【Redis】数据结构之dict

dict的基本结构

typedef struct dict {dictType *type;void *privdata;dictht ht[2];long rehashidx; /* rehashing not in progress if rehashidx == -1 */unsigned long iterators; /* number of iterators currently running */
} dict;

Redis中dict结构体包含了两个ditcht，这是为了rehash。

typedef struct dictType {uint64_t (*hashFunction)(const void *key);void *(*keyDup)(void *privdata, const void *key);void *(*valDup)(void *privdata, const void *obj);int (*keyCompare)(void *privdata, const void *key1, const void *key2);void (*keyDestructor)(void *privdata, void *key);void (*valDestructor)(void *privdata, void *obj);
} dictType;

提供了dictType，我认为这是用C语言实现的编译时多态，在创建dict时需要将dictType传入，不同的dictType可以提供不同的hashFunction、keyDup、keyCompare函数。

typedef struct dictht {dictEntry **table;unsigned long size;unsigned long sizemask;unsigned long used;
} dictht;

dicht的结构中有dictEntry **table这是一个指针数组，可以理解为是哈希表的array部分。

typedef struct dictEntry {void *key;union {void *val;uint64_t u64;int64_t s64;double d;} v;struct dictEntry *next;
} dictEntry;

这是dict中每个entry的结构，除了k和不同类型的v意外，还有struct dictEntry *next;体现了这是一个链哈希，即如果发生哈希冲突，就通过链表指针来组织起所有位于同一个哈希槽的entry。

dict的相关操作函数

底层通用的之查找插入key-value对应该放入ht表的哪个槽

/* Returns the index of a free slot that can be populated with* an hash entry for the given 'key'.* If the key already exists, -1 is returned. */
static int _dictKeyIndex(dict *ht, const void *key) {unsigned int h;dictEntry *he;/* Expand the hashtable if needed */if (_dictExpandIfNeeded(ht) == DICT_ERR)return -1;/* Compute the key hash value */h = dictHashKey(ht, key) & ht->sizemask;/* Search if this slot does not already contain the given key */he = ht->table[h];while(he) {if (dictCompareHashKeys(ht, key, he->key))return -1;he = he->next;}return h;
}

/* Expand the hash table if needed */
static int _dictExpandIfNeeded(dict *d)
{/* Incremental rehashing already in progress. Return. */if (dictIsRehashing(d)) return DICT_OK;/* If the hash table is empty expand it to the initial size. */if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);/* If we reached the 1:1 ratio, and we are allowed to resize the hash* table (global setting) or we should avoid it but the ratio between* elements/buckets is over the "safe" threshold, we resize doubling* the number of buckets. */if (d->ht[0].used >= d->ht[0].size &&(dict_can_resize ||d->ht[0].used/d->ht[0].size > dict_force_resize_ratio)){return dictExpand(d, d->ht[0].used*2);}return DICT_OK;
}

在dictExpand函数中，有

    _dictInit(&n, ht->type, ht->privdata);n.size = realsize;n.sizemask = realsize-1;n.table = calloc(realsize,sizeof(dictEntry*));

所以，一个key应该插入到ht的哪个槽呢？就是_dictKeyIndex中的这一句

 h = dictHashKey(ht, key) & ht->sizemask;

可以保证h的值在[0,size-1]之间，而这些槽已经被初始化了：

n.table = calloc(realsize,sizeof(dictEntry*));

常规的dictAdd、dictDelete比较简单。

rehash过程

值得一提的是rehash过程。

int dictRehash(dict *d, int n) {int empty_visits = n*10; /* Max number of empty buckets to visit. */if (!dictIsRehashing(d)) return 0;while(n-- && d->ht[0].used != 0) {dictEntry *de, *nextde;/* Note that rehashidx can't overflow as we are sure there are more* elements because ht[0].used != 0 */assert(d->ht[0].size > (unsigned long)d->rehashidx);while(d->ht[0].table[d->rehashidx] == NULL) {d->rehashidx++;if (--empty_visits == 0) return 1;}de = d->ht[0].table[d->rehashidx];/* Move all the keys in this bucket from the old to the new hash HT */while(de) {uint64_t h;nextde = de->next;/* Get the index in the new hash table */h = dictHashKey(d, de->key) & d->ht[1].sizemask;de->next = d->ht[1].table[h];d->ht[1].table[h] = de;d->ht[0].used--;d->ht[1].used++;de = nextde;}d->ht[0].table[d->rehashidx] = NULL;d->rehashidx++;}/* Check if we already rehashed the whole table... */if (d->ht[0].used == 0) {zfree(d->ht[0].table);d->ht[0] = d->ht[1];_dictReset(&d->ht[1]);d->rehashidx = -1;return 0;}/* More to rehash... */return 1;
}

整体来看就是ht[0]的尺寸太小了，为了效率，需要把ht[0]的所有元素都搬运到扩展了尺寸的ht[1]中。
返回值为1说明rehash还没有完成。
返回值为0说明rehash已经完成。并且已经交换了ht[0]和ht[1]，之后的命令写入可以往ht[0]里写了。

int dictRehashMilliseconds(dict *d, int ms) {long long start = timeInMilliseconds();int rehashes = 0;while(dictRehash(d,100)) {rehashes += 100;if (timeInMilliseconds()-start > ms) break;}return rehashes;
}

int incrementallyRehash(int dbid) {/* Keys dictionary */if (dictIsRehashing(server.db[dbid].dict)) {dictRehashMilliseconds(server.db[dbid].dict,1);return 1; /* already used our millisecond for this loop... */}/* Expires */if (dictIsRehashing(server.db[dbid].expires)) {dictRehashMilliseconds(server.db[dbid].expires,1);return 1; /* already used our millisecond for this loop... */}return 0;
}

实际上的rehash是在databasesCron函数里做的，incrementallyRehash指定了每次进行rehash的dict和时长(1 ms)。而dicthash()又设置了每次最多进行n个槽和n*10个空槽的遍历。