Populate first and last time in the chunk descriptor earlier

The First time is kind of trivial as we always know it when we create
a new chunkDesc.

The last time is only know when the chunk is closed, so we have to set
it at that time.

The change saves a lot of digging down into the chunk
itself. Especially the last time is relative expensive as it involves
the creation of an iterator. The first time access now doesn't require
locking, which is also a nice gain.

storage/local/chunk.go

@@ -56,11 +56,11 @@ const (
 // chunkDesc contains meta-data for a chunk. Many of its methods are
 // goroutine-safe proxies for chunk methods.
 type chunkDesc struct {
-	sync.Mutex
+	sync.Mutex           // TODO(beorn7): Try out if an RWMutex would help here.
 	c              chunk // nil if chunk is evicted.
 	rCnt           int
-	chunkFirstTime model.Time // Used if chunk is evicted.
-	chunkLastTime  model.Time // Used if chunk is evicted.
+	chunkFirstTime model.Time // Populated at creation.
+	chunkLastTime  model.Time // Populated on closing of the chunk, model.Earliest if unset.
 
 	// evictListElement is nil if the chunk is not in the evict list.
 	// evictListElement is _not_ protected by the chunkDesc mutex.
@@ -71,11 +71,16 @@ type chunkDesc struct {
 // newChunkDesc creates a new chunkDesc pointing to the provided chunk. The
 // provided chunk is assumed to be not persisted yet. Therefore, the refCount of
 // the new chunkDesc is 1 (preventing eviction prior to persisting).
-func newChunkDesc(c chunk) *chunkDesc {
+func newChunkDesc(c chunk, firstTime model.Time) *chunkDesc {
 	chunkOps.WithLabelValues(createAndPin).Inc()
 	atomic.AddInt64(&numMemChunks, 1)
 	numMemChunkDescs.Inc()
-	return &chunkDesc{c: c, rCnt: 1}
+	return &chunkDesc{
+		c:              c,
+		rCnt:           1,
+		chunkFirstTime: firstTime,
+		chunkLastTime:  model.Earliest,
+	}
 }
 
 func (cd *chunkDesc) add(s *model.SamplePair) []chunk {
@@ -124,23 +129,27 @@ func (cd *chunkDesc) refCount() int {
 }
 
 func (cd *chunkDesc) firstTime() model.Time {
+	// No lock required, will never be modified.
+	return cd.chunkFirstTime
+}
+
+func (cd *chunkDesc) lastTime() model.Time {
 	cd.Lock()
 	defer cd.Unlock()
 
-	if cd.c == nil {
-		return cd.chunkFirstTime
+	if cd.chunkLastTime != model.Earliest || cd.c == nil {
+		return cd.chunkLastTime
 	}
-	return cd.c.firstTime()
+	return cd.c.newIterator().lastTimestamp()
 }
 
-func (cd *chunkDesc) lastTime() model.Time {
+func (cd *chunkDesc) maybePopulateLastTime() {
 	cd.Lock()
 	defer cd.Unlock()
 
-	if cd.c == nil {
-		return cd.chunkLastTime
+	if cd.chunkLastTime == model.Earliest && cd.c != nil {
+		cd.chunkLastTime = cd.c.newIterator().lastTimestamp()
 	}
-	return cd.c.newIterator().lastTimestamp()
 }
 
 func (cd *chunkDesc) lastSamplePair() *model.SamplePair {
@@ -198,8 +207,10 @@ func (cd *chunkDesc) maybeEvict() bool {
 	if cd.rCnt != 0 {
 		return false
 	}
-	cd.chunkFirstTime = cd.c.firstTime()
-	cd.chunkLastTime = cd.c.newIterator().lastTimestamp()
+	// Last opportunity to populate chunkLastTime.
+	if cd.chunkLastTime == model.Earliest {
+		cd.chunkLastTime = cd.c.newIterator().lastTimestamp()
+	}
 	cd.c = nil
 	chunkOps.WithLabelValues(evict).Inc()
 	atomic.AddInt64(&numMemChunks, -1)

storage/local/persistence.go

@@ -856,7 +856,7 @@ func (p *persistence) loadSeriesMapAndHeads() (sm *seriesMap, chunksToPersist in
 					p.dirty = true
 					return sm, chunksToPersist, nil
 				}
-				chunkDescs[i] = newChunkDesc(chunk)
+				chunkDescs[i] = newChunkDesc(chunk, chunk.firstTime())
 				chunksToPersist++
 			}
 		}

storage/local/series.go

@@ -209,7 +209,7 @@ func newMemorySeries(m model.Metric, chunkDescs []*chunkDesc, modTime time.Time)
 // The caller must have locked the fingerprint of the series.
 func (s *memorySeries) add(v *model.SamplePair) int {
 	if len(s.chunkDescs) == 0 || s.headChunkClosed {
-		newHead := newChunkDesc(newChunk())
+		newHead := newChunkDesc(newChunk(), v.Timestamp)
 		s.chunkDescs = append(s.chunkDescs, newHead)
 		s.headChunkClosed = false
 	} else if s.headChunkUsedByIterator && s.head().refCount() > 1 {
@@ -233,7 +233,12 @@ func (s *memorySeries) add(v *model.SamplePair) int {
 	s.head().c = chunks[0]
 
 	for _, c := range chunks[1:] {
-		s.chunkDescs = append(s.chunkDescs, newChunkDesc(c))
+		s.chunkDescs = append(s.chunkDescs, newChunkDesc(c, c.firstTime()))
+	}
+
+	// Populate lastTime of now-closed chunks.
+	for _, cd := range s.chunkDescs[len(s.chunkDescs)-len(chunks) : len(s.chunkDescs)-1] {
+		cd.maybePopulateLastTime()
 	}
 
 	s.lastTime = v.Timestamp
@@ -254,6 +259,7 @@ func (s *memorySeries) maybeCloseHeadChunk() bool {
 		// Since we cannot modify the head chunk from now on, we
 		// don't need to bother with cloning anymore.
 		s.headChunkUsedByIterator = false
+		s.head().maybePopulateLastTime()
 		return true
 	}
 	return false