Avoid unnecessary cloning of the head chunk.

Change-Id: I5da774515d5493166a197b5814d0a720628cfaff

storage/local/chunk.go

@@ -68,6 +68,13 @@ func (cd *chunkDesc) unpin() {
 	}
 }
 
+func (cd *chunkDesc) getRefCount() int {
+	cd.Lock()
+	defer cd.Unlock()
+
+	return cd.refCount
+}
+
 func (cd *chunkDesc) firstTime() clientmodel.Timestamp {
 	cd.Lock()
 	defer cd.Unlock()

storage/local/series.go

@@ -144,6 +144,10 @@ type memorySeries struct {
 	// persisted. If true, the current head chunk must not be modified
 	// anymore.
 	headChunkPersisted bool
+	// Whether the current head chunk is used by an iterator. In that case,
+	// a non-persisted head chunk has to be cloned before more samples are
+	// appended.
+	headChunkUsedByIterator bool
 }
 
 // newMemorySeries returns a pointer to a newly allocated memorySeries for the
@@ -165,6 +169,21 @@ func (s *memorySeries) add(fp clientmodel.Fingerprint, v *metric.SamplePair, per
 		newHead := newChunkDesc(newDeltaEncodedChunk(d1, d0, true))
 		s.chunkDescs = append(s.chunkDescs, newHead)
 		s.headChunkPersisted = false
+	} else if s.headChunkUsedByIterator && s.head().getRefCount() > 1 {
+		// We only need to clone the head chunk if the current head
+		// chunk was used in an iterator at all and if the refCount is
+		// still greater than the 1 we always have because the head
+		// chunk is not yet persisted. The latter is just an
+		// approximation. We will still clone unnecessarily if an older
+		// iterator using a previous version of the head chunk is still
+		// around and keep the head chunk pinned. We needed to track
+		// pins by version of the head chunk, which is probably not
+		// worth the effort.
+		chunkOps.WithLabelValues(clone).Inc()
+		// No locking needed here because a non-persisted head chunk can
+		// not get evicted concurrently.
+		s.head().chunk = s.head().chunk.clone()
+		s.headChunkUsedByIterator = false
 	}
 
 	chunks := s.head().add(v)
@@ -258,6 +277,9 @@ func (s *memorySeries) evictOlderThan(
 			// This is a non-persisted head chunk that is old enough
 			// for eviction. Queue it to be persisted:
 			s.headChunkPersisted = true
+			// Since we cannot modify the head chunk from now on, we
+			// don't need to bother with cloning anymore.
+			s.headChunkUsedByIterator = false
 			persistQueue <- &persistRequest{
 				fingerprint: fp,
 				chunkDesc:   cd,
@@ -401,16 +423,16 @@ func (s *memorySeries) preloadChunksForRange(
 	return s.preloadChunks(pinIndexes, p)
 }
 
+// newIterator returns a new SeriesIterator. The caller must have locked the
+// fingerprint of the memorySeries.
 func (s *memorySeries) newIterator(lockFunc, unlockFunc func()) SeriesIterator {
 	chunks := make([]chunk, 0, len(s.chunkDescs))
 	for i, cd := range s.chunkDescs {
 		if !cd.isEvicted() {
-			if i == len(s.chunkDescs)-1 {
+			if i == len(s.chunkDescs)-1 && !s.headChunkPersisted {
-				chunkOps.WithLabelValues(clone).Inc()
+				s.headChunkUsedByIterator = true
-				chunks = append(chunks, cd.chunk.clone())
-			} else {
-				chunks = append(chunks, cd.chunk)
 			}
+			chunks = append(chunks, cd.chunk)
 		}
 	}