1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268
|
// Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: MPL-2.0
package arc
import (
"sync"
"github.com/hashicorp/golang-lru/v2/simplelru"
)
// ARCCache is a thread-safe fixed size Adaptive Replacement Cache (ARC).
// ARC is an enhancement over the standard LRU cache in that tracks both
// frequency and recency of use. This avoids a burst in access to new
// entries from evicting the frequently used older entries. It adds some
// additional tracking overhead to a standard LRU cache, computationally
// it is roughly 2x the cost, and the extra memory overhead is linear
// with the size of the cache. ARC has been patented by IBM, but is
// similar to the TwoQueueCache (2Q) which requires setting parameters.
type ARCCache[K comparable, V any] struct {
size int // Size is the total capacity of the cache
p int // P is the dynamic preference towards T1 or T2
t1 simplelru.LRUCache[K, V] // T1 is the LRU for recently accessed items
b1 simplelru.LRUCache[K, struct{}] // B1 is the LRU for evictions from t1
t2 simplelru.LRUCache[K, V] // T2 is the LRU for frequently accessed items
b2 simplelru.LRUCache[K, struct{}] // B2 is the LRU for evictions from t2
lock sync.RWMutex
}
// NewARC creates an ARC of the given size
func NewARC[K comparable, V any](size int) (*ARCCache[K, V], error) {
// Create the sub LRUs
b1, err := simplelru.NewLRU[K, struct{}](size, nil)
if err != nil {
return nil, err
}
b2, err := simplelru.NewLRU[K, struct{}](size, nil)
if err != nil {
return nil, err
}
t1, err := simplelru.NewLRU[K, V](size, nil)
if err != nil {
return nil, err
}
t2, err := simplelru.NewLRU[K, V](size, nil)
if err != nil {
return nil, err
}
// Initialize the ARC
c := &ARCCache[K, V]{
size: size,
p: 0,
t1: t1,
b1: b1,
t2: t2,
b2: b2,
}
return c, nil
}
// Get looks up a key's value from the cache.
func (c *ARCCache[K, V]) Get(key K) (value V, ok bool) {
c.lock.Lock()
defer c.lock.Unlock()
// If the value is contained in T1 (recent), then
// promote it to T2 (frequent)
if val, ok := c.t1.Peek(key); ok {
c.t1.Remove(key)
c.t2.Add(key, val)
return val, ok
}
// Check if the value is contained in T2 (frequent)
if val, ok := c.t2.Get(key); ok {
return val, ok
}
// No hit
return
}
// Add adds a value to the cache.
func (c *ARCCache[K, V]) Add(key K, value V) {
c.lock.Lock()
defer c.lock.Unlock()
// Check if the value is contained in T1 (recent), and potentially
// promote it to frequent T2
if c.t1.Contains(key) {
c.t1.Remove(key)
c.t2.Add(key, value)
return
}
// Check if the value is already in T2 (frequent) and update it
if c.t2.Contains(key) {
c.t2.Add(key, value)
return
}
// Check if this value was recently evicted as part of the
// recently used list
if c.b1.Contains(key) {
// T1 set is too small, increase P appropriately
delta := 1
b1Len := c.b1.Len()
b2Len := c.b2.Len()
if b2Len > b1Len {
delta = b2Len / b1Len
}
if c.p+delta >= c.size {
c.p = c.size
} else {
c.p += delta
}
// Potentially need to make room in the cache
if c.t1.Len()+c.t2.Len() >= c.size {
c.replace(false)
}
// Remove from B1
c.b1.Remove(key)
// Add the key to the frequently used list
c.t2.Add(key, value)
return
}
// Check if this value was recently evicted as part of the
// frequently used list
if c.b2.Contains(key) {
// T2 set is too small, decrease P appropriately
delta := 1
b1Len := c.b1.Len()
b2Len := c.b2.Len()
if b1Len > b2Len {
delta = b1Len / b2Len
}
if delta >= c.p {
c.p = 0
} else {
c.p -= delta
}
// Potentially need to make room in the cache
if c.t1.Len()+c.t2.Len() >= c.size {
c.replace(true)
}
// Remove from B2
c.b2.Remove(key)
// Add the key to the frequently used list
c.t2.Add(key, value)
return
}
// Potentially need to make room in the cache
if c.t1.Len()+c.t2.Len() >= c.size {
c.replace(false)
}
// Keep the size of the ghost buffers trim
if c.b1.Len() > c.size-c.p {
c.b1.RemoveOldest()
}
if c.b2.Len() > c.p {
c.b2.RemoveOldest()
}
// Add to the recently seen list
c.t1.Add(key, value)
}
// replace is used to adaptively evict from either T1 or T2
// based on the current learned value of P
func (c *ARCCache[K, V]) replace(b2ContainsKey bool) {
t1Len := c.t1.Len()
if t1Len > 0 && (t1Len > c.p || (t1Len == c.p && b2ContainsKey)) {
k, _, ok := c.t1.RemoveOldest()
if ok {
c.b1.Add(k, struct{}{})
}
} else {
k, _, ok := c.t2.RemoveOldest()
if ok {
c.b2.Add(k, struct{}{})
}
}
}
// Len returns the number of cached entries
func (c *ARCCache[K, V]) Len() int {
c.lock.RLock()
defer c.lock.RUnlock()
return c.t1.Len() + c.t2.Len()
}
// Keys returns all the cached keys
func (c *ARCCache[K, V]) Keys() []K {
c.lock.RLock()
defer c.lock.RUnlock()
k1 := c.t1.Keys()
k2 := c.t2.Keys()
return append(k1, k2...)
}
// Values returns all the cached values
func (c *ARCCache[K, V]) Values() []V {
c.lock.RLock()
defer c.lock.RUnlock()
v1 := c.t1.Values()
v2 := c.t2.Values()
return append(v1, v2...)
}
// Remove is used to purge a key from the cache
func (c *ARCCache[K, V]) Remove(key K) {
c.lock.Lock()
defer c.lock.Unlock()
if c.t1.Remove(key) {
return
}
if c.t2.Remove(key) {
return
}
if c.b1.Remove(key) {
return
}
if c.b2.Remove(key) {
return
}
}
// Purge is used to clear the cache
func (c *ARCCache[K, V]) Purge() {
c.lock.Lock()
defer c.lock.Unlock()
c.t1.Purge()
c.t2.Purge()
c.b1.Purge()
c.b2.Purge()
}
// Contains is used to check if the cache contains a key
// without updating recency or frequency.
func (c *ARCCache[K, V]) Contains(key K) bool {
c.lock.RLock()
defer c.lock.RUnlock()
return c.t1.Contains(key) || c.t2.Contains(key)
}
// Peek is used to inspect the cache value of a key
// without updating recency or frequency.
func (c *ARCCache[K, V]) Peek(key K) (value V, ok bool) {
c.lock.RLock()
defer c.lock.RUnlock()
if val, ok := c.t1.Peek(key); ok {
return val, ok
}
return c.t2.Peek(key)
}
|