runtime/metrics: change unit on *-by-size metrics to match bucket unit

This change modifies the *-by-size metrics' units to be based off the
bucket's unit (bytes) as opposed to the unit of the counts (objects).
This convention is more in-line with distributions in other metrics
systems.

Change-Id: Id3b68a09f52f0e1ff9f4346f613ae1cbd9f52f73
Reviewed-on: https://go-review.googlesource.com/c/go/+/282352
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics.go

@@ -86,7 +86,7 @@ func initMetrics() {
 				out.scalar = in.sysStats.gcCyclesDone
 			},
 		},
-		"/gc/heap/allocs-by-size:objects": {
+		"/gc/heap/allocs-by-size:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				hist := out.float64HistOrInit(sizeClassBuckets)
@@ -98,7 +98,7 @@ func initMetrics() {
 				}
 			},
 		},
-		"/gc/heap/frees-by-size:objects": {
+		"/gc/heap/frees-by-size:bytes": {
 			deps: makeStatDepSet(heapStatsDep),
 			compute: func(in *statAggregate, out *metricValue) {
 				hist := out.float64HistOrInit(sizeClassBuckets)

src/runtime/metrics/description.go

@@ -23,6 +23,11 @@ type Description struct {
 	// Examples of units might be "seconds", "bytes", "bytes/second", "cpu-seconds",
 	// "byte*cpu-seconds", and "bytes/second/second".
 	//
+	// For histograms, multiple units may apply. For instance, the units of the buckets and
+	// the count. By convention, for histograms, the units of the count are always "samples"
+	// with the type of sample evident by the metric's name, while the unit in the name
+	// specifies the buckets' unit.
+	//
 	// A complete name might look like "/memory/heap/free:bytes".
 	Name string
 
@@ -69,12 +74,12 @@ var allDesc = []Description{
 		Cumulative:  true,
 	},
 	{
-		Name:        "/gc/heap/allocs-by-size:objects",
+		Name:        "/gc/heap/allocs-by-size:bytes",
 		Description: "Distribution of all objects allocated by approximate size.",
 		Kind:        KindFloat64Histogram,
 	},
 	{
-		Name:        "/gc/heap/frees-by-size:objects",
+		Name:        "/gc/heap/frees-by-size:bytes",
 		Description: "Distribution of all objects freed by approximate size.",
 		Kind:        KindFloat64Histogram,
 	},

src/runtime/metrics/doc.go

@@ -61,10 +61,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 	/gc/cycles/total:gc-cycles
 		Count of all completed GC cycles.
 
-	/gc/heap/allocs-by-size:objects
+	/gc/heap/allocs-by-size:bytes
 		Distribution of all objects allocated by approximate size.
 
-	/gc/heap/frees-by-size:objects
+	/gc/heap/frees-by-size:bytes
 		Distribution of all objects freed by approximate size.
 
 	/gc/heap/goal:bytes

src/runtime/metrics_test.go

@@ -70,7 +70,7 @@ 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/allocs-by-size:objects":
+		case "/gc/heap/allocs-by-size:bytes":
 			hist := samples[i].Value.Float64Histogram()
 			// Skip size class 0 in BySize, because it's always empty and not represented
 			// in the histogram.
@@ -84,7 +84,7 @@ func TestReadMetrics(t *testing.T) {
 					t.Errorf("histogram counts do not much BySize for class %d: got %d, want %d", i, c, m)
 				}
 			}
-		case "/gc/heap/frees-by-size:objects":
+		case "/gc/heap/frees-by-size:bytes":
 			hist := samples[i].Value.Float64Histogram()
 			// Skip size class 0 in BySize, because it's always empty and not represented
 			// in the histogram.
@@ -161,9 +161,9 @@ func TestReadMetricsConsistency(t *testing.T) {
 			totalVirtual.got = samples[i].Value.Uint64()
 		case "/gc/heap/objects:objects":
 			objects.total = samples[i].Value.Uint64()
-		case "/gc/heap/allocs-by-size:objects":
+		case "/gc/heap/allocs-by-size:bytes":
 			objects.alloc = samples[i].Value.Float64Histogram()
-		case "/gc/heap/frees-by-size:objects":
+		case "/gc/heap/frees-by-size:bytes":
 			objects.free = samples[i].Value.Float64Histogram()
 		case "/gc/cycles:gc-cycles":
 			gc.numGC = samples[i].Value.Uint64()