Adaptively reduce the wait time for memory series maintenance.

This will make in-memory series maintenance the faster the more chunks
are waiting for persistence.

storage/local/storage.go

@@ -32,9 +32,10 @@ const (
 	chunkLen         = 1024
 
 	// See waitForNextFP.
-	fpMaxWaitDuration = 10 * time.Second
 	fpMaxSweepTime    = 6 * time.Hour
+	fpMaxWaitDuration = 10 * time.Second
 
+	// See waitForNextFP.
 	maxEvictInterval = time.Minute
 
 	// If numChunskToPersist is this percentage of maxChunksToPersist, we
@@ -532,21 +533,31 @@ func (s *memorySeriesStorage) maybeEvict() {
 // another fingerprint so that we will process all fingerprints in a tenth of
 // s.dropAfter assuming that the system is doing nothing else, e.g. if we want
 // to drop chunks after 40h, we want to cycle through all fingerprints within
-// 4h. However, the maximum sweep time is capped at fpMaxSweepTime. If
-// s.loopStopped is closed, it will return false immediately. The estimation is
-// based on the total number of fingerprints as passed in.
-func (s *memorySeriesStorage) waitForNextFP(numberOfFPs int) bool {
+// 4h.  The estimation is based on the total number of fingerprints as passed
+// in. However, the maximum sweep time is capped at fpMaxSweepTime. Also, the
+// method will never wait for longer than fpMaxWaitDuration.
+//
+// The maxWaitDurationFactor can be used to reduce the waiting time if a faster
+// processing is required (for example because unpersisted chunks pile up too
+// much).
+//
+// Normally, the method returns true once the wait duration has passed. However,
+// if s.loopStopped is closed, it will return false immediately.
+func (s *memorySeriesStorage) waitForNextFP(numberOfFPs int, maxWaitDurationFactor float64) bool {
 	d := fpMaxWaitDuration
 	if numberOfFPs != 0 {
 		sweepTime := s.dropAfter / 10
 		if sweepTime > fpMaxSweepTime {
 			sweepTime = fpMaxSweepTime
 		}
-		d = sweepTime / time.Duration(numberOfFPs)
-		if d > fpMaxWaitDuration {
-			d = fpMaxWaitDuration
+		calculatedWait := time.Duration(float64(sweepTime) / float64(numberOfFPs) * maxWaitDurationFactor)
+		if calculatedWait < d {
+			d = calculatedWait
 		}
 	}
+	if d == 0 {
+		return true
+	}
 	t := time.NewTimer(d)
 	select {
 	case <-t.C:
@@ -575,7 +586,7 @@ func (s *memorySeriesStorage) cycleThroughMemoryFingerprints() chan clientmodel.
 
 		for {
 			// Initial wait, also important if there are no FPs yet.
-			if !s.waitForNextFP(s.fpToSeries.length()) {
+			if !s.waitForNextFP(s.fpToSeries.length(), 1) {
 				return
 			}
 			begin := time.Now()
@@ -587,7 +598,8 @@ func (s *memorySeriesStorage) cycleThroughMemoryFingerprints() chan clientmodel.
 				case <-s.loopStopping:
 					return
 				}
-				s.waitForNextFP(s.fpToSeries.length())
+				// Reduce the wait time by the backlog score.
+				s.waitForNextFP(s.fpToSeries.length(), s.persistenceBacklogScore())
 				count++
 			}
 			if count > 0 {
@@ -616,11 +628,11 @@ func (s *memorySeriesStorage) cycleThroughArchivedFingerprints() chan clientmode
 			)
 			if err != nil {
 				glog.Error("Failed to lookup archived fingerprint ranges: ", err)
-				s.waitForNextFP(0)
+				s.waitForNextFP(0, 1)
 				continue
 			}
 			// Initial wait, also important if there are no FPs yet.
-			if !s.waitForNextFP(len(archivedFPs)) {
+			if !s.waitForNextFP(len(archivedFPs), 1) {
 				return
 			}
 			begin := time.Now()
@@ -630,7 +642,8 @@ func (s *memorySeriesStorage) cycleThroughArchivedFingerprints() chan clientmode
 				case <-s.loopStopping:
 					return
 				}
-				s.waitForNextFP(len(archivedFPs))
+				// Never speed up maintenance of archived FPs.
+				s.waitForNextFP(len(archivedFPs), 1)
 			}
 			if len(archivedFPs) > 0 {
 				glog.Infof(
@@ -945,7 +958,7 @@ func (s *memorySeriesStorage) isDegraded() bool {
 		glog.Warning("Storage has left graceful degradation mode. Things are back to normal.")
 	} else if !s.degraded && nowDegraded {
 		glog.Warningf(
-			"%d chunks waiting for persistence (%d%% of the allowed maximum %d). Storage is now in graceful degradation mode. Series files are not synced anymore if following the adaptive strategy. Checkpoints are not performed more often than every %v.",
+			"%d chunks waiting for persistence (%d%% of the allowed maximum %d). Storage is now in graceful degradation mode. Series files are not synced anymore if following the adaptive strategy. Checkpoints are not performed more often than every %v. Series maintenance happens as frequently as possible.",
 			s.getNumChunksToPersist(),
 			s.getNumChunksToPersist()*100/s.maxChunksToPersist,
 			s.maxChunksToPersist,
@@ -955,6 +968,18 @@ func (s *memorySeriesStorage) isDegraded() bool {
 	return s.degraded
 }
 
+// persistenceBacklogScore works similar to isDegraded, but returns a score
+// about how close we are to degradation. This score is 1.0 if no chunks are
+// waiting for persistence and 0.0 if we are at or above the degradation
+// threshold.
+func (s *memorySeriesStorage) persistenceBacklogScore() float64 {
+	score := 1 - float64(s.getNumChunksToPersist())/float64(s.maxChunksToPersist*percentChunksToPersistForDegradation/100)
+	if score < 0 {
+		return 0
+	}
+	return score
+}
+
 // Describe implements prometheus.Collector.
 func (s *memorySeriesStorage) Describe(ch chan<- *prometheus.Desc) {
 	s.persistence.Describe(ch)