runtime,runtime/metrics: add object size distribution metrics

This change adds metrics for the distribution of objects allocated and
freed by size, mirroring MemStats' BySize field.

For #37112.

Change-Id: Ibaf1812da93598b37265ec97abc6669c1a5efcbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/247045
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>

1 files changed, 52 insertions, 0 deletions

diff --git a/src/runtime/metrics.go b/src/runtime/metrics.go
index 6595a4342c..32d8ab461c 100644
--- a/src/runtime/metrics.go
+++ b/src/runtime/metrics.go
@@ -18,6 +18,8 @@ var (
 	metricsSema uint32 = 1
 	metricsInit bool
 	metrics     map[string]metricData
+
+	sizeClassBuckets []float64
 )
 
 type metricData struct {
@@ -38,6 +40,10 @@ func initMetrics() {
 	if metricsInit {
 		return
 	}
+	sizeClassBuckets = make([]float64, _NumSizeClasses)
+	for i := range sizeClassBuckets {
+		sizeClassBuckets[i] = float64(class_to_size[i])
+	}
 	metrics = map[string]metricData{
 		"/gc/cycles/automatic:gc-cycles": {
 			deps: makeStatDepSet(sysStatsDep),
@@ -60,6 +66,26 @@ func initMetrics() {
 				out.scalar = in.sysStats.gcCyclesDone
 			},
 		},
+		"/gc/heap/allocs-by-size:objects": {
+			deps: makeStatDepSet(heapStatsDep),
+			compute: func(in *statAggregate, out *metricValue) {
+				hist := out.float64HistOrInit(sizeClassBuckets)
+				hist.counts[len(hist.counts)-1] = uint64(in.heapStats.largeAllocCount)
+				for i := range hist.buckets {
+					hist.counts[i] = uint64(in.heapStats.smallAllocCount[i])
+				}
+			},
+		},
+		"/gc/heap/frees-by-size:objects": {
+			deps: makeStatDepSet(heapStatsDep),
+			compute: func(in *statAggregate, out *metricValue) {
+				hist := out.float64HistOrInit(sizeClassBuckets)
+				hist.counts[len(hist.counts)-1] = uint64(in.heapStats.largeFreeCount)
+				for i := range hist.buckets {
+					hist.counts[i] = uint64(in.heapStats.smallFreeCount[i])
+				}
+			},
+		},
 		"/gc/heap/goal:bytes": {
 			deps: makeStatDepSet(sysStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
@@ -370,6 +396,32 @@ type metricValue struct {
 	pointer unsafe.Pointer // contains non-scalar values.
 }
 
+// float64HistOrInit tries to pull out an existing float64Histogram
+// from the value, but if none exists, then it allocates one with
+// the given buckets.
+func (v *metricValue) float64HistOrInit(buckets []float64) *metricFloat64Histogram {
+	var hist *metricFloat64Histogram
+	if v.kind == metricKindFloat64Histogram && v.pointer != nil {
+		hist = (*metricFloat64Histogram)(v.pointer)
+	} else {
+		v.kind = metricKindFloat64Histogram
+		hist = new(metricFloat64Histogram)
+		v.pointer = unsafe.Pointer(hist)
+	}
+	hist.buckets = buckets
+	if len(hist.counts) != len(hist.buckets)+1 {
+		hist.counts = make([]uint64, len(buckets)+1)
+	}
+	return hist
+}
+
+// metricFloat64Histogram is a runtime copy of runtime/metrics.Float64Histogram
+// and must be kept structurally identical to that type.
+type metricFloat64Histogram struct {
+	counts  []uint64
+	buckets []float64
+}
+
 // agg is used by readMetrics, and is protected by metricsSema.
 //
 // Managed as a global variable because its pointer will be