store: Preallocate output buffer when encoding postings.

pkg/store/bucket.go

@@ -1705,7 +1705,8 @@ func (r *bucketIndexReader) fetchPostings(keys []labels.Label) ([]index.Postings
 					// Errors from corrupted postings will be reported when postings are used.
 					compressions++
 					s := time.Now()
-					data, err := diffVarintSnappyEncode(newBigEndianPostings(pBytes[4:]))
+					bep := newBigEndianPostings(pBytes[4:])
+					data, err := diffVarintSnappyEncode(bep, bep.count())
 					compressionTime = time.Since(s)
 					if err == nil {
 						dataToCache = data
@@ -1803,6 +1804,11 @@ func (it *bigEndianPostings) Err() error {
 	return nil
 }
 
+// Returns number of remaining postings values.
+func (it *bigEndianPostings) count() int {
+	return len(it.list) / 4
+}
+
 func (r *bucketIndexReader) PreloadSeries(ids []uint64) error {
 	// Load series from cache, overwriting the list of ids to preload
 	// with the missing ones.

pkg/store/bucket_test.go

@@ -6,6 +6,7 @@ package store
 import (
 	"bytes"
 	"context"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -1922,3 +1923,21 @@ func mustMarshalAny(pb proto.Message) *types.Any {
 	}
 	return out
 }
+
+func TestBigEndianPostingsCount(t *testing.T) {
+	const count = 1000
+	raw := make([]byte, count*4)
+
+	for ix := 0; ix < count; ix++ {
+		binary.BigEndian.PutUint32(raw[4*ix:], rand.Uint32())
+	}
+
+	p := newBigEndianPostings(raw)
+	testutil.Equals(t, count, p.count())
+
+	c := 0
+	for p.Next() {
+		c++
+	}
+	testutil.Equals(t, count, c)
+}

pkg/store/postings_codec.go

@@ -33,8 +33,9 @@ func isDiffVarintSnappyEncodedPostings(input []byte) bool {
 // diffVarintSnappyEncode encodes postings into diff+varint representation,
 // and applies snappy compression on the result.
 // Returned byte slice starts with codecHeaderSnappy header.
-func diffVarintSnappyEncode(p index.Postings) ([]byte, error) {
-	buf, err := diffVarintEncodeNoHeader(p)
+// Values argument is expected number of postings, used for preallocating buffer.
+func diffVarintSnappyEncode(p index.Postings, values int) ([]byte, error) {
+	buf, err := diffVarintEncodeNoHeader(p, values)
 	if err != nil {
 		return nil, err
 	}
@@ -52,9 +53,16 @@ func diffVarintSnappyEncode(p index.Postings) ([]byte, error) {
 
 // diffVarintEncodeNoHeader encodes postings into diff+varint representation.
 // It doesn't add any header to the output bytes.
-func diffVarintEncodeNoHeader(p index.Postings) ([]byte, error) {
+// Values argument is expected number of postings, used for preallocating buffer.
+func diffVarintEncodeNoHeader(p index.Postings, values int) ([]byte, error) {
 	buf := encoding.Encbuf{}
 
+	// This encoding uses around ~1 bytes per posting, but let's use
+	// conservative 1.25 bytes per posting to avoid extra allocations.
+	if values > 0 {
+		buf.B = make([]byte, 0, 5*values/4)
+	}
+
 	prev := uint64(0)
 	for p.Next() {
 		v := p.At()

pkg/store/postings_codec_test.go

@@ -5,7 +5,10 @@ package store
 
 import (
 	"io/ioutil"
+	"math"
+	"math/rand"
 	"os"
+	"strconv"
 	"testing"
 
 	"github.com/prometheus/prometheus/pkg/labels"
@@ -49,7 +52,7 @@ func TestDiffVarintCodec(t *testing.T) {
 	}
 
 	codecs := map[string]struct {
-		codingFunction   func(index.Postings) ([]byte, error)
+		codingFunction   func(index.Postings, int) ([]byte, error)
 		decodingFunction func([]byte) (index.Postings, error)
 	}{
 		"raw":    {codingFunction: diffVarintEncodeNoHeader, decodingFunction: func(bytes []byte) (index.Postings, error) { return newDiffVarintPostings(bytes), nil }},
@@ -68,11 +71,11 @@ func TestDiffVarintCodec(t *testing.T) {
 				t.Log("original size (4*entries):", 4*p.len(), "bytes")
 				p.reset() // We reuse postings between runs, so we need to reset iterator.
 
-				data, err := codec.codingFunction(p)
+				data, err := codec.codingFunction(p, p.len())
 				testutil.Ok(t, err)
 
 				t.Log("encoded size", len(data), "bytes")
-				t.Logf("ratio: %0.3f", (float64(len(data)) / float64(4*p.len())))
+				t.Logf("ratio: %0.3f", float64(len(data))/float64(4*p.len()))
 
 				decodedPostings, err := codec.decodingFunction(data)
 				testutil.Ok(t, err)
@@ -188,3 +191,31 @@ func (p *uint64Postings) reset() {
 func (p *uint64Postings) len() int {
 	return len(p.vals)
 }
+
+func BenchmarkEncodePostings(b *testing.B) {
+	const max = 1000000
+	r := rand.New(rand.NewSource(0))
+
+	p := make([]uint64, max)
+
+	for ix := 1; ix < len(p); ix++ {
+		// Use normal distribution, with stddev=64 (i.e. most values are < 64).
+		// This is very rough approximation of experiments with real blocks.v
+		d := math.Abs(r.NormFloat64()*64) + 1
+
+		p[ix] = p[ix-1] + uint64(d)
+	}
+
+	for _, count := range []int{10000, 100000, 1000000} {
+		b.Run(strconv.Itoa(count), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				ps := &uint64Postings{vals: p[:count]}
+
+				_, err := diffVarintEncodeNoHeader(ps, ps.len())
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}