[redis设计与实现][3]基本数据结构——字典

Redis字典采用哈希表实现。
哈希表:
[cce lang="c"]
typedef struct dictht {
//哈希表数组
dictEntry **table;
//哈希表大小
unsigned long size;
//哈希表掩码,用于计算索引值,总是等于size - 1
unsigned long size mask;
//已有的节点数量
unsigned long used;
} dictht;
[/cce]

哈希表节点:
[cce lang="c"]
typedef struct dictEntry {
//键
void *key;
//值
union {
void *val;
uint64_t u64;
int64_t s64;
double d;
} v;
//下一个哈希表节点
struct dictEntry *next;
} dictEntry;
[/cce]

字典:
[cce lang="c"]
typedef struct dict {
//类型特定的函数
dictType *type;
//私有数据
void *privdata;
//哈希表
dictht ht[2];
long rehashidx; /* rehashing not in progress if rehashidx == -1 */
int iterators; /* number of iterators currently running */
} dict;
[/cce]

* type属性是一个指向dictType结构的指针,每个dictType结构保存了一簇用于操作特定类型键值对的函数
* privdata保存了需要传给类型特定函数的可选参数

[cce lang="c"]
typedef struct dictType {
//计算哈希值的函数
unsigned int (*hashFunction)(const void *key);
//复制键的函数
void *(*keyDup)(void *privdata, const void *key);
//复制值的函数
void *(*valDup)(void *privdata, const void *obj);
//对比键的函数
int (*keyCompare)(void *privdata, const void *key1, const void *key2);
//销毁键的函数
void (*keyDestructor)(void *privdata, void *key);
//销毁值的函数
void (*valDestructor)(void *privdata, void *obj);
} dictType;
[/cce]

ht属性包含两个项的数组。一般情况下只使用ht[0]哈希表,ht[1]哈希表只在对ht[0]进行rehash的时候才会使用。

哈希算法:
计算:
hash = dict->type->hashFunction(key)
index = hash & dict->ht[x].sizemask
当字典被用作数据库的底层实现,或者哈希键的底层实现时,Redis使用MurmurHash2(https://code.google.com/p/smhasher/wiki/MurmurHash2)算法计算键的哈希值。

int dictAdd(dict *d, void *key, void *val);
[cce lang="c"]
int dictAdd(dict *d, void *key, void *val)
{
dictEntry *entry = dictAddRaw(d,key);

if (!entry) return DICT_ERR;
dictSetVal(d, entry, val);
return DICT_OK;
}
dictEntry *dictAddRaw(dict *d, void *key)
{
int index;
dictEntry *entry;
dictht *ht;
//#define dictIsRehashing(d) ((d)->rehashidx != -1)
if (dictIsRehashing(d)) _dictRehashStep(d);

/* Get the index of the new element, or -1 if
* the element already exists. */
if ((index = _dictKeyIndex(d, key)) == -1)
return NULL;

/* Allocate the memory and store the new entry */
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;
}
static int _dictKeyIndex(dict *d, const void *key)
{
unsigned int h, idx, table;
dictEntry *he;

/* Expand the hash table if needed */
if (_dictExpandIfNeeded(d) == DICT_ERR)
return -1;
/* Compute the key hash value */
//#define dictHashKey(d, key) (d)->type->hashFunction(key)
h = dictHashKey(d, key);
for (table = 0; table <= 1; table++) { idx = h & d->ht[table].sizemask;
/* Search if this slot does not already contain the given key */
he = d->ht[table].table[idx];
while(he) {
//比较key是否已经存在,已经存在返回-1
if (dictCompareKeys(d, key, he->key))
return -1;
he = he->next;
}
if (!dictIsRehashing(d)) break;
}
return idx;
}
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. */
//#define DICT_HT_INITIAL_SIZE 4
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. */
//static unsigned int dict_force_resize_ratio = 5;
/*
dict_can_resize设置:
void updateDictResizePolicy(void) {
if (server.rdb_child_pid == -1 && server.aof_child_pid == -1)
dictEnableResize();
else
dictDisableResize();
}
当有同步硬盘进程的时候改成不能扩充
*/
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;
}
int dictExpand(dict *d, unsigned long size)
{
dictht n; /* the new hash table */
unsigned long realsize = _dictNextPower(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;

/* 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;
}
int dictReplace(dict *d, void *key, void *val);

/* Add an element, discarding the old 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, auxentry;

/* Try to add the element. If the key
* does not exists dictAdd will suceed. */
if (dictAdd(d, key, val) == DICT_OK)
return 1;
/* It already exists, get the entry */
entry = dictFind(d, key);
/* 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 = *entry;
dictSetVal(d, entry, val);
dictFreeVal(d, &auxentry);
return 0;
}
dictEntry *dictFind(dict *d, const void *key)
{
dictEntry *he;
unsigned int h, idx, table;

if (d->ht[0].size == 0) return NULL; /* We don't have a table at all */
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 (dictCompareKeys(d, key, he->key))
return he;
he = he->next;
}
if (!dictIsRehashing(d)) return NULL;
}
return NULL;
}
[/cce]

int dictRehash(dict *d, int n);
[cce lang="c"]
int dictRehash(dict *d, int n) {
if (!dictIsRehashing(d)) return 0;

while(n--) {
dictEntry *de, *nextde;

/* Check if we already rehashed the whole table... */
//已经完成hash,释放ht[0]。将ht[0]指向ht[1]
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;
}

/* 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->rehashed);
//如果rehash索引为空,跳过
while(d->ht[0].table[d->rehashidx] == NULL) d->rehashidx++;
de = d->ht[0].table[d->rehashidx];
/* Move all the keys in this bucket from the old to the new hash HT */
//移动一个桶里面的所有key到新的哈希表
while(de) {
unsigned int 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++;
}
return 1;
}

//为了防止占用太多的CPU时间,限制一次rehash的CPU时间
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;
}
[/cce]

调用者(redis.c):每次尝试渐进式rehash执行1ms
[cce lang="c"]
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;
}
[/cce]

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