Compact head block early

Let older head blocks be compacted once the newest once has samples at
50% of its total range. This allows the memory of the compacted blocks
to be released and garbage collected before a new head block gets
created. Thereby the number of head blocks is 1 or 2 instead of 2 or 3
and memory spikes are reduced.

block.go

@@ -60,6 +60,11 @@ type Block interface {
 type headBlock interface {
 	Block
 	Appendable
+
+	// ActiveWriters returns the number of currently active appenders.
+	ActiveWriters() int
+	// HighTimestamp returns the highest currently inserted timestamp.
+	HighTimestamp() int64
 }
 
 // Snapshottable defines an entity that can be backedup online.
@@ -71,9 +76,6 @@ type Snapshottable interface {
 type Appendable interface {
 	// Appender returns a new Appender against an underlying store.
 	Appender() Appender
-
-	// Busy returns whether there are any currently active appenders.
-	Busy() bool
 }
 
 // Queryable defines an entity which provides a Querier.

db.go

@@ -305,6 +305,15 @@ func (db *DB) retentionCutoff() (bool, error) {
 	return retentionCutoff(db.dir, mint)
 }
 
+// headFullness returns up to which fraction of a blocks time range samples
+// were already inserted.
+func headFullness(h headBlock) float64 {
+	m := h.Meta()
+	a := float64(h.HighTimestamp() - m.MinTime)
+	b := float64(m.MaxTime - m.MinTime)
+	return a / b
+}
+
 func (db *DB) compact() (changes bool, err error) {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
@@ -319,12 +328,14 @@ func (db *DB) compact() (changes bool, err error) {
 	// returning the lock to not block Appenders.
 	// Selected blocks are semantically ensured to not be written to afterwards
 	// by appendable().
-	if len(db.heads) > 2 {
-		for _, h := range db.heads[:len(db.heads)-2] {
+	if len(db.heads) > 1 {
+		f := headFullness(db.heads[len(db.heads)-1])
+
+		for _, h := range db.heads[:len(db.heads)-1] {
 			// Blocks that won't be appendable when instantiating a new appender
 			// might still have active appenders on them.
 			// Abort at the first one we encounter.
-			if h.Busy() {
+			if h.ActiveWriters() > 0 || f < 0.5 {
 				break
 			}
 			singles = append(singles, h)
@@ -849,6 +860,8 @@ func (db *DB) createHeadBlock(mint, maxt int64) (headBlock, error) {
 		return nil, err
 	}
 
+	db.logger.Log("msg", "created head block", "ulid", newHead.meta.ULID, "mint", mint, "maxt", maxt)
+
 	db.blocks = append(db.blocks, newHead) // TODO(fabxc): this is a race!
 	db.heads = append(db.heads, newHead)
 

head.go

@@ -57,6 +57,7 @@ type HeadBlock struct {
 	wal WAL
 
 	activeWriters uint64
+	highTimestamp int64
 	closed        bool
 
 	// descs holds all chunk descs for the head block. Each chunk implicitly
@@ -389,9 +390,14 @@ func (h *HeadBlock) Appender() Appender {
 	return &headAppender{HeadBlock: h, samples: getHeadAppendBuffer()}
 }
 
-// Busy returns true if the block has open write transactions.
-func (h *HeadBlock) Busy() bool {
-	return atomic.LoadUint64(&h.activeWriters) > 0
+// ActiveWriters returns true if the block has open write transactions.
+func (h *HeadBlock) ActiveWriters() int {
+	return int(atomic.LoadUint64(&h.activeWriters))
+}
+
+// HighTimestamp returns the highest inserted sample timestamp.
+func (h *HeadBlock) HighTimestamp() int64 {
+	return atomic.LoadInt64(&h.highTimestamp)
 }
 
 var headPool = sync.Pool{}
@@ -416,6 +422,7 @@ type headAppender struct {
 	newHashes map[uint64]uint64
 
 	samples       []RefSample
+	highTimestamp int64
 }
 
 type hashedLabels struct {
@@ -513,6 +520,10 @@ func (a *headAppender) AddFast(ref string, t int64, v float64) error {
 		}
 	}
 
+	if t > a.highTimestamp {
+		a.highTimestamp = t
+	}
+
 	a.samples = append(a.samples, RefSample{
 		Ref: refn,
 		T:   t,
@@ -593,6 +604,16 @@ func (a *headAppender) Commit() error {
 	atomic.AddUint64(&a.meta.Stats.NumSamples, total)
 	atomic.AddUint64(&a.meta.Stats.NumSeries, uint64(len(a.newSeries)))
 
+	for {
+		ht := a.HeadBlock.HighTimestamp()
+		if a.highTimestamp <= ht {
+			break
+		}
+		if atomic.CompareAndSwapInt64(&a.HeadBlock.highTimestamp, ht, a.highTimestamp) {
+			break
+		}
+	}
+
 	return nil
 }
 

querier.go

@@ -38,7 +38,7 @@ type Querier interface {
 	Close() error
 }
 
-// Series represents a single time series.
+// Series exposes a single time series.
 type Series interface {
 	// Labels returns the complete set of labels identifying the series.
 	Labels() labels.Labels