Reduce memory taken up by posting/symbol tables.

Reuse the string already allocated for symbols
in the posting tables.

Use a slice for symbols in v2 format.

Move symbol size logic into the index code.
Avoid duplication of lookupSymbol logic.

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

block.go

@@ -15,7 +15,6 @@
 package tsdb
 
 import (
-	"encoding/binary"
 	"encoding/json"
 	"io/ioutil"
 	"os"
@@ -278,23 +277,13 @@ func OpenBlock(dir string, pool chunkenc.Pool) (*Block, error) {
 		return nil, err
 	}
 
-	// Calculating symbol table size.
-	tmp := make([]byte, 8)
-	symTblSize := uint64(0)
-	for _, v := range ir.SymbolTable() {
-		// Size of varint length of the symbol.
-		symTblSize += uint64(binary.PutUvarint(tmp, uint64(len(v))))
-		// Size of the symbol.
-		symTblSize += uint64(len(v))
-	}
-
 	pb := &Block{
 		dir:             dir,
 		meta:            *meta,
 		chunkr:          cr,
 		indexr:          ir,
 		tombstones:      tr,
-		symbolTableSize: symTblSize,
+		symbolTableSize: ir.SymbolTableSize(),
 	}
 	return pb, nil
 }

index/index.go

@@ -549,8 +549,10 @@ type Reader struct {
 	// block are always backed by true strings held in here rather than
 	// strings that are backed by byte slices from the mmap'd index file. This
 	// prevents memory faults when applications work with read symbols after
-	// the block has been unmapped.
-	symbols map[uint32]string
+	// the block has been unmapped. The older format has sparse indexes so a map
+	// must be used, but the new format is not so we can use a slice.
+	symbols     map[uint32]string
+	symbolSlice []string
 
 	dec *Decoder
 
@@ -631,11 +633,21 @@ func newReader(b ByteSlice, c io.Closer) (*Reader, error) {
 	}
 	var err error
 
+	// Use the strings already allocated by symbols, rather than
+	// re-allocating them again below.
+	symbols := make(map[string]string, len(r.symbols)+len(r.symbolSlice))
+	for _, s := range r.symbols {
+		symbols[s] = s
+	}
+	for _, s := range r.symbolSlice {
+		symbols[s] = s
+	}
+
 	err = r.readOffsetTable(r.toc.labelIndicesTable, func(key []string, off uint64) error {
 		if len(key) != 1 {
 			return errors.Errorf("unexpected key length %d", len(key))
 		}
-		r.labels[key[0]] = off
+		r.labels[symbols[key[0]]] = off
 		return nil
 	})
 	if err != nil {
@@ -645,14 +657,14 @@ func newReader(b ByteSlice, c io.Closer) (*Reader, error) {
 		if len(key) != 2 {
 			return errors.Errorf("unexpected key length %d", len(key))
 		}
-		r.postings[labels.Label{Name: key[0], Value: key[1]}] = off
+		r.postings[labels.Label{Name: symbols[key[0]], Value: symbols[key[1]]}] = off
 		return nil
 	})
 	if err != nil {
 		return nil, errors.Wrap(err, "read postings table")
 	}
 
-	r.dec = &Decoder{symbols: r.symbols}
+	r.dec = &Decoder{lookupSymbol: r.lookupSymbol}
 
 	return r, nil
 }
@@ -777,18 +789,17 @@ func (r *Reader) readSymbols(off int) error {
 		basePos = uint32(off) + 4
 		nextPos = basePos + uint32(origLen-d.len())
 	)
-
 	if r.version == 2 {
-		nextPos = 0
+		r.symbolSlice = make([]string, 0, cnt)
 	}
 
 	for d.err() == nil && d.len() > 0 && cnt > 0 {
 		s := d.uvarintStr()
-		r.symbols[nextPos] = s
 
 		if r.version == 2 {
-			nextPos++
+			r.symbolSlice = append(r.symbolSlice, s)
 		} else {
+			r.symbols[nextPos] = s
 			nextPos = basePos + uint32(origLen-d.len())
 		}
 		cnt--
@@ -828,6 +839,9 @@ func (r *Reader) Close() error {
 }
 
 func (r *Reader) lookupSymbol(o uint32) (string, error) {
+	if int(o) < len(r.symbolSlice) {
+		return r.symbolSlice[o], nil
+	}
 	s, ok := r.symbols[o]
 	if !ok {
 		return "", errors.Errorf("unknown symbol offset %d", o)
@@ -842,12 +856,22 @@ func (r *Reader) Symbols() (map[string]struct{}, error) {
 	for _, s := range r.symbols {
 		res[s] = struct{}{}
 	}
+	for _, s := range r.symbolSlice {
+		res[s] = struct{}{}
+	}
 	return res, nil
 }
 
 // SymbolTable returns the symbol table that is used to resolve symbol references.
-func (r *Reader) SymbolTable() map[uint32]string {
-	return r.symbols
+func (r *Reader) SymbolTableSize() uint64 {
+	var size int
+	for _, s := range r.symbols {
+		size += len(s) + 8
+	}
+	for _, s := range r.symbolSlice {
+		size += len(s) + 8
+	}
+	return uint64(size)
 }
 
 // LabelValues returns value tuples that exist for the given label name tuples.
@@ -1031,21 +1055,7 @@ func (t *serializedStringTuples) At(i int) ([]string, error) {
 // It currently does not contain decoding methods for all entry types but can be extended
 // by them if there's demand.
 type Decoder struct {
-	symbols map[uint32]string
-}
-
-func (dec *Decoder) lookupSymbol(o uint32) (string, error) {
-	s, ok := dec.symbols[o]
-	if !ok {
-		return "", errors.Errorf("unknown symbol offset %d", o)
-	}
-	return s, nil
-}
-
-// SetSymbolTable set the symbol table to be used for lookups when decoding series
-// and label indices
-func (dec *Decoder) SetSymbolTable(t map[uint32]string) {
-	dec.symbols = t
+	lookupSymbol func(uint32) (string, error)
 }
 
 // Postings returns a postings list for b and its number of elements.