Only send WAL read workers the samples they need.

Calculating the modulus in each worker was a hotspot,
and meant that you had more work to do the more cores you had.
This cuts CPU usage (on my 8 core, 4 real core machine) by
33%, and walltime by 3%

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

head.go

@@ -237,7 +237,6 @@ func NewHead(r prometheus.Registerer, l log.Logger, wal *wal.WAL, chunkRange int
 // Samples before the mint timestamp are discarded.
 func (h *Head) processWALSamples(
 	minValidTime int64,
-	partition, total uint64,
 	input <-chan []RefSample, output chan<- []RefSample,
 ) (unknownRefs uint64) {
 	defer close(output)
@@ -249,7 +248,7 @@ func (h *Head) processWALSamples(
 
 	for samples := range input {
 		for _, s := range samples {
-			if s.T < minValidTime || s.Ref%total != partition {
+			if s.T < minValidTime {
 				continue
 			}
 			ms := refSeries[s.Ref]
@@ -317,25 +316,22 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 	// They are connected through a ring of channels which ensures that all sample batches
 	// read from the WAL are processed in order.
 	var (
-		wg         sync.WaitGroup
-		n          = runtime.GOMAXPROCS(0)
-		firstInput = make(chan []RefSample, 300)
-		input      = firstInput
+		wg      sync.WaitGroup
+		n       = runtime.GOMAXPROCS(0)
+		inputs  = make([]chan []RefSample, n)
+		outputs = make([]chan []RefSample, n)
 	)
 	wg.Add(n)
 
 	for i := 0; i < n; i++ {
-		output := make(chan []RefSample, 300)
+		outputs[i] = make(chan []RefSample, 300)
+		inputs[i] = make(chan []RefSample, 300)
 
-		go func(i int, input <-chan []RefSample, output chan<- []RefSample) {
-			unknown := h.processWALSamples(minValidTime, uint64(i), uint64(n), input, output)
+		go func(input <-chan []RefSample, output chan<- []RefSample) {
+			unknown := h.processWALSamples(minValidTime, input, output)
 			atomic.AddUint64(&unknownRefs, unknown)
 			wg.Done()
-		}(i, input, output)
-
-		// The output feeds the next worker goroutine. For the last worker,
-		// it feeds the initial input again to reuse the RefSample slices.
-		input = output
+		}(inputs[i], outputs[i])
 	}
 
 	var (
@@ -373,17 +369,27 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 			// cause thousands of very large in flight buffers occupying large amounts
 			// of unused memory.
 			for len(samples) > 0 {
-				n := 5000
-				if len(samples) < n {
-					n = len(samples)
+				m := 5000
+				if len(samples) < m {
+					m = len(samples)
 				}
-				var buf []RefSample
-				select {
-				case buf = <-input:
-				default:
+				shards := make([][]RefSample, n)
+				for i := 0; i < n; i++ {
+					var buf []RefSample
+					select {
+					case buf = <-outputs[i]:
+					default:
+					}
+					shards[i] = buf[:0]
 				}
-				firstInput <- append(buf[:0], samples[:n]...)
-				samples = samples[n:]
+				for _, sam := range samples[:m] {
+					mod := sam.Ref % uint64(n)
+					shards[mod] = append(shards[mod], sam)
+				}
+				for i := 0; i < n; i++ {
+					inputs[i] <- shards[i]
+				}
+				samples = samples[m:]
 			}
 			samples = s // Keep whole slice for reuse.
 		case RecordTombstones:
@@ -407,9 +413,11 @@ func (h *Head) loadWAL(r *wal.Reader) error {
 		return errors.Wrap(r.Err(), "read records")
 	}
 
-	// Signal termination to first worker and wait for last one to close its output channel.
-	close(firstInput)
-	for range input {
+	// Signal termination to each worker and wait for it to close its output channel.
+	for i := 0; i < n; i++ {
+		close(inputs[i])
+		for range outputs[i] {
+		}
 	}
 	wg.Wait()