runtime,runtime/metrics: add heap object count metric

For #37112.

Change-Id: Idd3dd5c84215ddd1ab05c2e76e848aa0a4d40fb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/247043
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/metrics.go

@@ -38,6 +38,13 @@ func initMetrics() {
 		return
 	}
 	metrics = map[string]metricData{
+		"/gc/heap/objects:objects": {
+			deps: makeStatDepSet(heapStatsDep),
+			compute: func(in *statAggregate, out *metricValue) {
+				out.kind = metricKindUint64
+				out.scalar = in.heapStats.numObjects
+			},
+		},
 		"/memory/classes/heap/free:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
@@ -210,9 +217,13 @@ func (s *statDepSet) has(d statDep) bool {
 type heapStatsAggregate struct {
 	heapStatsDelta
 
+	// Derived from values in heapStatsDelta.
+
 	// inObjects is the bytes of memory occupied by objects,
-	// derived from other values in heapStats.
 	inObjects uint64
+
+	// numObjects is the number of live objects in the heap.
+	numObjects uint64
 }
 
 // compute populates the heapStatsAggregate with values from the runtime.
@@ -221,8 +232,11 @@ func (a *heapStatsAggregate) compute() {
 
 	// Calculate derived stats.
 	a.inObjects = uint64(a.largeAlloc - a.largeFree)
+	a.numObjects = uint64(a.largeAllocCount - a.largeFreeCount)
 	for i := range a.smallAllocCount {
-		a.inObjects += uint64(a.smallAllocCount[i]-a.smallFreeCount[i]) * uint64(class_to_size[i])
+		n := uint64(a.smallAllocCount[i] - a.smallFreeCount[i])
+		a.inObjects += n * uint64(class_to_size[i])
+		a.numObjects += n
 	}
 }
 

src/runtime/metrics/description.go

@@ -50,6 +50,11 @@ type Description struct {
 // The English language descriptions below must be kept in sync with the
 // descriptions of each metric in doc.go.
 var allDesc = []Description{
+	{
+		Name:        "/gc/heap/objects:objects",
+		Description: "Number of objects, live or unswept, occupying heap memory.",
+		Kind:        KindUint64,
+	},
 	{
 		Name:        "/memory/classes/heap/free:bytes",
 		Description: "Memory that is available for allocation, and may be returned to the underlying system.",

src/runtime/metrics/doc.go

@@ -44,6 +44,9 @@ the documentation of the Name field of the Description struct.
 
 Supported metrics
 
+	/gc/heap/objects:objects
+		Number of objects, live or unswept, occupying heap memory.
+
 	/memory/classes/heap/free:bytes
 		Memory that is available for allocation, and may be returned
 		to the underlying system.

src/runtime/metrics_test.go

@@ -70,6 +70,8 @@ func TestReadMetrics(t *testing.T) {
 			checkUint64(t, name, samples[i].Value.Uint64(), mstats.BuckHashSys)
 		case "/memory/classes/total:bytes":
 			checkUint64(t, name, samples[i].Value.Uint64(), mstats.Sys)
+		case "/gc/heap/objects:objects":
+			checkUint64(t, name, samples[i].Value.Uint64(), mstats.HeapObjects)
 		}
 	}
 }