Remove symbols map from TSDB head (#9301)

This saves memory, effort and locking.

Since every symbol is also added to postings, `Symbols()` can be
implemented there instead. This now has to build a map for
deduplication, but `Symbols()` is only called for compaction, and `gc()`
used to rebuild the symbols map after every compaction so not an
additional cost.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head.go

@@ -79,9 +79,6 @@ type Head struct {
 	// All series addressable by their ID or hash.
 	series *stripeSeries
 
-	symMtx  sync.RWMutex
-	symbols map[string]struct{}
-
 	deletedMtx sync.Mutex
 	deleted    map[uint64]int // Deleted series, and what WAL segment they must be kept until.
 
@@ -223,7 +220,6 @@ func (h *Head) resetInMemoryState() error {
 	h.exemplarMetrics = em
 	h.exemplars = es
 	h.series = newStripeSeries(h.opts.StripeSize, h.opts.SeriesCallback)
-	h.symbols = map[string]struct{}{}
 	h.postings = index.NewUnorderedMemPostings()
 	h.tombstones = tombstones.NewMemTombstones()
 	h.iso = newIsolation()
@@ -1120,22 +1116,6 @@ func (h *Head) gc() int64 {
 		h.deletedMtx.Unlock()
 	}
 
-	// Rebuild symbols and label value indices from what is left in the postings terms.
-	// symMtx ensures that append of symbols and postings is disabled for rebuild time.
-	h.symMtx.Lock()
-	defer h.symMtx.Unlock()
-
-	symbols := make(map[string]struct{}, len(h.symbols))
-	if err := h.postings.Iter(func(l labels.Label, _ index.Postings) error {
-		symbols[l.Name] = struct{}{}
-		symbols[l.Value] = struct{}{}
-		return nil
-	}); err != nil {
-		// This should never happen, as the iteration function only returns nil.
-		panic(err)
-	}
-	h.symbols = symbols
-
 	return actualMint
 }
 
@@ -1234,14 +1214,6 @@ func (h *Head) getOrCreateWithID(id, hash uint64, lset labels.Labels) (*memSerie
 	h.metrics.seriesCreated.Inc()
 	h.numSeries.Inc()
 
-	h.symMtx.Lock()
-	defer h.symMtx.Unlock()
-
-	for _, l := range lset {
-		h.symbols[l.Name] = struct{}{}
-		h.symbols[l.Value] = struct{}{}
-	}
-
 	h.postings.Add(id, lset)
 	return s, true, nil
 }

tsdb/head_read.go

@@ -54,16 +54,7 @@ func (h *headIndexReader) Close() error {
 }
 
 func (h *headIndexReader) Symbols() index.StringIter {
-	h.head.symMtx.RLock()
-	res := make([]string, 0, len(h.head.symbols))
-
-	for s := range h.head.symbols {
-		res = append(res, s)
-	}
-	h.head.symMtx.RUnlock()
-
-	sort.Strings(res)
-	return index.NewStringListIter(res)
+	return h.head.postings.Symbols()
 }
 
 // SortedLabelValues returns label values present in the head for the
@@ -88,8 +79,6 @@ func (h *headIndexReader) LabelValues(name string, matchers ...*labels.Matcher)
 	}
 
 	if len(matchers) == 0 {
-		h.head.symMtx.RLock()
-		defer h.head.symMtx.RUnlock()
 		return h.head.postings.LabelValues(name), nil
 	}
 
@@ -104,10 +93,7 @@ func (h *headIndexReader) LabelNames(matchers ...*labels.Matcher) ([]string, err
 	}
 
 	if len(matchers) == 0 {
-		h.head.symMtx.RLock()
 		labelNames := h.head.postings.LabelNames()
-		h.head.symMtx.RUnlock()
-
 		sort.Strings(labelNames)
 		return labelNames, nil
 	}

tsdb/head_test.go

@@ -469,13 +469,14 @@ func TestHead_Truncate(t *testing.T) {
 	require.Nil(t, postingsB2)
 	require.Nil(t, postingsC1)
 
-	require.Equal(t, map[string]struct{}{
-		"":  {}, // from 'all' postings list
-		"a": {},
-		"b": {},
-		"1": {},
-		"2": {},
-	}, h.symbols)
+	iter := h.postings.Symbols()
+	symbols := []string{}
+	for iter.Next() {
+		symbols = append(symbols, iter.At())
+	}
+	require.Equal(t,
+		[]string{"" /* from 'all' postings list */, "1", "2", "a", "b"},
+		symbols)
 
 	values := map[string]map[string]struct{}{}
 	for _, name := range h.postings.LabelNames() {

tsdb/index/postings.go

@@ -58,6 +58,29 @@ func NewUnorderedMemPostings() *MemPostings {
 	}
 }
 
+// Symbols returns an iterator over all unique name and value strings, in order.
+func (p *MemPostings) Symbols() StringIter {
+	p.mtx.RLock()
+
+	// Add all the strings to a map to de-duplicate.
+	symbols := make(map[string]struct{}, 512)
+	for n, e := range p.m {
+		symbols[n] = struct{}{}
+		for v := range e {
+			symbols[v] = struct{}{}
+		}
+	}
+	p.mtx.RUnlock()
+
+	res := make([]string, 0, len(symbols))
+	for k := range symbols {
+		res = append(res, k)
+	}
+
+	sort.Strings(res)
+	return NewStringListIter(res)
+}
+
 // SortedKeys returns a list of sorted label keys of the postings.
 func (p *MemPostings) SortedKeys() []labels.Label {
 	p.mtx.RLock()