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

weed/storage/needle_map/compact_map.go

@@ -1,330 +1,222 @@
 package needle_map
 
 import (
+	"fmt"
 	"sort"
 	"sync"
 
-	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
 )
 
 const (
-	MaxSectionBucketSize = 1024 * 8
-	LookBackWindowSize   = 1024 // how many entries to look back when inserting into a section
+	SegmentChunkSize = 25000
 )
 
-type SectionalNeedleId uint32
+type CompactMapSegment struct {
+	// TODO: maybe a compact-er structure for needle values?
+	list     []NeedleValue
+	firstKey types.NeedleId
+	lastKey  types.NeedleId
+}
 
-const SectionalNeedleIdLimit = 1<<32 - 1
+type CompactMap struct {
+	sync.RWMutex
 
-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
+	segments map[int]*CompactMapSegment
 }
 
-type CompactSection struct {
-	sync.RWMutex
-	values   []SectionalNeedleValue
-	overflow Overflow
-	start    NeedleId
-	end      NeedleId
+func newCompactMapSegment(chunk int) *CompactMapSegment {
+	startKey := types.NeedleId(chunk * SegmentChunkSize)
+	return &CompactMapSegment{
+		list:     []NeedleValue{},
+		firstKey: startKey + SegmentChunkSize - 1,
+		lastKey:  startKey,
+	}
 }
 
-type Overflow []SectionalNeedleValue
+func (cs *CompactMapSegment) len() int {
+	return len(cs.list)
+}
 
-func NewCompactSection(start NeedleId) *CompactSection {
-	return &CompactSection{
-		values:   make([]SectionalNeedleValue, 0),
-		overflow: Overflow(make([]SectionalNeedleValue, 0)),
-		start:    start,
-	}
+func (cs *CompactMapSegment) cap() int {
+	return cap(cs.list)
 }
 
-// return old entry size
-func (cs *CompactSection) Set(key NeedleId, offset Offset, size Size) (oldOffset Offset, oldSize Size) {
-	cs.Lock()
-	defer cs.Unlock()
+// bsearchKey returns the NeedleValue index for a given ID key.
+// If the key is not found, it returns the index where it should be inserted instead.
+func (cs *CompactMapSegment) bsearchKey(key types.NeedleId) (int, bool) {
+	switch {
+	case len(cs.list) == 0:
+		return 0, false
+	case key == cs.firstKey:
+		return 0, true
+	case key <= cs.firstKey:
+		return 0, false
+	case key == cs.lastKey:
+		return len(cs.list) - 1, true
+	case key > cs.lastKey:
+		return len(cs.list), false
+	}
+
+	i := sort.Search(len(cs.list), func(i int) bool {
+		return cs.list[i].Key >= key
+	})
+	return i, cs.list[i].Key == key
+}
 
-	if key > cs.end {
-		cs.end = key
-	}
-	skey := SectionalNeedleId(key - cs.start)
-	if i := cs.binarySearchValues(skey); i >= 0 {
+// set inserts/updates a NeedleValue.
+// If the operation is an update, returns the overwritten value's previous offset and size.
+func (cs *CompactMapSegment) set(key types.NeedleId, offset types.Offset, size types.Size) (oldOffset types.Offset, oldSize types.Size) {
+	i, found := cs.bsearchKey(key)
+	if found {
 		// 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
-	}
+		oldOffset.OffsetLower = cs.list[i].Offset.OffsetLower
+		oldOffset.OffsetHigher = cs.list[i].Offset.OffsetHigher
+		oldSize = cs.list[i].Size
 
-	var lkey SectionalNeedleId
-	if len(cs.values) > 0 {
-		lkey = cs.values[len(cs.values)-1].Key
+		cs.list[i].Size = size
+		cs.list[i].Offset.OffsetLower = offset.OffsetLower
+		cs.list[i].Offset.OffsetHigher = offset.OffsetHigher
+		return
 	}
 
-	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
+	// insert
+	if len(cs.list) >= SegmentChunkSize {
+		panic(fmt.Sprintf("attempted to write more than %d entries on CompactMapSegment %p!!!", SegmentChunkSize, cs))
 	}
+	if len(cs.list) == SegmentChunkSize-1 {
+		// if we max out our segment storage, pin its capacity to minimize memory usage
+		nl := make([]NeedleValue, SegmentChunkSize, SegmentChunkSize)
+		copy(nl, cs.list[:i])
+		copy(nl[i+1:], cs.list[i:])
+		cs.list = nl
+	} else {
+		cs.list = append(cs.list, NeedleValue{})
+		copy(cs.list[i+1:], cs.list[i:])
 	}
 
-	// overflow insert
-	if !hasAdded {
-		if oldValue, found := cs.findOverflowEntry(skey); found {
-			oldOffset.OffsetHigher, oldOffset.OffsetLower, oldSize = oldValue.OffsetHigher, oldValue.OffsetLower, oldValue.Size
+	cs.list[i] = NeedleValue{
+		Key:    key,
+		Offset: offset,
+		Size:   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
+	if key < cs.firstKey {
+		cs.firstKey = key
 	}
+	if key > cs.lastKey {
+		cs.lastKey = key
 	}
 
 	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
+// get seeks a map entry by key. Returns an entry pointer, with a boolean specifiying if the entry was found.
+func (cs *CompactMapSegment) get(key types.NeedleId) (*NeedleValue, bool) {
+	if i, found := cs.bsearchKey(key); found {
+		return &cs.list[i], true
 	}
 
-	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
+	return nil, 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
+// delete deletes a map entry by key. Returns the entries' previous Size, if available.
+func (cs *CompactMapSegment) delete(key types.NeedleId) types.Size {
+	if i, found := cs.bsearchKey(key); found {
+		if cs.list[i].Size > 0 && cs.list[i].Size.IsValid() {
+			ret := cs.list[i].Size
+			cs.list[i].Size = -cs.list[i].Size
+			return ret
 		}
 	}
+
+	return types.Size(0)
 }
 
-// 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
+func NewCompactMap() *CompactMap {
+	return &CompactMap{
+		segments: map[int]*CompactMapSegment{},
 	}
-	return ret
 }
-func (cs *CompactSection) Get(key NeedleId) (*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
+
+func (cm *CompactMap) Len() int {
+	l := 0
+	for _, s := range cm.segments {
+		l += s.len()
 	}
-	return x
+	return l
 }
 
-// This map assumes mostly inserting increasing keys
-// This map assumes mostly inserting increasing keys
-type CompactMap struct {
-	list []*CompactSection
+func (cm *CompactMap) Cap() int {
+	c := 0
+	for _, s := range cm.segments {
+		c += s.cap()
+	}
+	return c
 }
 
-func NewCompactMap() *CompactMap {
-	return &CompactMap{}
+func (cm *CompactMap) String() string {
+	return fmt.Sprintf(
+		"%d/%d elements on %d segments, %.02f%% efficiency",
+		cm.Len(), cm.Cap(), len(cm.segments),
+		float64(100)*float64(cm.Len())/float64(cm.Cap()))
 }
 
-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
-			}
+func (cm *CompactMap) segmentForKey(key types.NeedleId) *CompactMapSegment {
+	chunk := int(key / SegmentChunkSize)
+	if cs, ok := cm.segments[chunk]; ok {
+		return cs
 	}
-	}
-	// println(key, "set to section[", x, "].start", cm.list[x].start)
-	return cm.list[x].Set(key, offset, size)
+
+	cs := newCompactMapSegment(chunk)
+	cm.segments[chunk] = cs
+	return cs
 }
-func (cm *CompactMap) Delete(key NeedleId) Size {
-	x := cm.binarySearchCompactSection(key)
-	if x < 0 {
-		return Size(0)
-	}
-	return cm.list[x].Delete(key)
+
+// Set inserts/updates a NeedleValue.
+// If the operation is an update, returns the overwritten value's previous offset and size.
+func (cm *CompactMap) Set(key types.NeedleId, offset types.Offset, size types.Size) (oldOffset types.Offset, oldSize types.Size) {
+	cm.RLock()
+	defer cm.RUnlock()
+
+	cs := cm.segmentForKey(key)
+	return cs.set(key, offset, size)
 }
-func (cm *CompactMap) Get(key NeedleId) (*NeedleValue, bool) {
-	x := cm.binarySearchCompactSection(key)
-	if x < 0 {
-		return nil, false
-	}
-	return cm.list[x].Get(key)
+
+// Get seeks a map entry by key. Returns an entry pointer, with a boolean specifiying if the entry was found.
+func (cm *CompactMap) Get(key types.NeedleId) (*NeedleValue, bool) {
+	cm.RLock()
+	defer cm.RUnlock()
+
+	cs := cm.segmentForKey(key)
+	return cs.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
+
+// Delete deletes a map entry by key. Returns the entries' previous Size, if available.
+func (cm *CompactMap) Delete(key types.NeedleId) types.Size {
+	cm.RLock()
+	defer cm.RUnlock()
+
+	cs := cm.segmentForKey(key)
+	return cs.delete(key)
 }
 
-// Visit visits all entries or stop if any error when visiting
+// AscendingVisit runs a function on all entries, in ascending key order. Returns any errors hit while visiting.
 func (cm *CompactMap) AscendingVisit(visit func(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
-			}
+	cm.RLock()
+	defer cm.RUnlock()
+
+	chunks := []int{}
+	for c := range cm.segments {
+		chunks = append(chunks, c)
 	}
-		for ; j < len(cs.values); j++ {
-			if err := visit(toNeedleValue(cs.values[j], cs)); err != nil {
-				cs.RUnlock()
+	sort.Ints(chunks)
+
+	for _, c := range chunks {
+		for _, nv := range cm.segments[c].list {
+			if err := visit(nv); err != nil {
 				return err
 			}
 		}
-		cs.RUnlock()
 	}
 	return nil
 }
-
-func toNeedleValue(snv SectionalNeedleValue, cs *CompactSection) NeedleValue {
-	offset := Offset{
-		OffsetHigher: snv.OffsetHigher,
-		OffsetLower:  snv.OffsetLower,
-	}
-	return NeedleValue{Key: NeedleId(snv.Key) + cs.start, Offset: offset, Size: snv.Size}
-}
-
-func (nv NeedleValue) toSectionalNeedleValue(cs *CompactSection) SectionalNeedleValue {
-	return SectionalNeedleValue{
-		Key:          SectionalNeedleId(nv.Key - cs.start),
-		OffsetLower:  nv.Offset.OffsetLower,
-		Size:         nv.Size,
-		OffsetHigher: nv.Offset.OffsetHigher,
-	}
-}

weed/storage/needle_map/compact_map_perf_test.go

@@ -43,6 +43,7 @@ func TestMemoryUsage(t *testing.T) {
 
 		PrintMemUsage(totalRowCount)
 		now := time.Now()
+		fmt.Printf("\tCompactMap = %s", m.String())
 		fmt.Printf("\tTaken = %v\n", now.Sub(startTime))
 		startTime = now
 	}

weed/storage/needle_map/compact_map_test.go

@@ -2,240 +2,470 @@ package needle_map
 
 import (
 	"fmt"
-	"log"
-	"os"
+	"math/rand"
+	"reflect"
 	"testing"
 
-	"github.com/seaweedfs/seaweedfs/weed/sequence"
-	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
 )
 
-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 TestSegmentBsearchKey(t *testing.T) {
+	testSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 10},
+			NeedleValue{Key: 20},
+			NeedleValue{Key: 21},
+			NeedleValue{Key: 26},
+			NeedleValue{Key: 30},
+		},
+		firstKey: 10,
+		lastKey:  30,
+	}
+
+	testCases := []struct {
+		name      string
+		cs        *CompactMapSegment
+		key       types.NeedleId
+		wantIndex int
+		wantFound bool
+	}{
+		{
+			name:      "empty segment",
+			cs:        newCompactMapSegment(0),
+			key:       123,
+			wantIndex: 0,
+			wantFound: false,
+		},
+		{
+			name:      "new key, insert at beggining",
+			cs:        testSegment,
+			key:       5,
+			wantIndex: 0,
+			wantFound: false,
+		},
+		{
+			name:      "new key, insert at end",
+			cs:        testSegment,
+			key:       100,
+			wantIndex: 5,
+			wantFound: false,
+		},
+		{
+			name:      "new key, insert second",
+			cs:        testSegment,
+			key:       12,
+			wantIndex: 1,
+			wantFound: false,
+		},
+		{
+			name:      "new key, insert in middle",
+			cs:        testSegment,
+			key:       23,
+			wantIndex: 3,
+			wantFound: false,
+		},
+		{
+			name:      "key #1",
+			cs:        testSegment,
+			key:       10,
+			wantIndex: 0,
+			wantFound: true,
+		},
+		{
+			name:      "key #2",
+			cs:        testSegment,
+			key:       20,
+			wantIndex: 1,
+			wantFound: true,
+		},
+		{
+			name:      "key #3",
+			cs:        testSegment,
+			key:       21,
+			wantIndex: 2,
+			wantFound: true,
+		},
+		{
+			name:      "key #4",
+			cs:        testSegment,
+			key:       26,
+			wantIndex: 3,
+			wantFound: true,
+		},
+		{
+			name:      "key #5",
+			cs:        testSegment,
+			key:       30,
+			wantIndex: 4,
+			wantFound: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			index, found := tc.cs.bsearchKey(tc.key)
+			if got, want := index, tc.wantIndex; got != want {
+				t.Errorf("expected %v, got %v", want, got)
+			}
+			if got, want := found, tc.wantFound; got != want {
+				t.Errorf("expected %v, got %v", want, got)
 			}
-
-}
-
-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 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))
+func TestSegmentSet(t *testing.T) {
+	testSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 10, Offset: types.Uint32ToOffset(0), Size: 100},
+			NeedleValue{Key: 20, Offset: types.Uint32ToOffset(100), Size: 200},
+			NeedleValue{Key: 30, Offset: types.Uint32ToOffset(300), Size: 300},
+		},
+		firstKey: 10,
+		lastKey:  30,
+	}
+
+	if got, want := testSegment.len(), 3; got != want {
+		t.Errorf("got starting size %d, want %d", got, want)
+	}
+	if got, want := testSegment.cap(), 3; got != want {
+		t.Errorf("got starting capacity %d, want %d", got, want)
+	}
+
+	testSets := []struct {
+		name       string
+		key        types.NeedleId
+		offset     types.Offset
+		size       types.Size
+		wantOffset types.Offset
+		wantSize   types.Size
+	}{
+		{
+			name: "insert at beggining",
+			key:  5, offset: types.Uint32ToOffset(1000), size: 123,
+			wantOffset: types.Uint32ToOffset(0), wantSize: 0,
+		},
+		{
+			name: "insert at end",
+			key:  51, offset: types.Uint32ToOffset(7000), size: 456,
+			wantOffset: types.Uint32ToOffset(0), wantSize: 0,
+		},
+		{
+			name: "insert in middle",
+			key:  25, offset: types.Uint32ToOffset(8000), size: 789,
+			wantOffset: types.Uint32ToOffset(0), wantSize: 0,
+		},
+		{
+			name: "update existing",
+			key:  30, offset: types.Uint32ToOffset(9000), size: 999,
+			wantOffset: types.Uint32ToOffset(300), wantSize: 300,
+		},
+	}
+
+	for _, ts := range testSets {
+		offset, size := testSegment.set(ts.key, ts.offset, ts.size)
+		if offset != ts.wantOffset {
+			t.Errorf("%s: got offset %v, want %v", ts.name, offset, ts.wantOffset)
+		}
+		if size != ts.wantSize {
+			t.Errorf("%s: got size %v, want %v", ts.name, size, ts.wantSize)
+		}
+	}
+
+	wantSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 5, Offset: types.Uint32ToOffset(1000), Size: 123},
+			NeedleValue{Key: 10, Offset: types.Uint32ToOffset(0), Size: 100},
+			NeedleValue{Key: 20, Offset: types.Uint32ToOffset(100), Size: 200},
+			NeedleValue{Key: 25, Offset: types.Uint32ToOffset(8000), Size: 789},
+			NeedleValue{Key: 30, Offset: types.Uint32ToOffset(9000), Size: 999},
+			NeedleValue{Key: 51, Offset: types.Uint32ToOffset(7000), Size: 456},
+		},
+		firstKey: 5,
+		lastKey:  51,
+	}
+	if !reflect.DeepEqual(testSegment, wantSegment) {
+		t.Errorf("got result segment %v, want %v", testSegment, wantSegment)
+	}
+
+	if got, want := testSegment.len(), 6; got != want {
+		t.Errorf("got result size %d, want %d", got, want)
+	}
+	if got, want := testSegment.cap(), 6; got != want {
+		t.Errorf("got result capacity %d, want %d", got, want)
 	}
+}
 
-	for i := uint32(0); i < 10*MaxSectionBucketSize; i += 3 {
-		m.Set(NeedleId(i), ToOffset(int64(i+11)), Size(i+5))
+func TestSegmentSetOrdering(t *testing.T) {
+	keys := []types.NeedleId{}
+	for i := 0; i < SegmentChunkSize; i++ {
+		keys = append(keys, types.NeedleId(i))
 	}
 
-	//	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)
-	//		}
-	//	}
+	r := rand.New(rand.NewSource(123456789))
+	r.Shuffle(len(keys), func(i, j int) { keys[i], keys[j] = keys[j], keys[i] })
 
-	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!")
+	cs := newCompactMapSegment(0)
+	for _, k := range keys {
+		_, _ = cs.set(k, types.Uint32ToOffset(123), 456)
 	}
-			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)
+	if got, want := cs.len(), SegmentChunkSize; got != want {
+		t.Errorf("expected size %d, got %d", want, got)
 	}
+	for i := 1; i < cs.len(); i++ {
+		if ka, kb := cs.list[i-1].Key, cs.list[i].Key; ka >= kb {
+			t.Errorf("found out of order entries at (%d, %d) = (%d, %d)", i-1, i, ka, kb)
 		}
 	}
+}
 
-	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 TestSegmentGet(t *testing.T) {
+	testSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 10, Offset: types.Uint32ToOffset(0), Size: 100},
+			NeedleValue{Key: 20, Offset: types.Uint32ToOffset(100), Size: 200},
+			NeedleValue{Key: 30, Offset: types.Uint32ToOffset(300), Size: 300},
+		},
+		firstKey: 10,
+		lastKey:  30,
+	}
+
+	testCases := []struct {
+		name      string
+		key       types.NeedleId
+		wantValue *NeedleValue
+		wantFound bool
+	}{
+		{
+			name:      "invalid key",
+			key:       99,
+			wantValue: nil,
+			wantFound: false,
+		},
+		{
+			name:      "key #1",
+			key:       10,
+			wantValue: &testSegment.list[0],
+			wantFound: true,
+		},
+		{
+			name:      "key #2",
+			key:       20,
+			wantValue: &testSegment.list[1],
+			wantFound: true,
+		},
+		{
+			name:      "key #3",
+			key:       30,
+			wantValue: &testSegment.list[2],
+			wantFound: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			value, found := testSegment.get(tc.key)
+			if got, want := value, tc.wantValue; got != want {
+				t.Errorf("got %v, want %v", got, want)
+			}
+			if got, want := found, tc.wantFound; got != want {
+				t.Errorf("got %v, want %v", got, want)
 			}
+		})
 	}
-
 }
 
-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)
+func TestSegmentDelete(t *testing.T) {
+	testSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 10, Offset: types.Uint32ToOffset(0), Size: 100},
+			NeedleValue{Key: 20, Offset: types.Uint32ToOffset(100), Size: 200},
+			NeedleValue{Key: 30, Offset: types.Uint32ToOffset(300), Size: 300},
+			NeedleValue{Key: 40, Offset: types.Uint32ToOffset(600), Size: 400},
+		},
+		firstKey: 10,
+		lastKey:  40,
+	}
+
+	testDeletes := []struct {
+		name string
+		key  types.NeedleId
+		want types.Size
+	}{
+		{
+			name: "invalid key",
+			key:  99,
+			want: 0,
+		},
+		{
+			name: "delete key #2",
+			key:  20,
+			want: 200,
+		},
+		{
+			name: "delete key #4",
+			key:  40,
+			want: 400,
+		},
+	}
+
+	for _, td := range testDeletes {
+		size := testSegment.delete(td.key)
+		if got, want := size, td.want; got != want {
+			t.Errorf("%s: got %v, want %v", td.name, got, want)
+		}
+	}
+
+	wantSegment := &CompactMapSegment{
+		list: []NeedleValue{
+			NeedleValue{Key: 10, Offset: types.Uint32ToOffset(0), Size: 100},
+			NeedleValue{Key: 20, Offset: types.Uint32ToOffset(100), Size: -200},
+			NeedleValue{Key: 30, Offset: types.Uint32ToOffset(300), Size: 300},
+			NeedleValue{Key: 40, Offset: types.Uint32ToOffset(600), Size: -400},
+		},
+		firstKey: 10,
+		lastKey:  40,
+	}
+	if !reflect.DeepEqual(testSegment, wantSegment) {
+		t.Errorf("got result segment %v, want %v", testSegment, wantSegment)
 	}
+}
 
-	if cs.overflow[2].Size != 24 {
-		t.Fatalf("expecting o[2] has size 24: %+v", cs.overflow[2].Size)
+func TestSegmentForKey(t *testing.T) {
+	testMap := NewCompactMap()
+
+	tests := []struct {
+		name string
+		key  types.NeedleId
+		want *CompactMapSegment
+	}{
+		{
+			name: "first segment",
+			key:  12,
+			want: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: SegmentChunkSize - 1,
+				lastKey:  0,
+			},
+		},
+		{
+			name: "second segment, gapless",
+			key:  SegmentChunkSize + 34,
+			want: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: (2 * SegmentChunkSize) - 1,
+				lastKey:  SegmentChunkSize,
+			},
+		},
+		{
+			name: "gapped segment",
+			key:  (5 * SegmentChunkSize) + 56,
+			want: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: (6 * SegmentChunkSize) - 1,
+				lastKey:  5 * SegmentChunkSize,
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			cs := testMap.segmentForKey(tc.key)
+			if !reflect.DeepEqual(cs, tc.want) {
+				t.Errorf("got segment %v, want %v", cs, tc.want)
 			}
-
-	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)
+	wantMap := &CompactMap{
+		segments: map[int]*CompactMapSegment{
+			0: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: SegmentChunkSize - 1,
+				lastKey:  0,
+			},
+			1: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: (2 * SegmentChunkSize) - 1,
+				lastKey:  SegmentChunkSize,
+			},
+			5: &CompactMapSegment{
+				list:     []NeedleValue{},
+				firstKey: (6 * SegmentChunkSize) - 1,
+				lastKey:  5 * SegmentChunkSize,
+			},
+		},
+	}
+	if !reflect.DeepEqual(testMap, wantMap) {
+		t.Errorf("got map %v, want %v", testMap, wantMap)
 	}
+}
 
-	for i, x := range cs.overflow {
-		println("overflow[", i, "]:", x.Key)
+func TestAscendingVisit(t *testing.T) {
+	cm := NewCompactMap()
+	for _, nid := range []types.NeedleId{20, 7, 40000, 300000, 0, 100, 500, 10000, 200000} {
+		cm.Set(nid, types.Uint32ToOffset(123), 456)
 	}
-	println()
-
-	cs.deleteOverflowEntry(1)
 
-	for i, x := range cs.overflow {
-		println("overflow[", i, "]:", x.Key, "size", x.Size)
+	got := []NeedleValue{}
+	err := cm.AscendingVisit(func(nv NeedleValue) error {
+		got = append(got, nv)
+		return nil
+	})
+	if err != nil {
+		t.Errorf("got error %v, expected none", err)
 	}
-	println()
 
-	cs.setOverflowEntry(4, ToOffset(44), 44)
-	for i, x := range cs.overflow {
-		println("overflow[", i, "]:", x.Key)
+	want := []NeedleValue{
+		NeedleValue{Key: 0, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 7, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 20, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 100, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 500, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 10000, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 40000, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 200000, Offset: types.Uint32ToOffset(123), Size: 456},
+		NeedleValue{Key: 300000, Offset: types.Uint32ToOffset(123), Size: 456},
 	}
-	println()
-
-	cs.setOverflowEntry(1, ToOffset(11), 11)
-
-	for i, x := range cs.overflow {
-		println("overflow[", i, "]:", x.Key)
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got values %v, want %v", got, want)
 	}
-	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)
+func TestOrdering(t *testing.T) {
+	count := 8 * SegmentChunkSize
+	keys := []types.NeedleId{}
+	for i := 0; i < count; i++ {
+		keys = append(keys, types.NeedleId(i))
 	}
 
-	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))
-	}
+	r := rand.New(rand.NewSource(123456789))
+	r.Shuffle(len(keys), func(i, j int) { keys[i], keys[j] = keys[j], keys[i] })
 
-	nv1, ok := m.Get(1574318350048481283)
-	if ok {
-		t.Error(uint64(nv1.Key))
+	cm := NewCompactMap()
+	for _, k := range keys {
+		_, _ = cm.Set(k, types.Uint32ToOffset(123), 456)
 	}
-
-	m.Set(1574318350048481283, ToOffset(10002), 10002)
-	nv2, ok1 := m.Get(1574318350048481283)
-	if ok1 {
-		t.Log(uint64(nv2.Key))
+	if got, want := cm.Len(), count; got != want {
+		t.Errorf("expected size %d, got %d", want, got)
 	}
 
-	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))
+	last := -1
+	err := cm.AscendingVisit(func(nv NeedleValue) error {
+		key := int(nv.Key)
+		if key <= last {
+			return fmt.Errorf("found out of order entries (%d vs %d)", key, last)
 		}
+		last = key
+		return nil
+	})
+	if err != nil {
+		t.Errorf("got error %v, expected none", err)
 	}
 
-	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)
+	// Given that we've written a integer multiple of SegmentChunkSize, all
+	// segments should be fully utilized and capacity-adjusted.
+	if l, c := cm.Len(), cm.Cap(); l != c {
+		t.Errorf("map length (%d) doesn't match capacity (%d)", l, c)
 	}
 }