index: Optimize generatePackList

name                 old time/op    new time/op    delta
EncodeIndex/100-8      1.56ms ± 2%    1.48ms ± 3%   -5.37%  (p=0.000 n=10+10)
EncodeIndex/1000-8     14.5ms ± 2%    13.1ms ± 2%   -9.49%  (p=0.000 n=9+10)
EncodeIndex/10000-8     120ms ± 2%     116ms ± 2%   -3.58%  (p=0.000 n=10+10)

name                 old alloc/op   new alloc/op   delta
EncodeIndex/100-8       306kB ± 1%     275kB ± 1%  -10.28%  (p=0.000 n=10+10)
EncodeIndex/1000-8     3.69MB ±11%    2.88MB ± 5%  -22.07%  (p=0.000 n=10+9)
EncodeIndex/10000-8    35.9MB ±11%    31.9MB ±10%  -11.13%  (p=0.005 n=10+10)

name                 old allocs/op  new allocs/op  delta
EncodeIndex/100-8       3.39k ± 0%     2.39k ± 0%  -29.61%  (p=0.000 n=10+10)
EncodeIndex/1000-8      32.6k ± 0%     22.9k ± 0%  -29.63%  (p=0.000 n=10+9)
EncodeIndex/10000-8      326k ± 0%      229k ± 0%  -29.71%  (p=0.000 n=10+10)

The bulk of the allocation rate improvement comes from just removing the
debug.Log calls: every one of those copied a restic.ID to the heap.

internal/index/index.go

@@ -317,9 +317,9 @@ type blobJSON struct {
 }
 
 // generatePackList returns a list of packs.
-func (idx *Index) generatePackList() ([]*packJSON, error) {
-	list := []*packJSON{}
-	packs := make(map[restic.ID]*packJSON)
+func (idx *Index) generatePackList() ([]packJSON, error) {
+	list := make([]packJSON, 0, len(idx.packs))
+	packs := make(map[restic.ID]int, len(list)) // Maps to index in list.
 
 	for typ := range idx.byType {
 		m := &idx.byType[typ]
@@ -329,18 +329,13 @@ func (idx *Index) generatePackList() ([]*packJSON, error) {
 				panic("null pack id")
 			}
 
-			debug.Log("handle blob %v", e.id)
-
-			// see if pack is already in map
-			p, ok := packs[packID]
+			i, ok := packs[packID]
 			if !ok {
-				// else create new pack
-				p = &packJSON{ID: packID}
-
-				// and append it to the list and map
-				list = append(list, p)
-				packs[p.ID] = p
+				i = len(list)
+				list = append(list, packJSON{ID: packID})
+				packs[packID] = i
 			}
+			p := &list[i]
 
 			// add blob
 			p.Blobs = append(p.Blobs, blobJSON{
@@ -355,14 +350,12 @@ func (idx *Index) generatePackList() ([]*packJSON, error) {
 		})
 	}
 
-	debug.Log("done")
-
 	return list, nil
 }
 
 type jsonIndex struct {
-	Supersedes restic.IDs  `json:"supersedes,omitempty"`
-	Packs      []*packJSON `json:"packs"`
+	Supersedes restic.IDs `json:"supersedes,omitempty"`
+	Packs      []packJSON `json:"packs"`
 }
 
 // Encode writes the JSON serialization of the index to the writer w.

internal/index/index_test.go

@@ -3,6 +3,7 @@ package index_test
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"math/rand"
 	"sync"
 	"testing"
@@ -405,6 +406,26 @@ func BenchmarkDecodeIndexParallel(b *testing.B) {
 	})
 }
 
+func BenchmarkEncodeIndex(b *testing.B) {
+	for _, n := range []int{100, 1000, 10000} {
+		idx, _ := createRandomIndex(rand.New(rand.NewSource(0)), n)
+
+		b.Run(fmt.Sprint(n), func(b *testing.B) {
+			buf := new(bytes.Buffer)
+			err := idx.Encode(buf)
+			rtest.OK(b, err)
+
+			b.ResetTimer()
+			b.ReportAllocs()
+
+			for i := 0; i < b.N; i++ {
+				buf.Reset()
+				_ = idx.Encode(buf)
+			}
+		})
+	}
+}
+
 func TestIndexUnserializeOld(t *testing.T) {
 	idx, oldFormat, err := index.DecodeIndex(docOldExample, restic.NewRandomID())
 	rtest.OK(t, err)