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sieve.go
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sieve.go
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// Copyright 2024 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package infoschema
import (
"container/list"
"context"
"sync"
"github.com/pingcap/tidb/pkg/infoschema/internal"
)
// entry holds the key and value of a cache entry.
type entry[K comparable, V any] struct {
key K
value V
visited bool
element *list.Element
size uint64
}
func (t *entry[K, V]) Size() uint64 {
if t.size == 0 {
size := internal.Sizeof(t)
if size > 0 {
t.size = uint64(size)
}
}
return t.size
}
// Sieve is an efficient turn-Key eviction algorithm for web caches.
// See blog post https://cachemon.github.io/SIEVE-website/blog/2023/12/17/sieve-is-simpler-than-lru/
// and also the academic paper "SIEVE is simpler than LRU"
type Sieve[K comparable, V any] struct {
ctx context.Context
cancel context.CancelFunc
mu sync.Mutex
size uint64
capacity uint64
items map[K]*entry[K, V]
ll *list.List
hand *list.Element
}
func newSieve[K comparable, V any](capacity uint64) *Sieve[K, V] {
ctx, cancel := context.WithCancel(context.Background())
cache := &Sieve[K, V]{
ctx: ctx,
cancel: cancel,
capacity: capacity,
items: make(map[K]*entry[K, V]),
ll: list.New(),
}
return cache
}
func (s *Sieve[K, V]) SetCapacity(capacity uint64) {
s.mu.Lock()
defer s.mu.Unlock()
s.capacity = capacity
}
func (s *Sieve[K, V]) Capacity() uint64 {
s.mu.Lock()
defer s.mu.Unlock()
return s.capacity
}
func (s *Sieve[K, V]) Set(key K, value V) {
s.mu.Lock()
defer s.mu.Unlock()
if e, ok := s.items[key]; ok {
e.value = value
e.visited = true
return
}
for i := 0; s.size > s.capacity && i < 10; i++ {
s.evict()
}
e := &entry[K, V]{
key: key,
value: value,
}
s.size += e.Size() // calculate the size first without putting to the list.
e.element = s.ll.PushFront(key)
s.items[key] = e
}
func (s *Sieve[K, V]) Get(key K) (value V, ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
if e, ok := s.items[key]; ok {
e.visited = true
return e.value, true
}
return
}
func (s *Sieve[K, V]) Remove(key K) (ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
if e, ok := s.items[key]; ok {
// if the element to be removed is the hand,
// then move the hand to the previous one.
if e.element == s.hand {
s.hand = s.hand.Prev()
}
s.removeEntry(e)
return true
}
return false
}
func (s *Sieve[K, V]) Contains(key K) (ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
_, ok = s.items[key]
return
}
func (s *Sieve[K, V]) Peek(key K) (value V, ok bool) {
s.mu.Lock()
defer s.mu.Unlock()
if e, ok := s.items[key]; ok {
return e.value, true
}
return
}
func (s *Sieve[K, V]) Size() uint64 {
s.mu.Lock()
defer s.mu.Unlock()
return s.size
}
func (s *Sieve[K, V]) Len() int {
s.mu.Lock()
defer s.mu.Unlock()
return s.ll.Len()
}
func (s *Sieve[K, V]) Purge() {
s.mu.Lock()
defer s.mu.Unlock()
for _, e := range s.items {
s.removeEntry(e)
}
s.ll.Init()
}
func (s *Sieve[K, V]) Close() {
s.Purge()
s.mu.Lock()
s.cancel()
s.mu.Unlock()
}
func (s *Sieve[K, V]) removeEntry(e *entry[K, V]) {
s.ll.Remove(e.element)
delete(s.items, e.key)
s.size -= e.Size()
}
func (s *Sieve[K, V]) evict() {
o := s.hand
// if o is nil, then assign it to the tail element in the list
if o == nil {
o = s.ll.Back()
}
el, ok := s.items[o.Value.(K)]
if !ok {
panic("sieve: evicting non-existent element")
}
for el.visited {
el.visited = false
o = o.Prev()
if o == nil {
o = s.ll.Back()
}
el, ok = s.items[o.Value.(K)]
if !ok {
panic("sieve: evicting non-existent element")
}
}
s.hand = o.Prev()
s.removeEntry(el)
}