\u5728leveldb\u4e2d\uff0c\u4e3a\u4e86\u63d0\u5347\u5bf9\u6570\u636e\u7684\u68c0\u7d22\u901f\u5ea6\uff0c\u4e5f\u8bbe\u8ba1\u4e86\u7f13\u5b58\uff0c\u5bf9\u5e94\u7684\u4ee3\u7801\u5728db\/table_cache.h\u548cdb\/table_cache.cc\u4e2d\uff0c\u8fd9\u4e2atable cache\u7684\u5b9e\u73b0\u4e3b\u8981\u501f\u52a9\u4e8eCache\u7c7b\uff0c\u5173\u4e8eCache\u7c7b\uff0c\u662f\u53ef\u4ee5\u7528\u6237\u81ea\u5b9a\u4e49\u5b9e\u73b0\u7684\uff0c\u4f46leveldb\u4e5f\u6709\u4e00\u4e2a\u5185\u7f6e\u7684Cache\u7684\u5b9e\u73b0\uff0c\u6587\u4ef6\u662futil\/cache.cc\u3002\u4e3b\u8981\u662fLRU\uff08\u6700\u8fd1\u6700\u5c11\u4f7f\u7528\uff09\u7b97\u6cd5\uff0c\u539f\u7406\u662f\u201c\u5982\u679c\u4e00\u4e2a\u6570\u636e\u6700\u8fd1\u88ab\u4f7f\u7528\uff0c\u90a3\u4e48\u5c06\u6765\u88ab\u4f7f\u7528\u7684\u6982\u7387\u540c\u6837\u4e5f\u5f88\u5927\u201d\u3002\u540c\u6837\u4e5f\u5b9e\u73b0\u4e86\u4e00\u4e2aHashTable\uff0cleveldb\u4e2d\u63d0\u4f9b\u7684\u6570\u636e\u663e\u793a\u5728g++ 4.4.3\u4e0b\u6bd4built-in\u7684\u54c8\u5e0c\u8868\u6027\u80fd\u7a0d\u9ad8\uff0c\u5927\u69825%\u5de6\u53f3\u3002<\/p>\n
\u4e8c\u3001\u6982\u89c8<\/h2>\n2.1 hash table\u6570\u636e\u7ed3\u6784<\/h3>\n
\u54c8\u5e0c\u8868\u662f\u4e00\u4e2a\u5f88\u5e38\u89c1\u7684\u7ed3\u6784\u4e86\uff0c\u5b58\u50a8\u7684\u662fkey-value\u7ed3\u6784\uff0c\u4e00\u4e2akey-value\u5bf9\u5e38\u88ab\u79f0\u4f5centry\uff0c\u5b83\u6700\u5927\u7684\u7279\u70b9\u662f\u67e5\u627e\u5feb\u3002\u5728\u7f51\u4e0a\u627e\u4e86\u4e00\u5f20hash table\u7684\u56fe
\n![\"hash](\"https:\/\/www.myway5.com\/wp-content\/uploads\/2017\/07\/450px-Hash_table_5_0_1_1_1_1_1_LL.svg_.png\")
<\/p>\n
\u9996\u5148\u5b83\u662f\u4e00\u4e2a\u957f\u4e3alen\u7684\u6570\u7ec4\uff0c\u6bcf\u4e2a\u6570\u7ec4\u4e2d\u7684\u5143\u7d20\u90fd\u662f\u4e00\u4e2a\u94fe\u8868\u3002\u5982\u56fe\u6240\u793a\uff0cjohn Smith\u548cSandra Dee\u90fd\u88abhash\u5230152\u8fd9\u4e2a\u5730\u65b9\uff0c\u6240\u4ee5\u5728152\u8fd9\u4e2a\u5730\u65b9\u4f7f\u7528\u94fe\u8868\u6765\u5b58\u50a8\u8fd9\u4e24\u4e2aentry.\u67e5\u627e\u7684\u65f6\u5019Sandra Dee\u7684\u65f6\u5019\uff0c\u5148\u627e\u5230152\u8fd9\u4e2a\u5730\u65b9\uff0c\u5728\u904d\u5386\u94fe\u8868\uff0c\u76f4\u5230\u627e\u5230key\u662fSandra\u7684entry\u3002<\/p>\n
2.2 LRU\u7b97\u6cd5<\/h3>\n
\u5728leveldb\u4e2d,LRU\u7b97\u6cd5\u7684\u5b9e\u73b0\u662f\u4f7f\u7528\u4e24\u4e2a\u53cc\u5411\u73af\u5f62\u94fe\u8868\uff0c\u4e00\u4e2a\u94fe\u8868(in-use)\u5b58\u50a8\u5f53\u524d\u6b63\u5728\u4f7f\u7528\u7684\u6570\u636e\uff0c\u53e6\u4e00\u4e2a\u94fe\u8868(lru)\u6309\u7167\u8bbf\u95ee\u65f6\u95f4\u5148\u540e\u987a\u5e8f\u5b58\u50a8\u7f13\u5b58\u6570\u636e\uff0c\u6bcf\u4e2a\u6570\u636e\u90fd\u53ef\u4ee5\u5728in-use\u548clru\u4e4b\u95f4\u5207\u6362\u3002\u5f53\u6211\u4eec\u9700\u8981\u4f7f\u7528LRU\u7b97\u6cd5\u6765\u6dd8\u6c70\u6570\u636e\u65f6\uff0c\u53ea\u9700\u8981\u5728lru\u4e0a\u6dd8\u6c70\u6392\u5e8f\u9760\u540e\u7684\u6570\u636e\u5373\u53ef\u3002<\/p>\n
2.3 \u5206\u7247LRU\u7f13\u5b58<\/h3>\n
\u5206\u7247LRU\u7f13\u5b58\u5f88\u7b80\u5355\uff0c\u5176\u5b9e\u5c31\u662f\u540c\u65f6\u521b\u5efa\u591a\u4e2aLRU\u7f13\u5b58\u5bf9\u8c61\uff0c\u7136\u540e\u4f7f\u7528hash\u5c06\u7279\u5b9a\u7684\u7f13\u5b58\u6570\u636e\u653e\u7f6e\u5230\u76f8\u5e94\u7684LRU\u7f13\u5b58\u5bf9\u8c61\u4e2d\u3002\u8fd9\u4e2a\u65b9\u5f0f\u53ef\u4ee5\u907f\u514d\u4e00\u4e2aLRU\u7f13\u5b58\u4e2d\u5b58\u50a8\u8fc7\u591a\u7684\u6570\u636e\u3002<\/p>\n
\u4e09\u3001\u8be6\u7ec6\u5206\u6790<\/h2>\n3.1 Hash Table<\/h3>\n
leveldb\u4e2d\u5b9e\u73b0\u4e86HashTable\uff0c\u4f7f\u7528\u7684\u662f\u6700\u7ecf\u5178\u7684\u6570\u7ec4+\u94fe\u8868\u7684\u5b9e\u73b0\u3002\u901a\u8fc7hash\u503c\u5b9a\u4f4d\u5230\u6570\u7ec4\u4e2d\u7684\u67d0\u4e00\u5904\uff0c\u7136\u540e\u5728\u8fd9\u4e00\u5904\u7684\u94fe\u8868\u4e0a\u904d\u5386\u67e5\u627e\u3002<\/p>\n
HashTable\u7684\u957f\u5ea6\u662f\u54c8\u5e0c\u7b97\u6cd5\u6548\u7387\u7684\u6700\u5927\u5f71\u54cd\u56e0\u7d20\uff0c\u5982\u679c\u5b58\u5728\u8f83\u591a\u7684hash\u51b2\u7a81\uff0c\u5219\u5728\u94fe\u8868\u4e0a\u904d\u5386\u67e5\u627e\u7684\u65f6\u95f4\u82b1\u8d39\u4f1a\u5f88\u957f\u3002\u6240\u4ee5\u8fd9\u4e2a\u957f\u5ea6\u7684\u9009\u62e9\u662f\u5f88\u91cd\u8981\u7684\uff0c\u6bd4\u5982\u5728Java\u4e2d\u7684HashMap,\u5b9e\u73b0\u4e2d\u6709\u4e00\u4e2a\u8d1f\u8f7d\u56e0\u5b500.7\uff0c\u5982\u679c\u6570\u7ec4\u4e0a\u5df2\u7ecf\u6709\u503c\u7684\u6570\u91cf\u8d85\u8fc7\u603b\u957f\u5ea6\u76840.7\u5c31\u4f1a\u5bf9\u6574\u4e2a\u54c8\u5e0c\u8868resize\u3002\u5728leveldb\u4e2d\uff0c\u8fd9\u4e2aresize\u7684\u9608\u503c\u662f\u5f53\u524d\u6240\u6709Insert\u8fdb\u6765\u7684\u5143\u7d20\u4e2a\u6570\u8d85\u8fc7\u4e86\u6570\u7ec4\u7684\u957f\u5ea6length\uff0c\u5c31\u4f1a\u8fdb\u884cresize\u3002\u8fd9\u91cc\u7b80\u5355\u7684\u5206\u6790\u4e00\u4e0b\u6700\u91cd\u8981\u7684\u67e5\u627e\u548cresize\u8fc7\u7a0b\u3002<\/p>\n
- \n
- \u54c8\u5e0c\u8868\u67e5\u627e\u8fc7\u7a0b<\/li>\n<\/ul>\n
\/\/ Return a pointer to slot that points to a cache entry that\n \/\/ matches key\/hash. If there is no such cache entry, return a\n \/\/ pointer to the trailing slot in the corresponding linked list.\n LRUHandle** FindPointer(const Slice& key, uint32_t hash) {\n LRUHandle** ptr = &list_[hash & (length_ - 1)]; \/\/\u8fd9\u91cchash & (length_ - 1)\u4f1a\u5b9a\u4f4d\u5230\u5c0f\u4e8elength\u7684\u4f4d\u7f6e,ptr\u5c31\u662fhash\u5230\u7684\u4f4d\u7f6e\n while (*ptr != NULL &&\n ((*ptr)->hash != hash || key != (*ptr)->key())) {\n \/\/hash\u5230\u7279\u5b9a\u4f4d\u7f6e\u540e\uff0c\u5982\u679c\u5f53\u524d\u4f4d\u7f6e\u7684hash\u548c\u5f53\u524dhash\u4e0d\u4e00\u6837\uff0c\u6216\u8005key\u4e0d\u4e00\u6837\uff0c\u5e76\u4e14\u6307\u9488\u4e5f\u4e0d\u4e3a\u7a7a\uff0c\u5219\u7ee7\u7eed\u5411\u4e0b\u627e\uff0c\u76f4\u5230\u627e\u5230\n ptr = &(*ptr)->next_hash;\n }\n return ptr;\n }\n<\/code><\/pre>\n- \n
- \u54c8\u5e0c\u8868resize\u7684\u8fc7\u7a0b<\/li>\n<\/ul>\n
void Resize() {\n uint32_t new_length = 4; \/\/\u54c8\u5e0c\u8868\u7684\u957f\u5ea6\u4ece4\u5f00\u59cb\uff0c\u9010\u500d\u589e\u957f\n while (new_length < elems_) {\n new_length *= 2;\n }\n LRUHandle** new_list = new LRUHandle*[new_length];\n memset(new_list, 0, sizeof(new_list[0]) * new_length); \/\/\u7533\u8bf7\u65b0\u7684\u54c8\u5e0c\u8868\u7684\u6240\u9700\u7684\u7a7a\u95f4\n uint32_t count = 0;\n for (uint32_t i = 0; i < length_; i++) { \/\/\u5c06\u65e7\u7684\u54c8\u5e0c\u8868\u7684\u6570\u636e\u91cd\u65b0\u8ba1\u7b97\u590d\u5236\u5230\u65b0\u7684\u54c8\u5e0c\u8868\u4e2d\n LRUHandle* h = list_[i];\n while (h != NULL) {\n LRUHandle* next = h->next_hash;\n uint32_t hash = h->hash;\n LRUHandle** ptr = &new_list[hash & (new_length - 1)];\n h->next_hash = *ptr;\n *ptr = h;\n h = next;\n count++;\n }\n }\n assert(elems_ == count);\n delete[] list_; \/\/\u5220\u9664\u65e7\u7684\u54c8\u5e0c\u8868\n list_ = new_list;\n length_ = new_length;\n }\n};\n<\/code><\/pre>\n3.2 LRUCache\u7684\u5b9e\u73b0<\/h3>\n
\u8fd9\u91cc\u7684LRUCache\u7684\u79c1\u6709\u6210\u5458\u53d8\u91cf\u5305\u62ec\u4ee5\u4e0b\u51e0\u4e2a<\/p>\n
\/\/ Initialized before use.\n size_t capacity_; \/\/LRUCache\u7684\u5927\u5c0f\u3002\n\n \/\/ mutex_ protects the following state.\n mutable port::Mutex mutex_; \/\/\u4e92\u65a5\u53d8\u91cf\uff0c\u7528\u6765\u540c\u6b65\u8bbf\u95ee\n size_t usage_; \/\/LRUCache\u5df2\u7ecf\u4f7f\u7528\u7684\u5927\u5c0f\n\n \/\/ Dummy head of LRU list.\n \/\/ lru.prev is newest entry, lru.next is oldest entry.\n \/\/ Entries have refs==1 and in_cache==true.\n LRUHandle lru_; \/\/LRU\u94fe\u8868\u7684\u5934\uff0clru.prev\u4ee3\u8868\u65b0\u7684\u8282\u70b9\uff0clru.next\u4ee3\u8868\u65e7\u7684\u8282\u70b9\uff0c\u8282\u70b9\u7684refs == 1,in_cache == true\n\n \/\/ Dummy head of in-use list.\n \/\/ Entries are in use by clients, and have refs >= 2 and in_cache==true.\n LRUHandle in_use_; \/\/\u6b63\u5728\u4f7f\u7528\u7684\u8282\u70b9\u94fe\u8868\u7684\u5934\uff0crefs >= 2\uff0cin_cache == true\n\n HandleTable table_; \/\/\u54c8\u5e0c\u8868\uff0c\u7528\u6765\u5728\u7f13\u5b58\u4e2d\u5b9e\u73b0\u5feb\u901f\u67e5\u627e\n<\/code><\/pre>\nLRU\u7684\u6784\u9020\u51fd\u6570\uff0c\u76f4\u63a5\u6784\u9020\u51fa\u7a7a\u7684\u73af\u5f62\u94fe\u8868<\/p>\n
LRUCache::LRUCache()\n : usage_(0) {\n \/\/ Make empty circular linked lists.\n lru_.next = &lru_;\n lru_.prev = &lru_;\n in_use_.next = &in_use_;\n in_use_.prev = &in_use_;\n}\n<\/code><\/pre>\nLRU\u4e2d\u8282\u70b9\u7684\u5f15\u7528\u548c\u89e3\u5f15\u7528<\/p>\n
\/\/\u589e\u52a0\u5f15\u7528\u65f6\uff0c\u5982\u679c\u5728\u7f13\u5b58\u4e2d\uff0c\u5219\u79fb\u52a8\u5230in_use\u4e2d\nvoid LRUCache::Ref(LRUHandle* e) {\n if (e->refs == 1 && e->in_cache) { \/\/ If on lru_ list, move to in_use_ list.\n LRU_Remove(e);\n LRU_Append(&in_use_, e);\n }\n e->refs++;\n}\n\n\/\/\u89e3\u5f15\u7528\u65f6\u4f1a\u51fa\u73b0\u4e24\u79cd\u60c5\u51b5\u30021.\u8282\u70b9\u4e0d\u518d\u9700\u8981\uff0c\u4f7f\u7528deleter\u6765\u5220\u9664\u8282\u70b9 2.\u4e0d\u518d\u88ab\u4f7f\u7528\uff0c\u79fb\u52a8\u5230\u7f13\u5b58\nvoid LRUCache::Unref(LRUHandle* e) {\n assert(e->refs > 0);\n e->refs--;\n if (e->refs == 0) { \/\/ Deallocate.\n assert(!e->in_cache);\n (*e->deleter)(e->key(), e->value);\n free(e);\n } else if (e->in_cache && e->refs == 1) { \/\/ No longer in use; move to lru_ list.\n LRU_Remove(e);\n LRU_Append(&lru_, e);\n }\n}\n<\/code><\/pre>\n\u7f13\u5b58\u7684\u63d2\u5165<\/p>\n
Cache::Handle* LRUCache::Insert(\n const Slice& key, uint32_t hash, void* value, size_t charge,\n void (*deleter)(const Slice& key, void* value)) {\n MutexLock l(&mutex_);\n\n LRUHandle* e = reinterpret_cast<LRUHandle*>(\n malloc(sizeof(LRUHandle)-1 + key.size())); \/\/\u8fd9\u4e2a\u5730\u65b9\u7533\u8bf7\u5185\u5b58\u9700\u8981\u6ce8\u610f\uff0c\u5728LRUHandle\u4e2d\uff0ckey_data\u53ea\u67091\u4e2a\u5b57\u8282\uff0c\u5176\u5b9e\u662f\u6574\u4e2akey\u7684\u5f00\u5934\u4e00\u4e2a\u5b57\u8282\uff0c\u6240\u4ee5\u7533\u8bf7\u7684\u7a7a\u95f4\u5b9e\u9645\u4e0a\u662f\u5305\u542b\u6574\u4e2akey\u7684\n e->value = value;\n e->deleter = deleter;\n e->charge = charge;\n e->key_length = key.size();\n e->hash = hash;\n e->in_cache = false;\n e->refs = 1; \/\/ for the returned handle.\n memcpy(e->key_data, key.data(), key.size());\n\n if (capacity_ > 0) { \/\/\u5982\u679ccapacity\u5927\u4e8e0\uff0c\u4e5f\u5c31\u662f\u9700\u8981\u8fdb\u884c\u7f13\u5b58\n e->refs++; \/\/ for the cache's reference.\n e->in_cache = true;\n LRU_Append(&in_use_, e);\n usage_ += charge;\n FinishErase(table_.Insert(e));\n } \/\/ else don't cache. (Tests use capacity_==0 to turn off caching.)\n\n \/\/\u5f53\u4f7f\u7528\u7684\u5185\u5b58\u5927\u4e8e\u5bb9\u91cf\u65f6\uff0c\u5219\u8981\u79fb\u9664\u65e7\u7684\u7f13\u5b58\uff0c\u76f4\u5230\u7f13\u5b58\u5c0f\u4e8e\u6307\u5b9a\u7684\u5bb9\u91cf\n while (usage_ > capacity_ && lru_.next != &lru_) {\n LRUHandle* old = lru_.next;\n assert(old->refs == 1);\n bool erased = FinishErase(table_.Remove(old->key(), old->hash));\n if (!erased) { \/\/ to avoid unused variable when compiled NDEBUG\n assert(erased);\n }\n }\n\n return reinterpret_cast<Cache::Handle*>(e);\n}\n<\/code><\/pre>\n\u7f13\u5b58\u7684\u67e5\u627e<\/p>\n
Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {\n MutexLock l(&mutex_);\n LRUHandle* e = table_.Lookup(key, hash); \/\/\u76f4\u63a5\u501f\u7528\u54c8\u5e0c\u8868\u5b8c\u6210\u7f13\u5b58\u7684\u5feb\u901f\u67e5\u627e\n if (e != NULL) {\n Ref(e);\n }\n return reinterpret_cast<Cache::Handle*>(e);\n}\n<\/code><\/pre>\n3.3 \u5206\u7247\u7f13\u5b58\u7684\u5b9e\u73b0(ShardedLRUCache)<\/h3>\n
\u5206\u7247\u7f13\u5b58\u7684\u5b9e\u73b0\u5176\u5b9e\u5c31\u662f\u501f\u52a9\u4e0a\u9762\u7684LRUCaChe\uff0c\u53ea\u4e0d\u8fc7\u540c\u65f6\u62e5\u6709\u591a\u4e2aLRUCache\uff0c\u7136\u540e\u7279\u5b9a\u7684\u7f13\u5b58\u6570\u636e\u4f1a\u88ab\u7f13\u5b58\u5230\u76f8\u5e94\u7684LRUCache\u4e2d\u3002<\/p>\n
static const int kNumShardBits = 4; \/\/\u53ef\u4ee5\u7406\u89e3\u4e3a\u7f13\u5b58\u7247\u6570\u91cf\u7684\u5bb9\u91cf\u56e0\u5b50(\u8fd9\u91cc\u662f4\u4e2abits\uff0c\u6240\u4ee5\u5171\u670916\u4e2a\u7f13\u5b58\u7247\uff09\nstatic const int kNumShards = 1 << kNumShardBits; \/\/\u4e00\u5171\u6709\u591a\u5c11\u4e2a\u7f13\u5b58\u7247\n\n\/\/\u901a\u8fc7Shard\u51fd\u6570\u53ef\u4ee5\u5c06\u4efb\u610f\u4e00\u4e2ahash\u5206\u914d\u523016\u4e2a\u7f13\u5b58\u7247\u4e2d\u7684\u4efb\u610f\u4e00\u4e2a)\nstatic uint32_t Shard(uint32_t hash) {\n return hash >> (32 - kNumShardBits);\n}\n<\/code><\/pre>\n\u6240\u6709\u7684\u7f13\u5b58\u5b58\u50a8\u548c\u8bfb\u53d6\u90fd\u4f1a\u901a\u8fc7\u4e0a\u9762\u7684Shard\u51fd\u6570\u6765\u627e\u5230\u7279\u5b9a\u7684\u7f13\u5b58\u7247\uff0c\u4ece\u800c\u5b9e\u73b0\u4e86\u5206\u7247\u7f13\u5b58\u3002<\/p>\n
\u4e94\u3001\u603b\u7ed3<\/h2>\n
\u5728\u8fd9\u4e2a\u90e8\u5206\uff0c400\u884c\u5de6\u53f3\u7684\u4ee3\u7801\u5c31\u5b9e\u73b0\u4e86\u54c8\u5e0c\u8868\uff0cLRU\u7f13\u5b58\uff0c\u5206\u7247LRU\u7f13\u5b58\u3002\u4e86\u89e3\u5230LRU\u7f13\u5b58\u7684\u5177\u4f53\u5b9e\u73b0\u65b9\u5f0f\uff0c\u7279\u522b\u662fLRU\u4e2dRef\u548cUnRef\u7684\u5b9e\u73b0\u975e\u5e38\u7cbe\u70bc\uff0c\u5f88\u597d\u7684\u89e3\u51b3\u4e86\u4e00\u4e2a\u6570\u636e\u5728LRU\u7f13\u5b58\u4e2d\u88ab\u5f15\u7528\u65f6\u8131\u79bb\u7f13\u5b58\uff0c\u4e0d\u4f7f\u7528\u65f6\u8fdb\u5165LRU\u7f13\u5b58\u7684\u529f\u80fd\u3002\u540c\u65f6\u5c06LRU\u7f13\u5b58\u7b80\u5355\u7684\u5c01\u88c5\uff0c\u5c31\u53ef\u4ee5\u5b9e\u73b0\u5206\u7247LRU\u7f13\u5b58\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"
- \u54c8\u5e0c\u8868resize\u7684\u8fc7\u7a0b<\/li>\n<\/ul>\n