New `needle_map.CompactMap()` implementation for reduced memory usage (#6842)

* Rework `needle_map.CompactMap()` to maximize memory efficiency.

* Use a memory-efficient structure for `CompactMap` needle value entries.

This slightly complicates the code, but makes a **massive** difference
in memory efficiency - preliminary results show a ~30% reduction in
heap usage, with no measurable performance impact otherwise.

* Clean up type for `CompactMap` chunk IDs.

* Add a small comment description for `CompactMap()`.

* Add the old version of `CompactMap()` for comparison purposes.

3 files changed, 667 insertions, 0 deletions

diff --git a/weed/storage/needle_map/old/compact_map.go b/weed/storage/needle_map/old/compact_map.go
new file mode 100644
index 000000000..ca9892b0f
--- /dev/null
+++ b/weed/storage/needle_map/old/compact_map.go
@@ -0,0 +1,332 @@
+package needle_map
+
+import (
+	"sort"
+	"sync"
+
+	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
+
+	new_map "github.com/seaweedfs/seaweedfs/weed/storage/needle_map"
+)
+
+const (
+	MaxSectionBucketSize = 1024 * 8
+	LookBackWindowSize   = 1024 // how many entries to look back when inserting into a section
+)
+
+type SectionalNeedleId uint32
+
+const SectionalNeedleIdLimit = 1<<32 - 1
+
+type SectionalNeedleValue struct {
+	Key          SectionalNeedleId
+	OffsetLower  OffsetLower `comment:"Volume offset"` //since aligned to 8 bytes, range is 4G*8=32G
+	Size         Size        `comment:"Size of the data portion"`
+	OffsetHigher OffsetHigher
+}
+
+type CompactSection struct {
+	sync.RWMutex
+	values   []SectionalNeedleValue
+	overflow Overflow
+	start    NeedleId
+	end      NeedleId
+}
+
+type Overflow []SectionalNeedleValue
+
+func NewCompactSection(start NeedleId) *CompactSection {
+	return &CompactSection{
+		values:   make([]SectionalNeedleValue, 0),
+		overflow: Overflow(make([]SectionalNeedleValue, 0)),
+		start:    start,
+	}
+}
+
+// return old entry size
+func (cs *CompactSection) Set(key NeedleId, offset Offset, size Size) (oldOffset Offset, oldSize Size) {
+	cs.Lock()
+	defer cs.Unlock()
+
+	if key > cs.end {
+		cs.end = key
+	}
+	skey := SectionalNeedleId(key - cs.start)
+	if i := cs.binarySearchValues(skey); i >= 0 {
+		// update
+		oldOffset.OffsetHigher, oldOffset.OffsetLower, oldSize = cs.values[i].OffsetHigher, cs.values[i].OffsetLower, cs.values[i].Size
+		cs.values[i].OffsetHigher, cs.values[i].OffsetLower, cs.values[i].Size = offset.OffsetHigher, offset.OffsetLower, size
+		return
+	}
+
+	var lkey SectionalNeedleId
+	if len(cs.values) > 0 {
+		lkey = cs.values[len(cs.values)-1].Key
+	}
+
+	hasAdded := false
+	switch {
+	case len(cs.values) < MaxSectionBucketSize && lkey <= skey:
+		// non-overflow insert
+		cs.values = append(cs.values, SectionalNeedleValue{
+			Key:          skey,
+			OffsetLower:  offset.OffsetLower,
+			Size:         size,
+			OffsetHigher: offset.OffsetHigher,
+		})
+		hasAdded = true
+	case len(cs.values) < MaxSectionBucketSize:
+		// still has capacity and only partially out of order
+		lookBackIndex := len(cs.values) - LookBackWindowSize
+		if lookBackIndex < 0 {
+			lookBackIndex = 0
+		}
+		if cs.values[lookBackIndex].Key <= skey {
+			for ; lookBackIndex < len(cs.values); lookBackIndex++ {
+				if cs.values[lookBackIndex].Key >= skey {
+					break
+				}
+			}
+			cs.values = append(cs.values, SectionalNeedleValue{})
+			copy(cs.values[lookBackIndex+1:], cs.values[lookBackIndex:])
+			cs.values[lookBackIndex].Key, cs.values[lookBackIndex].Size = skey, size
+			cs.values[lookBackIndex].OffsetLower, cs.values[lookBackIndex].OffsetHigher = offset.OffsetLower, offset.OffsetHigher
+			hasAdded = true
+		}
+	}
+
+	// overflow insert
+	if !hasAdded {
+		if oldValue, found := cs.findOverflowEntry(skey); found {
+			oldOffset.OffsetHigher, oldOffset.OffsetLower, oldSize = oldValue.OffsetHigher, oldValue.OffsetLower, oldValue.Size
+		}
+		cs.setOverflowEntry(skey, offset, size)
+	} else {
+		// if we maxed out our values bucket, pin its capacity to minimize memory usage
+		if len(cs.values) == MaxSectionBucketSize {
+			bucket := make([]SectionalNeedleValue, len(cs.values))
+			copy(bucket, cs.values)
+			cs.values = bucket
+		}
+	}
+
+	return
+}
+
+func (cs *CompactSection) setOverflowEntry(skey SectionalNeedleId, offset Offset, size Size) {
+	needleValue := SectionalNeedleValue{Key: skey, OffsetLower: offset.OffsetLower, Size: size, OffsetHigher: offset.OffsetHigher}
+	insertCandidate := sort.Search(len(cs.overflow), func(i int) bool {
+		return cs.overflow[i].Key >= needleValue.Key
+	})
+
+	if insertCandidate != len(cs.overflow) && cs.overflow[insertCandidate].Key == needleValue.Key {
+		cs.overflow[insertCandidate] = needleValue
+		return
+	}
+
+	cs.overflow = append(cs.overflow, SectionalNeedleValue{})
+	copy(cs.overflow[insertCandidate+1:], cs.overflow[insertCandidate:])
+	cs.overflow[insertCandidate] = needleValue
+}
+
+func (cs *CompactSection) findOverflowEntry(key SectionalNeedleId) (nv SectionalNeedleValue, found bool) {
+	foundCandidate := sort.Search(len(cs.overflow), func(i int) bool {
+		return cs.overflow[i].Key >= key
+	})
+	if foundCandidate != len(cs.overflow) && cs.overflow[foundCandidate].Key == key {
+		return cs.overflow[foundCandidate], true
+	}
+	return nv, false
+}
+
+func (cs *CompactSection) deleteOverflowEntry(key SectionalNeedleId) {
+	length := len(cs.overflow)
+	deleteCandidate := sort.Search(length, func(i int) bool {
+		return cs.overflow[i].Key >= key
+	})
+	if deleteCandidate != length && cs.overflow[deleteCandidate].Key == key {
+		if cs.overflow[deleteCandidate].Size.IsValid() {
+			cs.overflow[deleteCandidate].Size = -cs.overflow[deleteCandidate].Size
+		}
+	}
+}
+
+// return old entry size
+func (cs *CompactSection) Delete(key NeedleId) Size {
+	cs.Lock()
+	defer cs.Unlock()
+	ret := Size(0)
+	if key > cs.end {
+		return ret
+	}
+	skey := SectionalNeedleId(key - cs.start)
+	if i := cs.binarySearchValues(skey); i >= 0 {
+		if cs.values[i].Size > 0 && cs.values[i].Size.IsValid() {
+			ret = cs.values[i].Size
+			cs.values[i].Size = -cs.values[i].Size
+		}
+	}
+	if v, found := cs.findOverflowEntry(skey); found {
+		cs.deleteOverflowEntry(skey)
+		ret = v.Size
+	}
+	return ret
+}
+func (cs *CompactSection) Get(key NeedleId) (*new_map.NeedleValue, bool) {
+	cs.RLock()
+	defer cs.RUnlock()
+	if key > cs.end {
+		return nil, false
+	}
+	skey := SectionalNeedleId(key - cs.start)
+	if v, ok := cs.findOverflowEntry(skey); ok {
+		nv := toNeedleValue(v, cs)
+		return &nv, true
+	}
+	if i := cs.binarySearchValues(skey); i >= 0 {
+		nv := toNeedleValue(cs.values[i], cs)
+		return &nv, true
+	}
+	return nil, false
+}
+func (cs *CompactSection) binarySearchValues(key SectionalNeedleId) int {
+	x := sort.Search(len(cs.values), func(i int) bool {
+		return cs.values[i].Key >= key
+	})
+	if x >= len(cs.values) {
+		return -1
+	}
+	if cs.values[x].Key > key {
+		return -2
+	}
+	return x
+}
+
+// This map assumes mostly inserting increasing keys
+// This map assumes mostly inserting increasing keys
+type CompactMap struct {
+	list []*CompactSection
+}
+
+func NewCompactMap() *CompactMap {
+	return &CompactMap{}
+}
+
+func (cm *CompactMap) Set(key NeedleId, offset Offset, size Size) (oldOffset Offset, oldSize Size) {
+	x := cm.binarySearchCompactSection(key)
+	if x < 0 || (key-cm.list[x].start) > SectionalNeedleIdLimit {
+		// println(x, "adding to existing", len(cm.list), "sections, starting", key)
+		cs := NewCompactSection(key)
+		cm.list = append(cm.list, cs)
+		x = len(cm.list) - 1
+		//keep compact section sorted by start
+		for x >= 0 {
+			if x > 0 && cm.list[x-1].start > key {
+				cm.list[x] = cm.list[x-1]
+				// println("shift", x, "start", cs.start, "to", x-1)
+				x = x - 1
+			} else {
+				cm.list[x] = cs
+				// println("cs", x, "start", cs.start)
+				break
+			}
+		}
+	}
+	// println(key, "set to section[", x, "].start", cm.list[x].start)
+	return cm.list[x].Set(key, offset, size)
+}
+func (cm *CompactMap) Delete(key NeedleId) Size {
+	x := cm.binarySearchCompactSection(key)
+	if x < 0 {
+		return Size(0)
+	}
+	return cm.list[x].Delete(key)
+}
+func (cm *CompactMap) Get(key NeedleId) (*new_map.NeedleValue, bool) {
+	x := cm.binarySearchCompactSection(key)
+	if x < 0 {
+		return nil, false
+	}
+	return cm.list[x].Get(key)
+}
+func (cm *CompactMap) binarySearchCompactSection(key NeedleId) int {
+	l, h := 0, len(cm.list)-1
+	if h < 0 {
+		return -5
+	}
+	if cm.list[h].start <= key {
+		if len(cm.list[h].values) < MaxSectionBucketSize || key <= cm.list[h].end {
+			return h
+		}
+		return -4
+	}
+	for l <= h {
+		m := (l + h) / 2
+		if key < cm.list[m].start {
+			h = m - 1
+		} else { // cm.list[m].start <= key
+			if cm.list[m+1].start <= key {
+				l = m + 1
+			} else {
+				return m
+			}
+		}
+	}
+	return -3
+}
+
+// Visit visits all entries or stop if any error when visiting
+func (cm *CompactMap) AscendingVisit(visit func(new_map.NeedleValue) error) error {
+	for _, cs := range cm.list {
+		cs.RLock()
+		var i, j int
+		for i, j = 0, 0; i < len(cs.overflow) && j < len(cs.values); {
+			if cs.overflow[i].Key < cs.values[j].Key {
+				if err := visit(toNeedleValue(cs.overflow[i], cs)); err != nil {
+					cs.RUnlock()
+					return err
+				}
+				i++
+			} else if cs.overflow[i].Key == cs.values[j].Key {
+				j++
+			} else {
+				if err := visit(toNeedleValue(cs.values[j], cs)); err != nil {
+					cs.RUnlock()
+					return err
+				}
+				j++
+			}
+		}
+		for ; i < len(cs.overflow); i++ {
+			if err := visit(toNeedleValue(cs.overflow[i], cs)); err != nil {
+				cs.RUnlock()
+				return err
+			}
+		}
+		for ; j < len(cs.values); j++ {
+			if err := visit(toNeedleValue(cs.values[j], cs)); err != nil {
+				cs.RUnlock()
+				return err
+			}
+		}
+		cs.RUnlock()
+	}
+	return nil
+}
+
+func toNeedleValue(snv SectionalNeedleValue, cs *CompactSection) new_map.NeedleValue {
+	offset := Offset{
+		OffsetHigher: snv.OffsetHigher,
+		OffsetLower:  snv.OffsetLower,
+	}
+	return new_map.NeedleValue{Key: NeedleId(snv.Key) + cs.start, Offset: offset, Size: snv.Size}
+}
+
+func toSectionalNeedleValue(nv new_map.NeedleValue, cs *CompactSection) SectionalNeedleValue {
+	return SectionalNeedleValue{
+		Key:          SectionalNeedleId(nv.Key - cs.start),
+		OffsetLower:  nv.Offset.OffsetLower,
+		Size:         nv.Size,
+		OffsetHigher: nv.Offset.OffsetHigher,
+	}
+}
diff --git a/weed/storage/needle_map/old/compact_map_perf_test.go b/weed/storage/needle_map/old/compact_map_perf_test.go
new file mode 100644
index 000000000..4728930db
--- /dev/null
+++ b/weed/storage/needle_map/old/compact_map_perf_test.go
@@ -0,0 +1,92 @@
+package needle_map
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"runtime"
+	"testing"
+	"time"
+
+	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+/*
+
+To see the memory usage:
+
+go test -run TestMemoryUsage
+The Alloc section shows the in-use memory increase for each iteration.
+
+go test -run TestMemoryUsage -memprofile=mem.out
+go tool pprof --alloc_space needle.test mem.out
+
+
+*/
+
+func TestMemoryUsage(t *testing.T) {
+
+	var maps []*CompactMap
+	totalRowCount := uint64(0)
+
+	startTime := time.Now()
+	for i := 0; i < 10; i++ {
+		indexFile, ie := os.OpenFile("../../../../test/data/sample.idx", os.O_RDWR|os.O_RDONLY, 0644)
+		if ie != nil {
+			log.Fatalln(ie)
+		}
+		m, rowCount := loadNewNeedleMap(indexFile)
+		maps = append(maps, m)
+		totalRowCount += rowCount
+
+		indexFile.Close()
+
+		PrintMemUsage(totalRowCount)
+		now := time.Now()
+		fmt.Printf("\tTaken = %v\n", now.Sub(startTime))
+		startTime = now
+	}
+
+}
+
+func loadNewNeedleMap(file *os.File) (*CompactMap, uint64) {
+	m := NewCompactMap()
+	bytes := make([]byte, NeedleMapEntrySize)
+	rowCount := uint64(0)
+	count, e := file.Read(bytes)
+	for count > 0 && e == nil {
+		for i := 0; i < count; i += NeedleMapEntrySize {
+			rowCount++
+			key := BytesToNeedleId(bytes[i : i+NeedleIdSize])
+			offset := BytesToOffset(bytes[i+NeedleIdSize : i+NeedleIdSize+OffsetSize])
+			size := BytesToSize(bytes[i+NeedleIdSize+OffsetSize : i+NeedleIdSize+OffsetSize+SizeSize])
+
+			if !offset.IsZero() {
+				m.Set(NeedleId(key), offset, size)
+			} else {
+				m.Delete(key)
+			}
+		}
+
+		count, e = file.Read(bytes)
+	}
+
+	return m, rowCount
+
+}
+
+func PrintMemUsage(totalRowCount uint64) {
+
+	runtime.GC()
+	var m runtime.MemStats
+	runtime.ReadMemStats(&m)
+	// For info on each, see: https://golang.org/pkg/runtime/#MemStats
+	fmt.Printf("Each %.02f Bytes", float64(m.Alloc)/float64(totalRowCount))
+	fmt.Printf("\tAlloc = %v MiB", bToMb(m.Alloc))
+	fmt.Printf("\tTotalAlloc = %v MiB", bToMb(m.TotalAlloc))
+	fmt.Printf("\tSys = %v MiB", bToMb(m.Sys))
+	fmt.Printf("\tNumGC = %v", m.NumGC)
+}
+func bToMb(b uint64) uint64 {
+	return b / 1024 / 1024
+}
diff --git a/weed/storage/needle_map/old/compact_map_test.go b/weed/storage/needle_map/old/compact_map_test.go
new file mode 100644
index 000000000..d2d0f1569
--- /dev/null
+++ b/weed/storage/needle_map/old/compact_map_test.go
@@ -0,0 +1,243 @@
+package needle_map
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/sequence"
+	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
+
+	new_map "github.com/seaweedfs/seaweedfs/weed/storage/needle_map"
+)
+
+func TestSnowflakeSequencer(t *testing.T) {
+	m := NewCompactMap()
+	seq, _ := sequence.NewSnowflakeSequencer("for_test", 1)
+
+	for i := 0; i < 200000; i++ {
+		id := seq.NextFileId(1)
+		oldOffset, oldSize := m.Set(NeedleId(id), ToOffset(8), 3000073)
+		if oldSize != 0 {
+			t.Errorf("id %d oldOffset %v oldSize %d", id, oldOffset, oldSize)
+		}
+	}
+
+}
+
+func TestOverflow2(t *testing.T) {
+	m := NewCompactMap()
+	_, oldSize := m.Set(NeedleId(150088), ToOffset(8), 3000073)
+	if oldSize != 0 {
+		t.Fatalf("expecting no previous data")
+	}
+	_, oldSize = m.Set(NeedleId(150088), ToOffset(8), 3000073)
+	if oldSize != 3000073 {
+		t.Fatalf("expecting previous data size is %d, not %d", 3000073, oldSize)
+	}
+	m.Set(NeedleId(150073), ToOffset(8), 3000073)
+	m.Set(NeedleId(150089), ToOffset(8), 3000073)
+	m.Set(NeedleId(150076), ToOffset(8), 3000073)
+	m.Set(NeedleId(150124), ToOffset(8), 3000073)
+	m.Set(NeedleId(150137), ToOffset(8), 3000073)
+	m.Set(NeedleId(150147), ToOffset(8), 3000073)
+	m.Set(NeedleId(150145), ToOffset(8), 3000073)
+	m.Set(NeedleId(150158), ToOffset(8), 3000073)
+	m.Set(NeedleId(150162), ToOffset(8), 3000073)
+
+	m.AscendingVisit(func(value new_map.NeedleValue) error {
+		println("needle key:", value.Key)
+		return nil
+	})
+}
+
+func TestIssue52(t *testing.T) {
+	m := NewCompactMap()
+	m.Set(NeedleId(10002), ToOffset(10002), 10002)
+	if element, ok := m.Get(NeedleId(10002)); ok {
+		fmt.Printf("key %d ok %v %d, %v, %d\n", 10002, ok, element.Key, element.Offset, element.Size)
+	}
+	m.Set(NeedleId(10001), ToOffset(10001), 10001)
+	if element, ok := m.Get(NeedleId(10002)); ok {
+		fmt.Printf("key %d ok %v %d, %v, %d\n", 10002, ok, element.Key, element.Offset, element.Size)
+	} else {
+		t.Fatal("key 10002 missing after setting 10001")
+	}
+}
+
+func TestCompactMap(t *testing.T) {
+	m := NewCompactMap()
+	for i := uint32(0); i < 100*MaxSectionBucketSize; i += 2 {
+		m.Set(NeedleId(i), ToOffset(int64(i)), Size(i))
+	}
+
+	for i := uint32(0); i < 100*MaxSectionBucketSize; i += 37 {
+		m.Delete(NeedleId(i))
+	}
+
+	for i := uint32(0); i < 10*MaxSectionBucketSize; i += 3 {
+		m.Set(NeedleId(i), ToOffset(int64(i+11)), Size(i+5))
+	}
+
+	//	for i := uint32(0); i < 100; i++ {
+	//		if v := m.Get(Key(i)); v != nil {
+	//			glog.V(4).Infoln(i, "=", v.Key, v.Offset, v.Size)
+	//		}
+	//	}
+
+	for i := uint32(0); i < 10*MaxSectionBucketSize; i++ {
+		v, ok := m.Get(NeedleId(i))
+		if i%3 == 0 {
+			if !ok {
+				t.Fatal("key", i, "missing!")
+			}
+			if v.Size != Size(i+5) {
+				t.Fatal("key", i, "size", v.Size)
+			}
+		} else if i%37 == 0 {
+			if ok && v.Size.IsValid() {
+				t.Fatal("key", i, "should have been deleted needle value", v)
+			}
+		} else if i%2 == 0 {
+			if v.Size != Size(i) {
+				t.Fatal("key", i, "size", v.Size)
+			}
+		}
+	}
+
+	for i := uint32(10 * MaxSectionBucketSize); i < 100*MaxSectionBucketSize; i++ {
+		v, ok := m.Get(NeedleId(i))
+		if i%37 == 0 {
+			if ok && v.Size.IsValid() {
+				t.Fatal("key", i, "should have been deleted needle value", v)
+			}
+		} else if i%2 == 0 {
+			if v == nil {
+				t.Fatal("key", i, "missing")
+			}
+			if v.Size != Size(i) {
+				t.Fatal("key", i, "size", v.Size)
+			}
+		}
+	}
+
+}
+
+func TestOverflow(t *testing.T) {
+	cs := NewCompactSection(1)
+
+	cs.setOverflowEntry(1, ToOffset(12), 12)
+	cs.setOverflowEntry(2, ToOffset(12), 12)
+	cs.setOverflowEntry(3, ToOffset(12), 12)
+	cs.setOverflowEntry(4, ToOffset(12), 12)
+	cs.setOverflowEntry(5, ToOffset(12), 12)
+
+	if cs.overflow[2].Key != 3 {
+		t.Fatalf("expecting o[2] has key 3: %+v", cs.overflow[2].Key)
+	}
+
+	cs.setOverflowEntry(3, ToOffset(24), 24)
+
+	if cs.overflow[2].Key != 3 {
+		t.Fatalf("expecting o[2] has key 3: %+v", cs.overflow[2].Key)
+	}
+
+	if cs.overflow[2].Size != 24 {
+		t.Fatalf("expecting o[2] has size 24: %+v", cs.overflow[2].Size)
+	}
+
+	cs.deleteOverflowEntry(4)
+
+	if len(cs.overflow) != 5 {
+		t.Fatalf("expecting 5 entries now: %+v", cs.overflow)
+	}
+
+	x, _ := cs.findOverflowEntry(5)
+	if x.Key != 5 {
+		t.Fatalf("expecting entry 5 now: %+v", x)
+	}
+
+	for i, x := range cs.overflow {
+		println("overflow[", i, "]:", x.Key)
+	}
+	println()
+
+	cs.deleteOverflowEntry(1)
+
+	for i, x := range cs.overflow {
+		println("overflow[", i, "]:", x.Key, "size", x.Size)
+	}
+	println()
+
+	cs.setOverflowEntry(4, ToOffset(44), 44)
+	for i, x := range cs.overflow {
+		println("overflow[", i, "]:", x.Key)
+	}
+	println()
+
+	cs.setOverflowEntry(1, ToOffset(11), 11)
+
+	for i, x := range cs.overflow {
+		println("overflow[", i, "]:", x.Key)
+	}
+	println()
+
+}
+
+func TestCompactSection_Get(t *testing.T) {
+	var maps []*CompactMap
+	totalRowCount := uint64(0)
+	indexFile, ie := os.OpenFile("../../../../test/data/sample.idx",
+		os.O_RDWR|os.O_RDONLY, 0644)
+	defer indexFile.Close()
+	if ie != nil {
+		log.Fatalln(ie)
+	}
+
+	m, rowCount := loadNewNeedleMap(indexFile)
+	maps = append(maps, m)
+	totalRowCount += rowCount
+	m.Set(1574318345753513987, ToOffset(10002), 10002)
+	nv, ok := m.Get(1574318345753513987)
+	if ok {
+		t.Log(uint64(nv.Key))
+	}
+
+	nv1, ok := m.Get(1574318350048481283)
+	if ok {
+		t.Error(uint64(nv1.Key))
+	}
+
+	m.Set(1574318350048481283, ToOffset(10002), 10002)
+	nv2, ok1 := m.Get(1574318350048481283)
+	if ok1 {
+		t.Log(uint64(nv2.Key))
+	}
+
+	m.Delete(nv2.Key)
+	nv3, has := m.Get(nv2.Key)
+	if has && nv3.Size > 0 {
+		t.Error(uint64(nv3.Size))
+	}
+}
+
+// Test after putting 1 ~ LookBackWindowSize*3 items in sequential order, but missing item LookBackWindowSize
+// insert the item LookBackWindowSize in the middle of the sequence
+func TestCompactSection_PutOutOfOrderItemBeyondLookBackWindow(t *testing.T) {
+	m := NewCompactMap()
+
+	// put 1 ~ 10
+	for i := 1; i <= LookBackWindowSize*3; i++ {
+		if i != LookBackWindowSize {
+			m.Set(NeedleId(i), ToOffset(int64(i)), Size(i))
+		}
+	}
+
+	m.Set(NeedleId(LookBackWindowSize), ToOffset(int64(LookBackWindowSize)), Size(LookBackWindowSize))
+
+	// check if 8 is in the right place
+	if v, ok := m.Get(NeedleId(LookBackWindowSize)); !ok || v.Offset != ToOffset(LookBackWindowSize) || v.Size != Size(LookBackWindowSize) {
+		t.Fatalf("expected to find LookBackWindowSize at offset %d with size %d, but got %v", LookBackWindowSize, LookBackWindowSize, v)
+	}
+}