Added prefix match for consistent hash

Author: Bob Hansen

consistenthash/consistenthash.go

@@ -19,24 +19,32 @@ package consistenthash
 
 import (
 	"hash/crc32"
+	"math/bits"
 	"sort"
 	"strconv"
 )
 
 type Hash func(data []byte) uint32
 
 type Map struct {
-	hash     Hash
-	replicas int
-	keys     []int // Sorted
-	hashMap  map[int]string
+	hash                 Hash
+	replicas             int
+	prefixTableExpansion int
+
+	keys    []int // Sorted
+	hashMap map[int]string
+
+	prefixBits  uint32
+	prefixShift uint32
+	prefixTable []string
 }
 
-func New(replicas int, fn Hash) *Map {
+func New(replicas int, tableExpansion int, fn Hash) *Map {
 	m := &Map{
-		replicas: replicas,
-		hash:     fn,
-		hashMap:  make(map[int]string),
+		replicas:             replicas,
+		hash:                 fn,
+		hashMap:              make(map[int]string),
+		prefixTableExpansion: tableExpansion,
 	}
 	if m.hash == nil {
 		m.hash = crc32.ChecksumIEEE
@@ -59,6 +67,37 @@ func (m *Map) Add(keys ...string) {
 		}
 	}
 	sort.Ints(m.keys)
+
+	// Find minimum number of bits to hold |keys| * prefixTableExpansion
+	m.prefixBits = uint32(bits.Len32(uint32(len(m.keys) * m.prefixTableExpansion)))
+	m.prefixShift = 32 - m.prefixBits
+
+	prefixTableSize := 1 << m.prefixBits
+	m.prefixTable = make([]string, prefixTableSize)
+
+	previousKeyPrefix := -1 // Effectively -Inf
+	currentKeyIdx := 0
+	currentKeyPrefix := m.keys[currentKeyIdx] >> m.prefixShift
+
+	for i := range m.prefixTable {
+		if previousKeyPrefix < i && currentKeyPrefix > i {
+			// All keys with this prefix will map to a single value
+			m.prefixTable[i] = m.hashMap[m.keys[currentKeyIdx]]
+		} else {
+			// Several keys might have the same prefix.  Walk
+			// over them until it changes
+			previousKeyPrefix = currentKeyPrefix
+			for currentKeyPrefix == previousKeyPrefix {
+				currentKeyIdx++
+				if currentKeyIdx < len(m.keys) {
+					currentKeyPrefix = m.keys[currentKeyIdx] >> m.prefixShift
+				} else {
+					currentKeyIdx = 0
+					currentKeyPrefix = prefixTableSize + 1 // Effectively +Inf
+				}
+			}
+		}
+	}
 }
 
 // Gets the closest item in the hash to the provided key.
@@ -69,6 +108,13 @@ func (m *Map) Get(key string) string {
 
 	hash := int(m.hash([]byte(key)))
 
+	// Look for the hash prefix in the prefix table
+	prefixSlot := hash >> m.prefixShift
+	tableResult := m.prefixTable[prefixSlot]
+	if len(tableResult) > 0 {
+		return tableResult
+	}
+
 	// Binary search for appropriate replica.
 	idx := sort.Search(len(m.keys), func(i int) bool { return m.keys[i] >= hash })
 

consistenthash/consistenthash_test.go

@@ -26,7 +26,7 @@ func TestHashing(t *testing.T) {
 
 	// Override the hash function to return easier to reason about values. Assumes
 	// the keys can be converted to an integer.
-	hash := New(3, func(key []byte) uint32 {
+	hash := New(3, 6, func(key []byte) uint32 {
 		i, err := strconv.Atoi(string(key))
 		if err != nil {
 			panic(err)
@@ -66,8 +66,8 @@ func TestHashing(t *testing.T) {
 }
 
 func TestConsistency(t *testing.T) {
-	hash1 := New(1, nil)
-	hash2 := New(1, nil)
+	hash1 := New(1, 6, nil)
+	hash2 := New(1, 6, nil)
 
 	hash1.Add("Bill", "Bob", "Bonny")
 	hash2.Add("Bob", "Bonny", "Bill")
@@ -86,25 +86,31 @@ func TestConsistency(t *testing.T) {
 
 }
 
-func BenchmarkGet8(b *testing.B)   { benchmarkGet(b, 8) }
-func BenchmarkGet32(b *testing.B)  { benchmarkGet(b, 32) }
-func BenchmarkGet128(b *testing.B) { benchmarkGet(b, 128) }
-func BenchmarkGet512(b *testing.B) { benchmarkGet(b, 512) }
+func BenchmarkGet8(b *testing.B)   { benchmarkGet(b, 8, 6) }
+func BenchmarkGet32(b *testing.B)  { benchmarkGet(b, 32, 6) }
+func BenchmarkGet128(b *testing.B) { benchmarkGet(b, 128, 6) }
+func BenchmarkGet512(b *testing.B) { benchmarkGet(b, 512, 6) }
 
-func benchmarkGet(b *testing.B, shards int) {
+func benchmarkGet(b *testing.B, shards int, expansion int) {
 
-	hash := New(50, nil)
+	hash := New(50, expansion, nil)
 
 	var buckets []string
 	for i := 0; i < shards; i++ {
 		buckets = append(buckets, fmt.Sprintf("shard-%d", i))
 	}
 
+	testStringCount := shards
+	var testStrings []string
+	for i := 0; i < testStringCount; i++ {
+		testStrings = append(testStrings, fmt.Sprintf("%d", i))
+	}
+
 	hash.Add(buckets...)
 
 	b.ResetTimer()
 
 	for i := 0; i < b.N; i++ {
-		hash.Get(buckets[i&(shards-1)])
+		hash.Get(testStrings[i&(testStringCount-1)])
 	}
 }

http.go

@@ -34,6 +34,8 @@ const defaultBasePath = "/_groupcache/"
 
 const defaultReplicas = 50
 
+const defaultHashExpansion = 6
+
 // HTTPPool implements PeerPicker for a pool of HTTP peers.
 type HTTPPool struct {
 	// Context optionally specifies a context for the server to use when it
@@ -106,7 +108,7 @@ func NewHTTPPoolOpts(self string, o *HTTPPoolOptions) *HTTPPool {
 	if p.opts.Replicas == 0 {
 		p.opts.Replicas = defaultReplicas
 	}
-	p.peers = consistenthash.New(p.opts.Replicas, p.opts.HashFn)
+	p.peers = consistenthash.New(p.opts.Replicas, defaultHashExpansion, p.opts.HashFn)
 
 	RegisterPeerPicker(func() PeerPicker { return p })
 	return p
@@ -118,7 +120,7 @@ func NewHTTPPoolOpts(self string, o *HTTPPoolOptions) *HTTPPool {
 func (p *HTTPPool) Set(peers ...string) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
-	p.peers = consistenthash.New(p.opts.Replicas, p.opts.HashFn)
+	p.peers = consistenthash.New(p.opts.Replicas, defaultHashExpansion, p.opts.HashFn)
 	p.peers.Add(peers...)
 	p.httpGetters = make(map[string]*httpGetter, len(peers))
 	for _, peer := range peers {