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second_chance.go 2.72 KB
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/*
Copyright IBM Corp. All Rights Reserved.
SPDX-License-Identifier: Apache-2.0
*/
package cache
import (
"sync"
"sync/atomic"
)
// This package implements Second-Chance Algorithm, an approximate LRU algorithms.
// https://www.cs.jhu.edu/~yairamir/cs418/os6/tsld023.htm
// secondChanceCache holds key-value items with a limited size.
// When the number cached items exceeds the limit, victims are selected based on the
// Second-Chance Algorithm and get purged
type secondChanceCache struct {
// manages mapping between keys and items
table map[string]*cacheItem
// holds a list of cached items.
items []*cacheItem
// indicates the next candidate of a victim in the items list
position int
// read lock for get, and write lock for add
rwlock sync.RWMutex
}
type cacheItem struct {
key string
value interface{}
// set to 1 when get() is called. set to 0 when victim scan
referenced int32
}
func newSecondChanceCache(cacheSize int) *secondChanceCache {
var cache secondChanceCache
cache.position = 0
cache.items = make([]*cacheItem, cacheSize)
cache.table = make(map[string]*cacheItem)
return &cache
}
func (cache *secondChanceCache) len() int {
cache.rwlock.RLock()
defer cache.rwlock.RUnlock()
return len(cache.table)
}
func (cache *secondChanceCache) get(key string) (interface{}, bool) {
cache.rwlock.RLock()
defer cache.rwlock.RUnlock()
item, ok := cache.table[key]
if !ok {
return nil, false
}
// referenced bit is set to true to indicate that this item is recently accessed.
atomic.StoreInt32(&item.referenced, 1)
return item.value, true
}
func (cache *secondChanceCache) add(key string, value interface{}) {
cache.rwlock.Lock()
defer cache.rwlock.Unlock()
if old, ok := cache.table[key]; ok {
old.value = value
atomic.StoreInt32(&old.referenced, 1)
return
}
var item cacheItem
item.key = key
item.value = value
atomic.StoreInt32(&item.referenced, 1)
size := len(cache.items)
num := len(cache.table)
if num < size {
// cache is not full, so just store the new item at the end of the list
cache.table[key] = &item
cache.items[num] = &item
return
}
// starts victim scan since cache is full
for {
// checks whether this item is recently accsessed or not
victim := cache.items[cache.position]
if atomic.LoadInt32(&victim.referenced) == 0 {
// a victim is found. delete it, and store the new item here.
delete(cache.table, victim.key)
cache.table[key] = &item
cache.items[cache.position] = &item
cache.position = (cache.position + 1) % size
return
}
// referenced bit is set to false so that this item will be get purged
// unless it is accessed until a next victim scan
atomic.StoreInt32(&victim.referenced, 0)
cache.position = (cache.position + 1) % size
}
}
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