benchmarks/gc_latency: add new microbenchmark for tricky mutator late…

…ncy issues

This emits statistical latency measurements, not the worst case, because as
a benchmark, worst-case is too noisy.  This retains latency debugging features.

Updates golang/go#27732.

Change-Id: I19b9060f24cda1547b8d75f762316dd5271e32c6

gc_latency/main.go

@@ -0,0 +1,169 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/trace"
+	"sort"
+	"time"
+	"unsafe"
+)
+
+const (
+	bufferLen = 200000
+	msgCount0 = 1000000 // warmup
+	msgCount1 = 5000000 // run (5 million)
+)
+
+type kbyte []byte
+type circularBuffer [bufferLen]kbyte
+
+var worst time.Duration
+
+// For making sense of the bad outcome.
+var total time.Duration
+var worstIndex int
+var allStart time.Time
+var worstElapsed time.Duration
+
+var fluff kbyte
+var doFluff bool
+
+// newSlice allocates a 1k slice of bytes and initializes them all to byte(n)
+func newSlice(n int) kbyte {
+	m := make(kbyte, 1024)
+	if doFluff && n&63 == 0 {
+		fluff = make(kbyte, 1024)
+	}
+	for i := range m {
+		m[i] = byte(n)
+	}
+	return m
+}
+
+var delays []time.Duration
+
+// storeSlice stores a newly allocated 1k slice of bytes at circularBuffer[count%bufferLen]
+// (bufferLen is the length of the array circularBuffer)
+// It also checks the time needed to do this and records the worst case.
+func storeSlice(c *circularBuffer, highID int) {
+	start := time.Now()
+	c[highID%bufferLen] = newSlice(highID)
+	elapsed := time.Since(start)
+
+	candElapsed := time.Since(allStart) // Record location of worst in trace
+	if elapsed > worst {
+		worst = elapsed
+		worstIndex = highID
+		worstElapsed = candElapsed
+	}
+	total = time.Duration(total.Nanoseconds() + elapsed.Nanoseconds())
+	delays = append(delays, elapsed)
+}
+
+//go:noinline
+func work(c *circularBuffer, count int) {
+	for i := 0; i < count; i++ {
+		storeSlice(c, i)
+	}
+}
+
+//go:noinline
+func stack() {
+	var c circularBuffer
+	work(&c, msgCount1)
+}
+
+var global_c circularBuffer
+
+//go:noinline
+func global() {
+	work(&global_c, msgCount1)
+}
+
+var sink *circularBuffer
+
+//go:noinline
+func heap() {
+	c := &circularBuffer{}
+	sink = c // force to heap
+	work(c, msgCount1)
+}
+
+func main() {
+	var traceFile string
+	var howAllocated = "stack"
+	var asBench bool
+
+	flag.StringVar(&traceFile, "trace", traceFile, "name of trace file to create")
+	flag.StringVar(&howAllocated, "how", howAllocated, "how the buffer is allocated = {stack,heap,global}")
+	flag.BoolVar(&doFluff, "fluff", doFluff, "insert 'fluff' into allocation runs to break up sweeps")
+	flag.BoolVar(&asBench, "bench", asBench, "output in Go benchmark format")
+	flag.Parse()
+
+	var c *circularBuffer = &circularBuffer{}
+	delays = make([]time.Duration, 0, msgCount1)
+	// Warmup
+	work(c, msgCount0)
+	c = nil
+	delays = delays[:0]
+
+	total = time.Duration(0)
+	worstElapsed = time.Duration(0)
+	worst = time.Duration(0)
+	worstIndex = 0
+
+	if traceFile != "" {
+		f, err := os.Create(traceFile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Could not create trace file '%s'\n", traceFile)
+			os.Exit(1)
+		}
+		defer f.Close()
+		trace.Start(f)
+		defer trace.Stop()
+	}
+	allStart = time.Now()
+	c = nil
+
+	switch howAllocated {
+	case "stack":
+		stack()
+	case "heap":
+		heap()
+	case "global":
+		global()
+	default:
+		fmt.Fprintf(os.Stderr, "-how needs to be one of 'heap', 'stack' or 'global, saw '%s' instead\n", howAllocated)
+		os.Exit(1)
+	}
+
+	sort.Slice(delays, func(i, j int) bool { return delays[i] < delays[j] })
+
+	if asBench {
+		fmt.Printf("goos: %s\n", runtime.GOOS)
+		fmt.Printf("goarch: %s\n", runtime.GOARCH)
+
+		fmt.Printf("BenchmarkAverageLatency  1  %dns\n", time.Duration(total.Nanoseconds()/msgCount1))
+		fmt.Printf("BenchmarkMedianLatency   1  %dns\n", delays[len(delays)/2])
+		fmt.Printf("Benchmark99Latency       1  %dns\n", delays[int(0.99*float64(len(delays)))])
+		fmt.Printf("Benchmark999Latency      1  %dns\n", delays[int(0.999*float64(len(delays)))])
+		fmt.Printf("Benchmark9999Latency     1  %dns\n", delays[int(0.9999*float64(len(delays)))])
+		fmt.Printf("Benchmark99999Latency    1  %dns\n", delays[int(0.99999*float64(len(delays)))])
+	} else {
+
+		fmt.Println("Worst allocation latency:", worst)
+		fmt.Println("Worst allocation index:", worstIndex)
+		fmt.Println("Worst allocation occurs at run elapsed time:", worstElapsed)
+		fmt.Println("Average allocation latency:", time.Duration(total.Nanoseconds()/msgCount1))
+		fmt.Println("Median allocation latency:", delays[len(delays)/2])
+		fmt.Println("99% allocation latency:", delays[int(0.99*float64(len(delays)))])
+		fmt.Println("99.9% allocation latency:", delays[int(0.999*float64(len(delays)))])
+		fmt.Println("99.99% allocation latency:", delays[int(0.9999*float64(len(delays)))])
+		fmt.Println("99.999% allocation latency:", delays[int(0.99999*float64(len(delays)))])
+		fmt.Println("Sizeof(circularBuffer) =", unsafe.Sizeof(*c))
+		fmt.Println("Approximate live memory =", unsafe.Sizeof(*c)+bufferLen*1024)
+	}
+}