Load postings in batch on startup

This allows to insert IDs to postings out of order until
a trigger function is called. This avoids the insertion sort we usually
do which can be very costly since WAL entries are more out of order than
regular adds.

head.go

@@ -178,7 +178,7 @@ func NewHead(r prometheus.Registerer, l log.Logger, wal WAL, chunkRange int64) (
 		series:     newStripeSeries(),
 		values:     map[string]stringset{},
 		symbols:    map[string]struct{}{},
-		postings:   newMemPostings(),
+		postings:   newUnorderedMemPostings(),
 		tombstones: newEmptyTombstoneReader(),
 	}
 	h.metrics = newHeadMetrics(h, r)
@@ -188,6 +188,8 @@ func NewHead(r prometheus.Registerer, l log.Logger, wal WAL, chunkRange int64) (
 
 // ReadWAL initializes the head by consuming the write ahead log.
 func (h *Head) ReadWAL() error {
+	defer h.postings.ensureOrder()
+
 	r := h.wal.Reader()
 	mint := h.MinTime()
 

postings.go

@@ -15,6 +15,7 @@ package tsdb
 
 import (
 	"encoding/binary"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -22,14 +23,30 @@ import (
 	"github.com/prometheus/tsdb/labels"
 )
 
+// memPostings holds postings list for series ID per label pair. They may be written
+// to out of order.
+// ensureOrder() must be called once before any reads are done. This allows for quick
+// unordered batch fills on startup.
 type memPostings struct {
-	mtx sync.RWMutex
-	m   map[labels.Label][]uint64
+	mtx     sync.RWMutex
+	m       map[labels.Label][]uint64
+	ordered bool
 }
 
+// newMemPoistings returns a memPostings that's ready for reads and writes.
 func newMemPostings() *memPostings {
 	return &memPostings{
-		m: make(map[labels.Label][]uint64, 512),
+		m:       make(map[labels.Label][]uint64, 512),
+		ordered: true,
+	}
+}
+
+// newUnorderedMemPostings returns a memPostings that is not safe to be read from
+// until ensureOrder was called once.
+func newUnorderedMemPostings() *memPostings {
+	return &memPostings{
+		m:       make(map[labels.Label][]uint64, 512),
+		ordered: false,
 	}
 }
 
@@ -47,6 +64,40 @@ func (p *memPostings) get(name, value string) Postings {
 
 var allPostingsKey = labels.Label{}
 
+// ensurePostings ensures that all postings lists are sorted. After it returns all further
+// calls to add and addFor will insert new IDs in a sorted manner.
+func (p *memPostings) ensureOrder() {
+	p.mtx.Lock()
+	defer p.mtx.Unlock()
+
+	if p.ordered {
+		return
+	}
+
+	n := runtime.GOMAXPROCS(0)
+	workc := make(chan []uint64)
+
+	var wg sync.WaitGroup
+	wg.Add(n)
+
+	for i := 0; i < n; i++ {
+		go func() {
+			for l := range workc {
+				sort.Slice(l, func(i, j int) bool { return l[i] < l[j] })
+			}
+			wg.Done()
+		}()
+	}
+
+	for _, l := range p.m {
+		workc <- l
+	}
+	close(workc)
+	wg.Wait()
+
+	p.ordered = true
+}
+
 // add adds a document to the index. The caller has to ensure that no
 // term argument appears twice.
 func (p *memPostings) add(id uint64, lset labels.Labels) {
@@ -64,6 +115,9 @@ func (p *memPostings) addFor(id uint64, l labels.Label) {
 	list := append(p.m[l], id)
 	p.m[l] = list
 
+	if !p.ordered {
+		return
+	}
 	// There is no guarantee that no higher ID was inserted before as they may
 	// be generated independently before adding them to postings.
 	// We repair order violations on insert. The invariant is that the first n-1

postings_test.go

@@ -15,9 +15,12 @@ package tsdb
 
 import (
 	"encoding/binary"
+	"fmt"
 	"math/rand"
+	"sort"
 	"testing"
 
+	"github.com/prometheus/tsdb/labels"
 	"github.com/stretchr/testify/require"
 )
 
@@ -30,6 +33,31 @@ func TestMemPostings_addFor(t *testing.T) {
 	require.Equal(t, []uint64{1, 2, 3, 4, 5, 6, 7, 8}, p.m[allPostingsKey])
 }
 
+func TestMemPostings_ensureOrder(t *testing.T) {
+	p := newUnorderedMemPostings()
+
+	for i := 0; i < 100; i++ {
+		l := make([]uint64, 100)
+		for j := range l {
+			l[j] = rand.Uint64()
+		}
+		v := fmt.Sprintf("%d", i)
+
+		p.m[labels.Label{"a", v}] = l
+	}
+
+	p.ensureOrder()
+
+	for _, l := range p.m {
+		ok := sort.SliceIsSorted(l, func(i, j int) bool {
+			return l[i] < l[j]
+		})
+		if !ok {
+			t.Fatalf("postings list %v is not sorted", l)
+		}
+	}
+}
+
 type mockPostings struct {
 	next  func() bool
 	seek  func(uint64) bool