clarify Latency histogram

util/metric/metric.go

@@ -61,18 +61,6 @@ type periodic interface {
 var _ periodic = &Histogram{}
 var _ periodic = &Rate{}
 
-// Unit is a base unit for a histogram.
-type Unit int64
-
-// UnitMs is the number of milliseconds in a nanosecond.
-const UnitMs = Unit(time.Millisecond)
-
-// MaxVal is a maximum value for a histogram.
-type MaxVal int64
-
-// MaxMinute truncates histogram values larger than one minute.
-const MaxMinute = MaxVal(time.Minute)
-
 func maybeTick(m periodic) {
 	for m.nextTick().Before(time.Now()) {
 		m.tick()
@@ -81,7 +69,6 @@ func maybeTick(m periodic) {
 
 // A Histogram is a wrapper around an hdrhistogram.WindowedHistogram.
 type Histogram struct {
-	unit   int64
 	maxVal int64
 
 	mu       sync.Mutex
@@ -112,9 +99,8 @@ func (h *Histogram) RecordValue(v int64) {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 	maybeTick(h)
-	v /= h.unit
 	for h.windowed.Current.RecordValue(v) != nil {
-		v = h.maxVal / h.unit
+		v = h.maxVal
 	}
 }
 

util/metric/registry.go

@@ -90,27 +90,28 @@ func (r *Registry) MarshalJSON() ([]byte, error) {
 // Histogram registers a new windowed HDRHistogram with the given
 // parameters. Data is kept in the active window for approximately the given
 // duration.
-func (r *Registry) Histogram(name string, duration time.Duration, unit Unit, maxVal MaxVal, sigFigs int) *Histogram {
+func (r *Registry) Histogram(name string, duration time.Duration, maxVal int64, sigFigs int) *Histogram {
 	const n = 4
 	h := &Histogram{}
 	h.maxVal = int64(maxVal)
-	h.unit = int64(unit)
 	h.interval = duration / n
 	h.nextT = time.Now()
 
-	h.windowed = hdrhistogram.NewWindowed(n, 0, h.maxVal/h.unit, sigFigs)
+	h.windowed = hdrhistogram.NewWindowed(n, 0, h.maxVal, sigFigs)
 	r.MustAdd(name, h)
 	return h
 }
 
 // Latency is a convenience function which registers histograms with
-// suitable defaults for latency tracking on millisecond to minute time scales.
+// suitable defaults for latency tracking. Values are expressed in ns,
+// are truncated into the interval [0, time.Minute] and are recorded
+// with two digits of precision (i.e. errors of <1ms at 100ms, <.6s at 1m).
 // The generated names of the metric will begin with the given prefix.
 func (r *Registry) Latency(prefix string) Histograms {
 	windows := []timeScale{scale1M, scale10M, scale1H}
 	hs := make([]*Histogram, 0, 3)
 	for _, w := range windows {
-		h := r.Histogram(prefix+w.name, w.d, UnitMs, MaxMinute, 2)
+		h := r.Histogram(prefix+w.name, w.d, int64(time.Minute), 2)
 		hs = append(hs, h)
 	}
 	return hs