index: Remove pointers from within indexentrys

The indexEntry objects are now allocated in a separate array. References
to an indexEntry are now stored as array indices. This has the benefit
of allowing the garbage collector to ignore the indexEntry objects as
these do not contain pointers and are part of a single large allocation.

internal/index/indexmap.go

@@ -17,12 +17,12 @@ import (
 // needs to be resized when the table grows, preventing memory usage spikes.
 type indexMap struct {
 	// The number of buckets is always a power of two and never zero.
-	buckets    []*indexEntry
+	buckets    []uint
 	numentries uint
 
 	mh maphash.Hash
 
-	free *indexEntry // Free list.
+	blockList []indexEntry
 }
 
 const (
@@ -41,7 +41,7 @@ func (m *indexMap) add(id restic.ID, packIdx int, offset, length uint32, uncompr
 	}
 
 	h := m.hash(id)
-	e := m.newEntry()
+	e, idx := m.newEntry()
 	e.id = id
 	e.next = m.buckets[h] // Prepend to existing chain.
 	e.packIndex = packIdx
@@ -49,18 +49,19 @@ func (m *indexMap) add(id restic.ID, packIdx int, offset, length uint32, uncompr
 	e.length = length
 	e.uncompressedLength = uncompressedLength
 
-	m.buckets[h] = e
+	m.buckets[h] = idx
 	m.numentries++
 }
 
 // foreach calls fn for all entries in the map, until fn returns false.
 func (m *indexMap) foreach(fn func(*indexEntry) bool) {
-	for _, e := range m.buckets {
+	for _, ei := range m.buckets {
-		for e != nil {
+		for ei != 0 {
+			e := m.resolve(ei)
 			if !fn(e) {
 				return
 			}
-			e = e.next
+			ei = e.next
 		}
 	}
 }
@@ -72,7 +73,10 @@ func (m *indexMap) foreachWithID(id restic.ID, fn func(*indexEntry)) {
 	}
 
 	h := m.hash(id)
-	for e := m.buckets[h]; e != nil; e = e.next {
+	ei := m.buckets[h]
+	for ei != 0 {
+		e := m.resolve(ei)
+		ei = e.next
 		if e.id != id {
 			continue
 		}
@@ -87,25 +91,29 @@ func (m *indexMap) get(id restic.ID) *indexEntry {
 	}
 
 	h := m.hash(id)
-	for e := m.buckets[h]; e != nil; e = e.next {
+	ei := m.buckets[h]
+	for ei != 0 {
+		e := m.resolve(ei)
 		if e.id == id {
 			return e
 		}
+		ei = e.next
 	}
 	return nil
 }
 
 func (m *indexMap) grow() {
 	old := m.buckets
-	m.buckets = make([]*indexEntry, growthFactor*len(m.buckets))
+	m.buckets = make([]uint, growthFactor*len(m.buckets))
 
-	for _, e := range old {
+	for _, ei := range old {
-		for e != nil {
+		for ei != 0 {
+			e := m.resolve(ei)
 			h := m.hash(e.id)
 			next := e.next
 			e.next = m.buckets[h]
-			m.buckets[h] = e
+			m.buckets[h] = ei
-			e = next
+			ei = next
 		}
 	}
 }
@@ -124,45 +132,29 @@ func (m *indexMap) hash(id restic.ID) uint {
 
 func (m *indexMap) init() {
 	const initialBuckets = 64
-	m.buckets = make([]*indexEntry, initialBuckets)
+	m.buckets = make([]uint, initialBuckets)
+	// first entry in blockList serves as null byte
+	m.blockList = make([]indexEntry, 1)
 }
 
 func (m *indexMap) len() uint { return m.numentries }
 
-func (m *indexMap) newEntry() *indexEntry {
+func (m *indexMap) newEntry() (*indexEntry, uint) {
-	// We keep a free list of objects to speed up allocation and GC.
+	m.blockList = append(m.blockList, indexEntry{})
-	// There's an obvious trade-off here: allocating in larger batches
-	// means we allocate faster and the GC has to keep fewer bits to track
-	// what we have in use, but it means we waste some space.
-	//
-	// Then again, allocating each indexEntry separately also wastes space
-	// on 32-bit platforms, because the Go malloc has no size class for
-	// exactly 52 bytes, so it puts the indexEntry in a 64-byte slot instead.
-	// See src/runtime/sizeclasses.go in the Go source repo.
-	//
-	// The batch size of 4 means we hit the size classes for 4×64=256 bytes
-	// (64-bit) and 4×52=208 bytes (32-bit), wasting nothing in malloc on
-	// 64-bit and relatively little on 32-bit.
-	const entryAllocBatch = 4
 
-	e := m.free
+	idx := uint(len(m.blockList) - 1)
-	if e != nil {
+	e := &m.blockList[idx]
-		m.free = e.next
-	} else {
-		free := new([entryAllocBatch]indexEntry)
-		e = &free[0]
-		for i := 1; i < len(free)-1; i++ {
-			free[i].next = &free[i+1]
-		}
-		m.free = &free[1]
-	}
 
-	return e
+	return e, idx
+}
+
+func (m *indexMap) resolve(idx uint) *indexEntry {
+	return &m.blockList[idx]
 }
 
 type indexEntry struct {
 	id                 restic.ID
-	next               *indexEntry
+	next               uint
 	packIndex          int // Position in containing Index's packs field.
 	offset             uint32
 	length             uint32