetBlockProfileRate(int((40 * time.Microsecond).Nanoseconds()))
+	done := make(chan struct{})
+	g := errgroup.Group{}
+	g.Go(func() error {
+		return eventA(done)
+	})
+	g.Go(func() error {
+		return eventB(done)
+	})
+	time.Sleep(time.Second)
+	close(done)
+	if err := g.Wait(); err != nil {
+		return err
+	}
+
+	f, err := os.Create("block.pb.gz")
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if err := pprof.Lookup("block").WriteTo(f, 0); err != nil {
+		return err
+	}
+	return nil
+}
+
+func eventA(done chan struct{}) error {
+	return simulateBlockEvents(20*time.Microsecond, done)
+}
+
+func eventB(done chan struct{}) error {
+	return simulateBlockEvents(40*time.Microsecond, done)
+}
+
+const tolerance = 1.1
+
+func simulateBlockEvents(meanDuration time.Duration, done chan struct{}) error {
+	var (
+		prev   time.Time
+		sum    time.Duration
+		count  int
+		ticker = time.NewTicker(meanDuration)
+	)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ticker.C:
+			now := time.Now()
+			if !prev.IsZero() {
+				sum += now.Sub(prev)
+				count += 1
+				if count > 1000 {
+					actualMean := float64(sum) / float64(count)
+					max := tolerance * float64(meanDuration)
+					min := float64(meanDuration) / tolerance
+					if actualMean <= min || actualMean >= max {
+						return fmt.Errorf("low clock accuracy: got=%s want=%s", time.Duration(actualMean), meanDuration)
+					}
+				}
+			}
+			prev = now
+		case <-done:
+			return nil
+		}
+	}
+}