package lru import ( "sync" "github.com/hashicorp/golang-lru/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 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 // T1 is the LRU for recently accessed items b1 simplelru.LRUCache // B1 is the LRU for evictions from t1 t2 simplelru.LRUCache // T2 is the LRU for frequently accessed items b2 simplelru.LRUCache // B2 is the LRU for evictions from t2 lock sync.RWMutex } // NewARC creates an ARC of the given size func NewARC(size int) (*ARCCache, error) { // Create the sub LRUs b1, err := simplelru.NewLRU(size, nil) if err != nil { return nil, err } b2, err := simplelru.NewLRU(size, nil) if err != nil { return nil, err } t1, err := simplelru.NewLRU(size, nil) if err != nil { return nil, err } t2, err := simplelru.NewLRU(size, nil) if err != nil { return nil, err } // Initialize the ARC c := &ARCCache{ 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) Get(key interface{}) (value interface{}, 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 nil, false } // Add adds a value to the cache. func (c *ARCCache) Add(key, value interface{}) { 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) 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, nil) } } else { k, _, ok := c.t2.RemoveOldest() if ok { c.b2.Add(k, nil) } } } // Len returns the number of cached entries func (c *ARCCache) 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) Keys() []interface{} { c.lock.RLock() defer c.lock.RUnlock() k1 := c.t1.Keys() k2 := c.t2.Keys() return append(k1, k2...) } // Remove is used to purge a key from the cache func (c *ARCCache) Remove(key interface{}) { 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) 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) Contains(key interface{}) 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) Peek(key interface{}) (value interface{}, 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) }