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tlfu.go
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tlfu.go
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package internal
import (
"sync/atomic"
)
type TinyLfu[K comparable, V any] struct {
slru *Slru[K, V]
sketch *CountMinSketch
hasher *Hasher[K]
size uint
counter uint
total atomic.Uint32
hit atomic.Uint32
hr float32
threshold atomic.Int32
lruFactor uint8
step int8
}
func NewTinyLfu[K comparable, V any](size uint, hasher *Hasher[K]) *TinyLfu[K, V] {
tlfu := &TinyLfu[K, V]{
size: size,
slru: NewSlru[K, V](size),
sketch: NewCountMinSketch(),
step: 1,
hasher: hasher,
}
// default threshold to -1 so all entries are admitted until cache is full
tlfu.threshold.Store(-1)
return tlfu
}
func (t *TinyLfu[K, V]) climb() {
total := t.total.Load()
hit := t.hit.Load()
current := float32(hit) / float32(total)
delta := current - t.hr
var diff int8
if delta > 0.0 {
if t.step < 0 {
t.step -= 1
} else {
t.step += 1
}
if t.step < -13 {
t.step = -13
} else if t.step > 13 {
t.step = 13
}
newFactor := int8(t.lruFactor) + t.step
if newFactor < 0 {
newFactor = 0
} else if newFactor > 16 {
newFactor = 16
}
diff = newFactor - int8(t.lruFactor)
t.lruFactor = uint8(newFactor)
} else if delta < 0.0 {
// reset
if t.step > 0 {
t.step = -1
} else {
t.step = 1
}
newFactor := int8(t.lruFactor) + t.step
if newFactor < 0 {
newFactor = 0
} else if newFactor > 16 {
newFactor = 16
}
diff = newFactor - int8(t.lruFactor)
t.lruFactor = uint8(newFactor)
}
t.threshold.Add(-int32(diff))
t.hr = current
t.hit.Store(0)
t.total.Store(0)
}
func (t *TinyLfu[K, V]) Set(entry *Entry[K, V]) *Entry[K, V] {
t.counter++
if t.counter > 10*t.size {
t.climb()
t.counter = 0
}
if entry.meta.prev == nil {
if victim := t.slru.victim(); victim != nil {
freq := int(entry.frequency.Load())
if freq == -1 {
freq = int(t.sketch.Estimate(t.hasher.hash(entry.key)))
}
evictedCount := uint(freq) + uint(t.lruFactor)
victimCount := t.sketch.Estimate(t.hasher.hash(victim.key))
if evictedCount <= uint(victimCount) {
return entry
}
} else {
count := t.slru.probation.count + t.slru.protected.count
t.sketch.EnsureCapacity(uint(count + count/100))
}
evicted := t.slru.insert(entry)
return evicted
}
return nil
}
func (t *TinyLfu[K, V]) Access(item ReadBufItem[K, V]) {
t.counter++
if t.counter > 10*t.size {
t.climb()
t.counter = 0
}
if entry := item.entry; entry != nil {
reset := t.sketch.Add(item.hash)
if reset {
t.threshold.Store(t.threshold.Load() / 2)
}
if entry.meta.prev != nil {
var tail bool
if entry == t.slru.victim() {
tail = true
}
t.slru.access(entry)
if tail {
t.UpdateThreshold()
}
} else {
entry.frequency.Store(int32(t.sketch.Estimate(item.hash)))
}
} else {
reset := t.sketch.Add(item.hash)
if reset {
t.threshold.Store(t.threshold.Load() / 2)
}
}
}
func (t *TinyLfu[K, V]) Remove(entry *Entry[K, V]) {
t.slru.remove(entry)
}
func (t *TinyLfu[K, V]) UpdateCost(entry *Entry[K, V], delta int64) {
t.slru.updateCost(entry, delta)
}
func (t *TinyLfu[K, V]) EvictEntries() []*Entry[K, V] {
removed := []*Entry[K, V]{}
for t.slru.probation.Len()+t.slru.protected.Len() > int(t.slru.maxsize) {
entry := t.slru.probation.PopTail()
if entry == nil {
break
}
removed = append(removed, entry)
}
for t.slru.probation.Len()+t.slru.protected.Len() > int(t.slru.maxsize) {
entry := t.slru.protected.PopTail()
if entry == nil {
break
}
removed = append(removed, entry)
}
return removed
}
func (t *TinyLfu[K, V]) UpdateThreshold() {
if t.slru.probation.Len()+t.slru.protected.Len() < int(t.slru.maxsize) {
t.threshold.Store(-1)
} else {
tail := t.slru.victim()
if tail != nil {
t.threshold.Store(
int32(t.sketch.Estimate(t.hasher.hash(tail.key)) - uint(t.lruFactor)),
)
} else {
// cache is not full
t.threshold.Store(-1)
}
}
}