util/clientmetric: optimize memory layout for finding updates

Updates #3307

Change-Id: I2840b190583467cc3f00688b96ce3d170df46a46
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/clientmetric/clientmetric.go

@@ -26,8 +26,24 @@ var (
 	lastDelta   time.Time   // time of last call to EncodeLogTailMetricsDelta
 	sortedDirty bool        // whether sorted needs to be rebuilt
 	sorted      []*Metric   // by name
+	lastLogVal  []scanEntry // by Metric.regIdx
+	unsorted    []*Metric   // by Metric.regIdx
+
+	// valFreeList is a set of free contiguous int64s whose
+	// element addresses get assigned to Metric.v.
+	// Any memory address in len(valFreeList) is free for use.
+	// They're contiguous to reduce cache churn during diff scans.
+	// When out of length, a new backing array is made.
+	valFreeList []int64
 )
 
+// scanEntry contains the minimal data needed for quickly scanning
+// memory for changed values. It's small to reduce memory pressure.
+type scanEntry struct {
+	v          *int64 // Metric.v
+	lastLogged int64  // last logged value
+}
+
 // Type is a metric type: counter or gauge.
 type Type uint8
 
@@ -40,32 +56,40 @@ const (
 //
 // It's safe for concurrent use.
 type Metric struct {
-	v    int64 // atomic; the metric value
+	v      *int64 // atomic; the metric value
+	regIdx int    // index into lastLogVal and unsorted
 	name   string
 	typ    Type
 
-	// Owned by package-level 'mu'.
-	wireID     int // zero until named
+	// The following fields are owned by the package-level 'mu':
+
+	// wireID is the lazily-allocated "wire ID". Until a metric is encoded
+	// in the logs (by EncodeLogTailMetricsDelta), it has no wireID. This
+	// ensures that unused metrics don't waste valuable low numbers, which
+	// encode with varints with fewer bytes.
+	wireID int
+
+	// lastNamed is the last time the name of this metric was
+	// written on the wire.
 	lastNamed time.Time
-	lastLogVal int64
 }
 
 func (m *Metric) Name() string { return m.name }
-func (m *Metric) Value() int64 { return atomic.LoadInt64(&m.v) }
+func (m *Metric) Value() int64 { return atomic.LoadInt64(m.v) }
 func (m *Metric) Type() Type   { return m.typ }
 
 // Add increments m's value by n.
 //
 // If m is of type counter, n should not be negative.
 func (m *Metric) Add(n int64) {
-	atomic.AddInt64(&m.v, n)
+	atomic.AddInt64(m.v, n)
 }
 
 // Set sets m's value to v.
 //
 // If m is of type counter, Set should not be used.
 func (m *Metric) Set(v int64) {
-	atomic.StoreInt64(&m.v, v)
+	atomic.StoreInt64(m.v, v)
 }
 
 // Publish registers a metric in the global map.
@@ -81,6 +105,16 @@ func (m *Metric) Publish() {
 	}
 	metrics[m.name] = m
 	sortedDirty = true
+
+	if len(valFreeList) == 0 {
+		valFreeList = make([]int64, 256)
+	}
+	m.v = &valFreeList[0]
+	valFreeList = valFreeList[1:]
+
+	m.regIdx = len(unsorted)
+	unsorted = append(unsorted, m)
+	lastLogVal = append(lastLogVal, scanEntry{v: m.v})
 }
 
 // Metrics returns the sorted list of metrics.
@@ -190,17 +224,18 @@ func EncodeLogTailMetricsDelta() string {
 	lastDelta = now
 
 	var enc *deltaEncBuf // lazy
-	for _, m := range metrics {
-		val := m.Value()
-		delta := val - m.lastLogVal
+	for i, ent := range lastLogVal {
+		val := atomic.LoadInt64(ent.v)
+		delta := val - ent.lastLogged
 		if delta == 0 {
 			continue
 		}
+		lastLogVal[i].lastLogged = val
+		m := unsorted[i]
 		if enc == nil {
 			enc = deltaPool.Get().(*deltaEncBuf)
 			enc.buf.Reset()
 		}
-		m.lastLogVal = val
 		if m.wireID == 0 {
 			numWireID++
 			m.wireID = numWireID