// Copyright 2020 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package cache
import (
"fmt"
"math/bits"
"os"
"runtime/debug"
"strings"
"time"
"unsafe"
"github.com/cockroachdb/pebble/internal/invariants"
"github.com/cockroachdb/pebble/internal/manual"
)
var hashSeed = uint64(time.Now().UnixNano())
// Fibonacci hash: https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/
func robinHoodHash(k key, shift uint32) uint32 {
const m = 11400714819323198485
h := hashSeed
h ^= k.id * m
h ^= uint64(k.fileNum.FileNum()) * m
h ^= k.offset * m
return uint32(h >> shift)
}
type robinHoodEntry struct {
key key
// Note that value may point to a Go allocated object (if the "invariants"
// build tag was specified), even though the memory for the entry itself is
// manually managed. This is technically a volation of the Cgo pointer rules:
//
// https://golang.org/cmd/cgo/#hdr-Passing_pointers
//
// Specifically, Go pointers should not be stored in C allocated memory. The
// reason for this rule is that the Go GC will not look at C allocated memory
// to find pointers to Go objects. If the only reference to a Go object is
// stored in C allocated memory, the object will be reclaimed. What makes
// this "safe" is that the Cache guarantees that there are other pointers to
// the entry and shard which will keep them alive. In particular, every Go
// allocated entry in the cache is referenced by the shard.entries map. And
// every shard is referenced by the Cache.shards map.
value *entry
// The distance the entry is from its desired position.
dist uint32
}
type robinHoodEntries struct {
ptr unsafe.Pointer
len uint32
}
func newRobinHoodEntries(n uint32) robinHoodEntries {
size := uintptr(n) * unsafe.Sizeof(robinHoodEntry{})
return robinHoodEntries{
ptr: unsafe.Pointer(&(manual.New(int(size)))[0]),
len: n,
}
}
func (e robinHoodEntries) at(i uint32) *robinHoodEntry {
return (*robinHoodEntry)(unsafe.Pointer(uintptr(e.ptr) +
uintptr(i)*unsafe.Sizeof(robinHoodEntry{})))
}
func (e robinHoodEntries) free() {
size := uintptr(e.len) * unsafe.Sizeof(robinHoodEntry{})
buf := (*[manual.MaxArrayLen]byte)(e.ptr)[:size:size]
manual.Free(buf)
}
// robinHoodMap is an implementation of Robin Hood hashing. Robin Hood hashing
// is an open-address hash table using linear probing. The twist is that the
// linear probe distance is reduced by moving existing entries when inserting
// and deleting. This is accomplished by keeping track of how far an entry is
// from its "desired" slot (hash of key modulo number of slots). During
// insertion, if the new entry being inserted is farther from its desired slot
// than the target entry, we swap the target and new entry. This effectively
// steals from the "rich" target entry and gives to the "poor" new entry (thus
// the origin of the name).
//
// An extension over the base Robin Hood hashing idea comes from
// https://probablydance.com/2017/02/26/i-wrote-the-fastest-hashtable/. A cap
// is placed on the max distance an entry can be from its desired slot. When
// this threshold is reached during insertion, the size of the table is doubled
// and insertion is restarted. Additionally, the entries slice is given "max
// dist" extra entries on the end. The very last entry in the entries slice is
// never used and acts as a sentinel which terminates loops. The previous
// maxDist-1 entries act as the extra entries. For example, if the size of the
// table is 2, maxDist is computed as 4 and the actual size of the entry slice
// is 6.
//
// +---+---+---+---+---+---+
// | 0 | 1 | 2 | 3 | 4 | 5 |
// +---+---+---+---+---+---+
// ^
// size
//
// In this scenario, the target entry for a key will always be in the range
// [0,1]. Valid entries may reside in the range [0,4] due to the linear probing
// of up to maxDist entries. The entry at index 5 will never contain a value,
// and instead acts as a sentinel (its distance is always 0). The max distance
// threshold is set to log2(num-entries). This ensures that retrieval is O(log
// N), though note that N is the number of total entries, not the count of
// valid entries.
//
// Deletion is implemented via the backward shift delete mechanism instead of
// tombstones. This preserves the performance of the table in the presence of
// deletions. See
// http://codecapsule.com/2013/11/17/robin-hood-hashing-backward-shift-deletion
// for details.
type robinHoodMap struct {
entries robinHoodEntries
size uint32
shift uint32
count uint32
maxDist uint32
}
func maxDistForSize(size uint32) uint32 {
desired := uint32(bits.Len32(size))
if desired < 4 {
desired = 4
}
return desired
}
func newRobinHoodMap(initialCapacity int) *robinHoodMap {
m := &robinHoodMap{}
m.init(initialCapacity)
// Note: this is a no-op if invariants are disabled or race is enabled.
invariants.SetFinalizer(m, func(obj interface{}) {
m := obj.(*robinHoodMap)
if m.entries.ptr != nil {
fmt.Fprintf(os.Stderr, "%p: robin-hood map not freed\n", m)
os.Exit(1)
}
})
return m
}
func (m *robinHoodMap) init(initialCapacity int) {
if initialCapacity < 1 {
initialCapacity = 1
}
targetSize := 1 << (uint(bits.Len(uint(2*initialCapacity-1))) - 1)
m.rehash(uint32(targetSize))
}
func (m *robinHoodMap) free() {
if m.entries.ptr != nil {
m.entries.free()
m.entries.ptr = nil
}
}
func (m *robinHoodMap) rehash(size uint32) {
oldEntries := m.entries
m.size = size
m.shift = uint32(64 - bits.Len32(m.size-1))
m.maxDist = maxDistForSize(size)
m.entries = newRobinHoodEntries(size + m.maxDist)
m.count = 0
for i := uint32(0); i < oldEntries.len; i++ {
e := oldEntries.at(i)
if e.value != nil {
m.Put(e.key, e.value)
}
}
if oldEntries.ptr != nil {
oldEntries.free()
}
}
// Find an entry containing the specified value. This is intended to be used
// from debug and test code.
func (m *robinHoodMap) findByValue(v *entry) *robinHoodEntry {
for i := uint32(0); i < m.entries.len; i++ {
e := m.entries.at(i)
if e.value == v {
return e
}
}
return nil
}
func (m *robinHoodMap) Count() int {
return int(m.count)
}
func (m *robinHoodMap) Put(k key, v *entry) {
maybeExists := true
n := robinHoodEntry{key: k, value: v, dist: 0}
for i := robinHoodHash(k, m.shift); ; i++ {
e := m.entries.at(i)
if maybeExists && k == e.key {
// Entry already exists: overwrite.
e.value = n.value
m.checkEntry(i)
return
}
if e.value == nil {
// Found an empty entry: insert here.
*e = n
m.count++
m.checkEntry(i)
return
}
if e.dist < n.dist {
// Swap the new entry with the current entry because the current is
// rich. We then continue to loop, looking for a new location for the
// current entry. Note that this is also the not-found condition for
// retrieval, which means that "k" is not present in the map. See Get().
n, *e = *e, n
m.checkEntry(i)
maybeExists = false
}
// The new entry gradually moves away from its ideal position.
n.dist++
// If we've reached the max distance threshold, grow the table and restart
// the insertion.
if n.dist == m.maxDist {
m.rehash(2 * m.size)
i = robinHoodHash(n.key, m.shift) - 1
n.dist = 0
maybeExists = false
}
}
}
func (m *robinHoodMap) Get(k key) *entry {
var dist uint32
for i := robinHoodHash(k, m.shift); ; i++ {
e := m.entries.at(i)
if k == e.key {
// Found.
return e.value
}
if e.dist < dist {
// Not found.
return nil
}
dist++
}
}
func (m *robinHoodMap) Delete(k key) {
var dist uint32
for i := robinHoodHash(k, m.shift); ; i++ {
e := m.entries.at(i)
if k == e.key {
m.checkEntry(i)
// We found the entry to delete. Shift the following entries backwards
// until the next empty value or entry with a zero distance. Note that
// empty values are guaranteed to have "dist == 0".
m.count--
for j := i + 1; ; j++ {
t := m.entries.at(j)
if t.dist == 0 {
*e = robinHoodEntry{}
return
}
e.key = t.key
e.value = t.value
e.dist = t.dist - 1
e = t
m.checkEntry(j)
}
}
if dist > e.dist {
// Not found.
return
}
dist++
}
}
func (m *robinHoodMap) checkEntry(i uint32) {
if invariants.Enabled {
e := m.entries.at(i)
if e.value != nil {
pos := robinHoodHash(e.key, m.shift)
if (uint32(i) - pos) != e.dist {
fmt.Fprintf(os.Stderr, "%d: invalid dist=%d, expected %d: %s\n%s",
i, e.dist, uint32(i)-pos, e.key, debug.Stack())
os.Exit(1)
}
if e.dist > m.maxDist {
fmt.Fprintf(os.Stderr, "%d: invalid dist=%d > maxDist=%d: %s\n%s",
i, e.dist, m.maxDist, e.key, debug.Stack())
os.Exit(1)
}
}
}
}
func (m *robinHoodMap) String() string {
var buf strings.Builder
fmt.Fprintf(&buf, "count: %d\n", m.count)
for i := uint32(0); i < m.entries.len; i++ {
e := m.entries.at(i)
if e.value != nil {
fmt.Fprintf(&buf, "%d: [%s,%p,%d]\n", i, e.key, e.value, e.dist)
}
}
return buf.String()
}