Order postings lists in index file by key

Aligning postings list for similar keys close to each other improves
page cache hit rates in typical queries that select postings for
multiple label pairs with the same name.

compact.go

@@ -589,7 +589,7 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		}
 	}
 
-	for l := range postings.m {
+	for _, l := range postings.sortedKeys() {
 		if err := indexw.WritePostings(l.Name, l.Value, postings.get(l.Name, l.Value)); err != nil {
 			return errors.Wrap(err, "write postings")
 		}

postings.go

@@ -50,6 +50,25 @@ func newUnorderedMemPostings() *memPostings {
 	}
 }
 
+// sortedKeys returns a list of sorted label keys of the postings.
+func (p *memPostings) sortedKeys() []labels.Label {
+	p.mtx.RLock()
+	keys := make([]labels.Label, 0, len(p.m))
+
+	for l := range p.m {
+		keys = append(keys, l)
+	}
+	p.mtx.RUnlock()
+
+	sort.Slice(keys, func(i, j int) bool {
+		if d := strings.Compare(keys[i].Name, keys[j].Name); d != 0 {
+			return d < 0
+		}
+		return keys[i].Value < keys[j].Value
+	})
+	return keys
+}
+
 // Postings returns an iterator over the postings list for s.
 func (p *memPostings) get(name, value string) Postings {
 	p.mtx.RLock()