Redis 源码分析-内部数据结构 dict

Redis 源码分析-内部数据结构 dict

在上一篇 Redis 数据库源码分析 提到了 Redis 其实用了全局的 hash 表来存储所有的键值对，即下方图示的 dict，dict 中有两个数组，其中 ht[1] 只在 rehash 时候才真正用到，平时都是指向 null，hash 的底层又是使用数组来实现的，那就引出了接下来的几个问题：

• redis 启动初始化这个 dict 时数组容量是多少
• 如果发生了 hash 冲突怎么办
• 扩容机制（扩容的时机，扩容的实现)

dict 的创建

/* Create a new hash table */
dict *dictCreate(dictType *type, void *privDataPtr)
{dict *d = zmalloc(sizeof(*d));_dictInit(d,type,privDataPtr);return d;
}/* Initialize the hash table */
int _dictInit(dict *d, dictType *type, void *privDataPtr)
{_dictReset(&d->ht[0]);_dictReset(&d->ht[1]);d->type = type;			d->privdata = privDataPtr;d->rehashidx = -1;d->iterators = 0;return DICT_OK;
}/* Reset a hash table already initialized with ht_init().* NOTE: This function should only be called by ht_destroy(). */
static void _dictReset(dictht *ht)
{ht->table = NULL;ht->size = 0;		// table 的长度, 一定是2的倍数ht->sizemask = 0;	// size - 1,具体原因参见上篇ht->used = 0;		// 已经存的元素总数
}

代码分析：

• 2行，创建变量时的 dictType *type，包含很多函数指针，无论是添加或查找 key 时 hash 函数还是删除 dictEntry 时的析构函数
• 25行，ht->table = NULL，也就是说 dict 在被创建时 table 并没有分配内存空间，意味着第一个数据插入时才会真正分配空间。

dict 的扩容

/* 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;
}/* Expand or create the hash table */
int dictExpand(dict *d, unsigned long size)
{/* the size is invalid if it is smaller than the number of* elements already inside the hash table */if (dictIsRehashing(d) || d->ht[0].used > size)return DICT_ERR;dictht n; /* the new hash table */unsigned long realsize = _dictNextPower(size);	// _dictNextPower 根据传入的 size 获得最终分配的 size/* Rehashing to the same table size is not useful. */if (realsize == d->ht[0].size) return DICT_ERR;/* Allocate the new hash table and initialize all pointers to NULL */n.size = realsize;n.sizemask = realsize-1;n.table = zcalloc(realsize*sizeof(dictEntry*));n.used = 0;/* Is this the first initialization? If so it's not really a rehashing* we just set the first hash table so that it can accept keys. */if (d->ht[0].table == NULL) {d->ht[0] = n;return DICT_OK;}/* Prepare a second hash table for incremental rehashing */d->ht[1] = n;d->rehashidx = 0;return DICT_OK;
}/* Our hash table capability is a power of two */
static unsigned long _dictNextPower(unsigned long size)
{unsigned long i = DICT_HT_INITIAL_SIZE;if (size >= LONG_MAX) return LONG_MAX + 1LU;while(1) {if (i >= size)return i;i *= 2;}
}

代码分析：

• 8行，如果是第一次插入（ht[0].size == 0），用初始化大小（DICT_HT_INITIAL_SIZE）分配，这个值默认是 4
• 14行，扩容的判断条件，负载因子（d->ht[0].used / d->ht[0].size），即 ht 中元素总数与 bucket 个数的比值
  • 负载因子 >= 1，且 dict_can_resize，dict_can_resize 的值受此时 redis 状态的影响，例如 redis 在进行 aof 重写或 rdb生成时，不想造成太多的内存拷贝，就会关闭
  • 负载因子 >= 5，即 dict_force_resize_ratio 的默认值，此时 hash 冲突已经很严重了，不管此时处于什么状态，都会进行扩容

dict 的销毁

/* Clear & Release the hash table */
void dictRelease(dict *d)
{_dictClear(d,&d->ht[0],NULL);_dictClear(d,&d->ht[1],NULL);zfree(d);
}/* Destroy an entire dictionary */
int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {unsigned long i;/* Free all the elements */for (i = 0; i < ht->size && ht->used > 0; i++) {dictEntry *he, *nextHe;if (callback && (i & 65535) == 0) callback(d->privdata);if ((he = ht->table[i]) == NULL) continue;while(he) {nextHe = he->next;dictFreeKey(d, he);dictFreeVal(d, he);zfree(he);ht->used--;he = nextHe;}}/* Free the table and the allocated cache structure */zfree(ht->table);/* Re-initialize the table */_dictReset(ht);return DICT_OK; /* never fails */
}#define dictFreeKey(d, entry) \if ((d)->type->keyDestructor) \(d)->type->keyDestructor((d)->privdata, (entry)->key)
#define dictFreeVal(d, entry) \if ((d)->type->valDestructor) \(d)->type->valDestructor((d)->privdata, (entry)->v.val)

代码分析：

• 20-27行：删除每个 dictEntry 时会调用 key 和 value 的析构函数（keyDestructor 和 valDestructor），用于释放键和值所占的内存。

dict 添加元素

主要涉及两个函数，dictAdd 和 dictReplace

dictAdd

该函数向 hash table 中添加数据，如果 key 已经存在会返回失败

/* Add an element to the target hash table */
int dictAdd(dict *d, void *key, void *val)
{dictEntry *entry = dictAddRaw(d,key,NULL);if (!entry) return DICT_ERR;dictSetVal(d, entry, val);return DICT_OK;
}/* Low level add or find:* This function adds the entry but instead of setting a value returns the* dictEntry structure to the user, that will make sure to fill the value* field as he wishes.** This function is also directly exposed to the user API to be called* mainly in order to store non-pointers inside the hash value, example:** entry = dictAddRaw(dict,mykey,NULL);* if (entry != NULL) dictSetSignedIntegerVal(entry,1000);** Return values:** If key already exists NULL is returned, and "*existing" is populated* with the existing entry if existing is not NULL.** If key was added, the hash entry is returned to be manipulated by the caller.*/
dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing)
{long index;dictEntry *entry;dictht *ht;if (dictIsRehashing(d)) _dictRehashStep(d);/* Get the index of the new element, or -1 if* the element already exists. */if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1)return NULL;/* Allocate the memory and store the new entry.* Insert the element in top, with the assumption that in a database* system it is more likely that recently added entries are accessed* more frequently. */ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];entry = zmalloc(sizeof(*entry));entry->next = ht->table[index];ht->table[index] = entry;ht->used++;/* Set the hash entry fields. */dictSetKey(d, entry, key);return entry;
}

代码分析：

• 34行判断此时是否正在 rehash，如果在，则将 rehash 向前推进一步
• 38行，_dictKeyIndex函数根据传入的 key 寻找插入的位置，如果该 key 已经存在，会返回-1
• 45行，如果此时正在 rehash，就把数据插入到 ht[1]，否则插入到 ht[0]
• 47行-49行，使用头插法插入数据

上面有两个关键函数，一个是_dictKeyIndex，另一个是_dictRehashStep

_dictKeyIndex

/* Returns the index of a free slot that can be populated with* a hash entry for the given 'key'.* If the key already exists, -1 is returned* and the optional output parameter may be filled.** Note that if we are in the process of rehashing the hash table, the* index is always returned in the context of the second (new) hash table. */
static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing)
{unsigned long idx, table;dictEntry *he;if (existing) *existing = NULL;/* Expand the hash table if needed */if (_dictExpandIfNeeded(d) == DICT_ERR)return -1;for (table = 0; table <= 1; table++) {idx = hash & d->ht[table].sizemask;/* Search if this slot does not already contain the given key */he = d->ht[table].table[idx];while(he) {if (key==he->key || dictCompareKeys(d, key, he->key)) {if (existing) *existing = he;return -1;}he = he->next;}if (!dictIsRehashing(d)) break;}return idx;
}

代码分析：

• 15行，会判断此时是否需要扩容
• 28行，查找时会先从 ht[0] 查找，如果 ht[0] 中没有数据且此时不在进行 rehash，就直接返回

_dictKeyIndex 如果 key 已经存在会返回 -1，否则返回该 key 需要插入的桶的索引。

_dictRehashStep

/* This function performs just a step of rehashing, and only if there are* no safe iterators bound to our hash table. When we have iterators in the* middle of a rehashing we can't mess with the two hash tables otherwise* some element can be missed or duplicated.** This function is called by common lookup or update operations in the* dictionary so that the hash table automatically migrates from H1 to H2* while it is actively used. */
static void _dictRehashStep(dict *d) {if (d->iterators == 0) dictRehash(d,1);
}/* Performs N steps of incremental rehashing. Returns 1 if there are still* keys to move from the old to the new hash table, otherwise 0 is returned.** Note that a rehashing step consists in moving a bucket (that may have more* than one key as we use chaining) from the old to the new hash table, however* since part of the hash table may be composed of empty spaces, it is not* guaranteed that this function will rehash even a single bucket, since it* will visit at max N*10 empty buckets in total, otherwise the amount of* work it does would be unbound and the function may block for a long time. */
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;
}

代码分析：

• 32-35行，如果 rehashidx 指向的 bucket 里一个 dictEntry 也没有，那么它就没有可迁移的数据。这时它尝试在 ht[0].table 数组中不断向后遍历，直到找到下一个存有数据的 bucket 位置。如果一直找不到，则最多走 n*10 步（empty_visits）。避免在该操作上一直向后遍历，影响响应时间。

• 43行，原 bucket 里的元素重新 hash 计算后对新的 sizemask 进行余操作，确定在新 table 上的位置。

• 55-61行，如果 ht[0] 上的数据都迁移到 ht[1] 上了（即d->ht[0].used == 0），那么整个重哈希结束，ht[0] 变成 ht[1] 的内容，而 ht[1] 重置为空。

参考下面这张图：哈希桶即 bucket，entry 即 dictEntry

dictReplace

该函数向 hash table 中添加数据，如果 key 已经存在会替换

/* Add or Overwrite:* Add an element, discarding the old value if the key already exists.* Return 1 if the key was added from scratch, 0 if there was already an* element with such key and dictReplace() just performed a value update* operation. */
int dictReplace(dict *d, void *key, void *val)
{dictEntry *entry, *existing, auxentry;/* Try to add the element. If the key* does not exists dictAdd will succeed. */entry = dictAddRaw(d,key,&existing);if (entry) {dictSetVal(d, entry, val);return 1;}/* Set the new value and free the old one. Note that it is important* to do that in this order, as the value may just be exactly the same* as the previous one. In this context, think to reference counting,* you want to increment (set), and then decrement (free), and not the* reverse. */auxentry = *existing;dictSetVal(d, existing, val);dictFreeVal(d, &auxentry);return 0;
}

代码分析：

• 12行，如果插入成功（key不存在）entry 就是插入的对象，如果 key 已经存在，则 existing 变量就是对应的 dictEntry
• 23-26行，注意顺序是先创建并设置新值，再释放旧值。

dict 删除元素

/* Remove an element, returning DICT_OK on success or DICT_ERR if the* element was not found. */
int dictDelete(dict *ht, const void *key) {return dictGenericDelete(ht,key,0) ? DICT_OK : DICT_ERR;
}/* Search and remove an element. This is an helper function for* dictDelete() and dictUnlink(), please check the top comment* of those functions. */
static dictEntry *dictGenericDelete(dict *d, const void *key, int nofree) {uint64_t h, idx;dictEntry *he, *prevHe;int table;if (d->ht[0].used == 0 && d->ht[1].used == 0) return NULL;if (dictIsRehashing(d)) _dictRehashStep(d);h = dictHashKey(d, key);for (table = 0; table <= 1; table++) {idx = h & d->ht[table].sizemask;he = d->ht[table].table[idx];prevHe = NULL;while(he) {if (key==he->key || dictCompareKeys(d, key, he->key)) {/* Unlink the element from the list */if (prevHe)prevHe->next = he->next;elsed->ht[table].table[idx] = he->next;if (!nofree) {dictFreeKey(d, he);dictFreeVal(d, he);zfree(he);}d->ht[table].used--;return he;}prevHe = he;he = he->next;}if (!dictIsRehashing(d)) break;}return NULL; /* not found */
}

代码分析：

• 17行，如果在 rehash，也推进一步
• 25-38行，其实就是首先根据 hash 找到对应的 bucket，在链表中删除该元素，找不到则返回 null，根据是否在 rehash 决定只查找 ht[0] 还是 ht[1]
• 32-33行，删除后会调用 key 和 value 的析构函数（keyDestructor 和 valDestructor）

dict 查找元素

dictEntry *dictFind(dict *d, const void *key)
{dictEntry *he;uint64_t h, idx, table;if (d->ht[0].used + d->ht[1].used == 0) return NULL; /* dict is empty */if (dictIsRehashing(d)) _dictRehashStep(d);h = dictHashKey(d, key);for (table = 0; table <= 1; table++) {idx = h & d->ht[table].sizemask;he = d->ht[table].table[idx];while(he) {if (key==he->key || dictCompareKeys(d, key, he->key))return he;he = he->next;}if (!dictIsRehashing(d)) return NULL;}return NULL;
}

代码分析：

• 7行，如果此时在 rehash，它也会推进一步
• 10行，hash 值与 sizemask 取余 = hash % size，这样写的原因是 & 一步到位，而 % 是一直除下去
• 17行，如果 ht[0] 中没有，且当前不在 rehash，直接返回 null，否则去 ht[1] 中查找

参考资料

【1】Redis(5.0.8)源码

【2】哔哩哔哩-Redis底层设计与源码分析

【3】极客时间-Redis核心技术剖析与实战