Handle long ReadMemStats duration in Go collector

tl;dr: Return previous memstats if reading new ones takes longer than
1s.

See the doc comment of NewGoCollector for details.

Signed-off-by: beorn7 <bjoern@rabenste.in>

prometheus/go_collector.go

@@ -16,6 +16,7 @@ package prometheus
 import (
 	"runtime"
 	"runtime/debug"
+	"sync"
 	"time"
 )
 
@@ -25,16 +26,41 @@ type goCollector struct {
 	gcDesc         *Desc
 	goInfoDesc     *Desc
 
-	// metrics to describe and collect
-	metrics memStatsMetrics
+	// ms... are memstats related.
+	msLast          *runtime.MemStats // Previously collected memstats.
+	msLastTimestamp time.Time
+	msMtx           sync.Mutex // Protects msLast and msLastTimestamp.
+	msMetrics       memStatsMetrics
+	msRead          func(*runtime.MemStats) // For mocking in tests.
+	msMaxWait       time.Duration           // Wait time for fresh memstats.
+	msMaxAge        time.Duration           // Maximum allowed age of old memstats.
 }
 
 // NewGoCollector returns a collector which exports metrics about the current Go
 // process. This includes memory stats. To collect those, runtime.ReadMemStats
-// is called. This causes a stop-the-world, which is very short with Go1.9+
-// (~25µs). However, with older Go versions, the stop-the-world duration depends
-// on the heap size and can be quite significant (~1.7 ms/GiB as per
+// is called. This requires to “stop the world”, which usually only happens for
+// garbage collection (GC). Take the following implications into account when
+// deciding whether to use the Go collector:
+//
+// 1. The performance impact of stopping the world is the more relevant the more
+// frequently metrics are collected. However, with Go1.9 or later the
+// stop-the-world time per metrics collection is very short (~25µs) so that the
+// performance impact will only matter in rare cases. However, with older Go
+// versions, the stop-the-world duration depends on the heap size and can be
+// quite significant (~1.7 ms/GiB as per
 // https://go-review.googlesource.com/c/go/+/34937).
+//
+// 2. During an ongoing GC, nothing else can stop the world. Therefore, if the
+// metrics collection happens to coincide with GC, it will only complete after
+// GC has finished. Usually, GC is fast enough to not cause problems. However,
+// with a very large heap, GC might take multiple seconds, which is enough to
+// cause scrape timeouts in common setups. To avoid this problem, the Go
+// collector will use the memstats from a previous collection if
+// runtime.ReadMemStats takes more than 1s. However, if there are no previously
+// collected memstats, or their collection is more than 5m ago, the collection
+// will block until runtime.ReadMemStats succeeds. (The problem might be solved
+// in Go1.13, see https://github.com/golang/go/issues/19812 for the related Go
+// issue.)
 func NewGoCollector() Collector {
 	return &goCollector{
 		goroutinesDesc: NewDesc(
@@ -53,7 +79,11 @@ func NewGoCollector() Collector {
 			"go_info",
 			"Information about the Go environment.",
 			nil, Labels{"version": runtime.Version()}),
-		metrics: memStatsMetrics{
+		msLast:    &runtime.MemStats{},
+		msRead:    runtime.ReadMemStats,
+		msMaxWait: time.Second,
+		msMaxAge:  5 * time.Minute,
+		msMetrics: memStatsMetrics{
 			{
 				desc: NewDesc(
 					memstatNamespace("alloc_bytes"),
@@ -261,13 +291,27 @@ func (c *goCollector) Describe(ch chan<- *Desc) {
 	ch <- c.threadsDesc
 	ch <- c.gcDesc
 	ch <- c.goInfoDesc
-	for _, i := range c.metrics {
+	for _, i := range c.msMetrics {
 		ch <- i.desc
 	}
 }
 
 // Collect returns the current state of all metrics of the collector.
 func (c *goCollector) Collect(ch chan<- Metric) {
+	var (
+		ms   = &runtime.MemStats{}
+		done = make(chan struct{})
+	)
+	// Start reading memstats first as it might take a while.
+	go func() {
+		c.msRead(ms)
+		c.msMtx.Lock()
+		c.msLast = ms
+		c.msLastTimestamp = time.Now()
+		c.msMtx.Unlock()
+		close(done)
+	}()
+
 	ch <- MustNewConstMetric(c.goroutinesDesc, GaugeValue, float64(runtime.NumGoroutine()))
 	n, _ := runtime.ThreadCreateProfile(nil)
 	ch <- MustNewConstMetric(c.threadsDesc, GaugeValue, float64(n))
@@ -285,9 +329,31 @@ func (c *goCollector) Collect(ch chan<- Metric) {
 
 	ch <- MustNewConstMetric(c.goInfoDesc, GaugeValue, 1)
 
-	ms := &runtime.MemStats{}
-	runtime.ReadMemStats(ms)
-	for _, i := range c.metrics {
+	timer := time.NewTimer(c.msMaxWait)
+	select {
+	case <-done: // Our own ReadMemStats succeeded in time. Use it.
+		timer.Stop() // Important for high collection frequencies to not pile up timers.
+		c.msCollect(ch, ms)
+		return
+	case <-timer.C: // Time out, use last memstats if possible. Continue below.
+	}
+	c.msMtx.Lock()
+	if time.Since(c.msLastTimestamp) < c.msMaxAge {
+		// Last memstats are recent enough. Collect from them under the lock.
+		c.msCollect(ch, c.msLast)
+		c.msMtx.Unlock()
+		return
+	}
+	// If we are here, the last memstats are too old or don't exist. We have
+	// to wait until our own ReadMemStats finally completes. For that to
+	// happen, we have to release the lock.
+	c.msMtx.Unlock()
+	<-done
+	c.msCollect(ch, ms)
+}
+
+func (c *goCollector) msCollect(ch chan<- Metric, ms *runtime.MemStats) {
+	for _, i := range c.msMetrics {
 		ch <- MustNewConstMetric(i.desc, i.valType, i.eval(ms))
 	}
 }

prometheus/go_collector_test.go

@@ -150,3 +150,88 @@ func TestGoCollectorGC(t *testing.T) {
 		break
 	}
 }
+
+func TestGoCollectorMemStats(t *testing.T) {
+	var (
+		c   = NewGoCollector().(*goCollector)
+		got uint64
+	)
+
+	checkCollect := func(want uint64) {
+		metricCh := make(chan Metric)
+		endCh := make(chan struct{})
+
+		go func() {
+			c.Collect(metricCh)
+			close(endCh)
+		}()
+	Collect:
+		for {
+			select {
+			case metric := <-metricCh:
+				if metric.Desc().fqName != "go_memstats_alloc_bytes" {
+					continue Collect
+				}
+				pb := &dto.Metric{}
+				metric.Write(pb)
+				got = uint64(pb.GetGauge().GetValue())
+			case <-endCh:
+				break Collect
+			}
+		}
+		if want != got {
+			t.Errorf("unexpected value of go_memstats_alloc_bytes, want %d, got %d", want, got)
+		}
+	}
+
+	// Speed up the timing to make the tast faster.
+	c.msMaxWait = time.Millisecond
+	c.msMaxAge = 10 * time.Millisecond
+
+	// Scenario 1: msRead responds slowly, no previous memstats available,
+	// msRead is executed anyway.
+	c.msRead = func(ms *runtime.MemStats) {
+		time.Sleep(3 * time.Millisecond)
+		ms.Alloc = 1
+	}
+	checkCollect(1)
+	// Now msLast is set.
+	if want, got := uint64(1), c.msLast.Alloc; want != got {
+		t.Errorf("unexpected of msLast.Alloc, want %d, got %d", want, got)
+	}
+
+	// Scenario 2: msRead responds fast, previous memstats available, new
+	// value collected.
+	c.msRead = func(ms *runtime.MemStats) {
+		ms.Alloc = 2
+	}
+	checkCollect(2)
+	// msLast is set, too.
+	if want, got := uint64(2), c.msLast.Alloc; want != got {
+		t.Errorf("unexpected of msLast.Alloc, want %d, got %d", want, got)
+	}
+
+	// Scenario 3: msRead responds slowly, previous memstats available, old
+	// value collected.
+	c.msRead = func(ms *runtime.MemStats) {
+		time.Sleep(3 * time.Millisecond)
+		ms.Alloc = 3
+	}
+	checkCollect(2)
+	// After waiting, new value is still set in msLast.
+	time.Sleep(12 * time.Millisecond)
+	if want, got := uint64(3), c.msLast.Alloc; want != got {
+		t.Errorf("unexpected of msLast.Alloc, want %d, got %d", want, got)
+	}
+
+	// Scenario 4: msRead responds slowly, previous memstats is too old, new
+	// value collected.
+	c.msRead = func(ms *runtime.MemStats) {
+		time.Sleep(3 * time.Millisecond)
+		ms.Alloc = 4
+	}
+	checkCollect(4)
+	if want, got := uint64(4), c.msLast.Alloc; want != got {
+		t.Errorf("unexpected of msLast.Alloc, want %d, got %d", want, got)
+	}
+}