Switch to sync.Pool.

sync.Pool is after all faster than the home-made free list. Especially
under contention. To prove that, this commit also adds a benchmark for
concurrent fingerprint calculation.

Benchmark results:
(Run with -cpu=1,2,4. x-y -> x goroutines with GOMAXPROCS=y.)

benchmark                                     old ns/op     new ns/op     delta
BenchmarkMetricToFingerprintTripleConc4-4     320           138           -56.88%
BenchmarkMetricToFingerprintTripleConc8-4     314           141           -55.10%
BenchmarkMetricToFingerprintTripleConc4-2     344           264           -23.26%
BenchmarkMetricToFingerprintTripleConc2-4     331           256           -22.66%
BenchmarkMetricToFingerprintTripleConc8-2     338           263           -22.19%
BenchmarkMetricToFingerprintTripleConc1-4     599           505           -15.69%
BenchmarkMetricToFingerprintTripleConc4       553           493           -10.85%
BenchmarkMetricToFingerprintTripleConc2-2     327           292           -10.70%
BenchmarkMetricToFingerprintTripleConc8       554           496           -10.47%
BenchmarkMetricToFingerprintTripleConc2       555           501           -9.73%
BenchmarkMetricToFingerprintTripleConc1       554           509           -8.12%
BenchmarkMetricToFingerprintTripleConc1-2     551           513           -6.90%

model/signature.go

@@ -17,6 +17,7 @@ import (
 	"bytes"
 	"hash"
 	"hash/fnv"
+	"sync"
 )
 
 // SeparatorByte is a byte that cannot occur in valid UTF-8 sequences and is
@@ -28,7 +29,7 @@ var (
 	// cache the signature of an empty label set.
 	emptyLabelSignature = fnv.New64a().Sum64()
 
-	hashAndBufPool = make(chan *hashAndBuf, 1024)
+	hashAndBufPool sync.Pool
 )
 
 type hashAndBuf struct {
@@ -37,19 +38,15 @@ type hashAndBuf struct {
 }
 
 func getHashAndBuf() *hashAndBuf {
-	select {
-	case hb := <-hashAndBufPool:
-		return hb
-	default:
+	hb := hashAndBufPool.Get()
+	if hb == nil {
 		return &hashAndBuf{h: fnv.New64a()}
 	}
+	return hb.(*hashAndBuf)
 }
 
 func putHashAndBuf(hb *hashAndBuf) {
-	select {
-	case hashAndBufPool <- hb:
-	default:
-	}
+	hashAndBufPool.Put(hb)
 }
 
 // LabelsToSignature returns a unique signature (i.e., fingerprint) for a given

model/signature_test.go

@@ -15,6 +15,7 @@ package model
 
 import (
 	"runtime"
+	"sync"
 	"testing"
 )
 
@@ -216,3 +217,40 @@ func TestEmptyLabelSignature(t *testing.T) {
 		t.Fatal("expected LabelsToSignature with empty labels not to perform allocations")
 	}
 }
+
+func benchmarkMetricToFingerprintConc(b *testing.B, m Metric, e Fingerprint, concLevel int) {
+	var start, end sync.WaitGroup
+	start.Add(1)
+	end.Add(concLevel)
+
+	for i := 0; i < concLevel; i++ {
+		go func() {
+			start.Wait()
+			for j := b.N / concLevel; j >= 0; j-- {
+				if a := metricToFingerprint(m); a != e {
+					b.Fatalf("expected signature of %d for %s, got %d", e, m, a)
+				}
+			}
+			end.Done()
+		}()
+	}
+	b.ResetTimer()
+	start.Done()
+	end.Wait()
+}
+
+func BenchmarkMetricToFingerprintTripleConc1(b *testing.B) {
+	benchmarkMetricToFingerprintConc(b, Metric{"first-label": "first-label-value", "second-label": "second-label-value", "third-label": "third-label-value"}, 15738406913934009676, 1)
+}
+
+func BenchmarkMetricToFingerprintTripleConc2(b *testing.B) {
+	benchmarkMetricToFingerprintConc(b, Metric{"first-label": "first-label-value", "second-label": "second-label-value", "third-label": "third-label-value"}, 15738406913934009676, 2)
+}
+
+func BenchmarkMetricToFingerprintTripleConc4(b *testing.B) {
+	benchmarkMetricToFingerprintConc(b, Metric{"first-label": "first-label-value", "second-label": "second-label-value", "third-label": "third-label-value"}, 15738406913934009676, 4)
+}
+
+func BenchmarkMetricToFingerprintTripleConc8(b *testing.B) {
+	benchmarkMetricToFingerprintConc(b, Metric{"first-label": "first-label-value", "second-label": "second-label-value", "third-label": "third-label-value"}, 15738406913934009676, 8)
+}