rocksDB的LRU cache 三兩說

先講單shard的LRU cache，主要有3個數據結構
LRUHandle 作爲LRU的一個個體元素，其數據結構如下，next，prev是在LRU的雙向列表的next和prev，next_hash是在hash_table用來解決拉鍊地址衝突。key_data[1]被分配了連續的地址，用來存儲key值

struct LRUHandle {
  void* value;
  void (*deleter)(const Slice&, void* value);
  LRUHandle* next_hash;
  LRUHandle* next;
  LRUHandle* prev;
  size_t charge;  // TODO(opt): Only allow uint32_t?
  size_t key_length;
  // The hash of key(). Used for fast sharding and comparisons.
  uint32_t hash;
  // The number of external refs to this entry. The cache itself is not counted.
  uint32_t refs;

  enum Flags : uint8_t {
    // Whether this entry is referenced by the hash table.
    IN_CACHE = (1 << 0),
    // Whether this entry is high priority entry.
    IS_HIGH_PRI = (1 << 1),
    // Whether this entry is in high-pri pool.
    IN_HIGH_PRI_POOL = (1 << 2),
    // Wwhether this entry has had any lookups (hits).
    HAS_HIT = (1 << 3),
  };
   char key_data[1];

LRUHandleTable是個自建的Hash Table，通過key用來快速定位到LRUHandle，從而得到prev 還是next

class LRUHandleTable {
}

LRUCacheShard是LRU的核心數據結構， 包括了insert ，LookUp數據結構