redis的hashtable------dict.c
先了解基本的struct
typedef struct dictEntry { void *key; union { void *val; uint64_t u64; int64_t s64; } v; struct dictEntry *next; } dictEntry; typedef struct dictType { unsigned int (*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; /* This is our hash table structure. Every dictionary has two of this as we * implement incremental rehashing, for the old to the new table. */ typedef struct dictht { dictEntry **table; unsigned long size; unsigned long sizemask; unsigned long used; } dictht; typedef struct dict { dictType *type; void *privdata; dictht ht[2]; int rehashidx; /* rehashing not in progress if rehashidx == -1 */ int iterators; /* number of iterators currently running */ } dict;
这里先提醒一下dictType中都是hashtable dictEntry 绑定的指向哈希函数的指针变量,切不要混淆函数指针.
下面逐个分析dict.c中的方法
/* 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; ht->sizemask = 0; ht->used = 0; } /* 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; } zmalloc分配内存给*d,参数dictType *type为*d的哈希函数,reset *d的两个hashtable,设置rehashidx=-1(在上面的结构注释中已说明:如果rehashidex==-1就不在rehashing过程中),设置迭代数为0.
/* Expand or create the hash table */ int dictExpand(dict *d, unsigned long size) { dictht n; /* the new hash table */ unsigned long realsize = _dictNextPower(size); ******* /* 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; while(1) { if (i >= size) return i; i *= 2; } } DICT_HT_INITIAL_SIZE 在dict.h中define为4,hashtable容量都是2的次方,容量不能超过LONG_MAX ******* /* 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; ********** #define dictIsRehashing(ht) ((ht)->rehashidx != -1) 我们*d如果是刚创建的,那肯定是-1,所以dictIsRehashing(d)为false,如果是扩张的,为true,此处used不能大于刚刚扩张的容量 ********* /* 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; } 这里如果*d是新创建的将不会rehashing,否则将刚创建的hashtable n赋值给d的ht[1]以便于进行rehash /* Prepare a second hash table for incremental rehashing */ d->ht[1] = n; d->rehashidx = 0; return DICT_OK; }
/* 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 * thank one key as we use chaining) from the old to the new hash table. */ int dictRehash(dict *d, int n) { if (!dictIsRehashing(d)) return 0; while(n--) { dictEntry *de, *nextde; /* 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; } *********** 如果d->ht[0].used == 0,释放table内存,将1赋值给0,再将1给reset,将rehashidx重新标记为-1(检查我们刚刚已经rehash过table,这样就不用向下去rehash了) *********** /* 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)d->rehashidx); while(d->ht[0].table[d->rehashidx] == NULL) d->rehashidx++; // 取到最先开始有值的table,赋值给de,减少后面的while循环 de = d->ht[0].table[d->rehashidx]; /* Move all the keys in this bucket from the old to the new hash HT */ 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; //通过上述绑定的哈希函数并与sizemask进行位运算得到key 的索引,后面将ht[0]赋值给ht[1],ht[1]的作用就在于此,可以说它就是为rehash而创建的,它相当于一个salve,rehash时作ht[1]的一个备份,最后ht[1] 又会赋值给ht[0],再reset ht[1] 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;//清除ht[0] d->rehashidx++;//将rehash标识+1 } return 1; }
/* 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); } //迭代器为0时,rehash一次,目的就是将ht[1]赋值给ht[0] /* 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 intial size. */ if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE); //这里用到了上面讲到的dictexpand方法, /* 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)) //dict_force_resize_radio定义为5,有点不解,为什么是5倍才expand { return dictExpand(d, ((d->ht[0].size > d->ht[0].used) ? d->ht[0].size : d->ht[0].used)*2); } return DICT_OK; }
/* 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. * * 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 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 */ h = dictHashKey(d, key);//获取key的索引值 //遍历ht[0],ht[1] for (table = 0; table <= 1; table++) { idx = h & d->ht[table].sizemask; //rehash中的 /* Get the index in the new hash table */ h = dictHashKey(d, de->key) & d->ht[1].sizemask; 这里再&,取回idx /* Search if this slot does not already contain the given key */ he = d->ht[table].table[idx]; while(he) { if (dictCompareKeys(d, key, he->key))//哈希函数比较key,存在则返回-1 return -1; he = he->next; } if (!dictIsRehashing(d)) break; } return idx; } /* Low level add. 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 user API to be called * mainly in order to store non-pointers inside the hash value, example: * * entry = dictAddRaw(dict,mykey); * if (entry != NULL) dictSetSignedIntegerVal(entry,1000); * * Return values: * * If key already exists NULL is returned. * If key was added, the hash entry is returned to be manipulated by the caller. */ dictEntry *dictAddRaw(dict *d, void *key) { int index; dictEntry *entry; dictht *ht; if (dictIsRehashing(d)) _dictRehashStep(d);//这里rehash while1次 /* Get the index of the new element, or -1 if * the element already exists. */ if ((index = _dictKeyIndex(d, key)) == -1)//如果有相同的key就返回 return NULL; /* Allocate the memory and store the new entry */ ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];//上面_dictRehashStep将ht[0]赋值给了ht[1] entry = zmalloc(sizeof(*entry));//malloc新的内存 entry->next = ht->table[index];//新的next是ht的index,可见是插入table的头部 ht->table[index] = entry; ht->used++; /* Set the hash entry fields. */ dictSetKey(d, entry, key)//哈希函数设置key return entry; } /* Add an element to the target hash table */ int dictAdd(dict *d, void *key, void *val) { dictEntry *entry = dictAddRaw(d,key); if (!entry) return DICT_ERR; dictSetVal(d, entry, val);//哈希函数设置val return DICT_OK; }
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 没有东东,找个屁啊,呵呵*/ if (dictIsRehashing(d)) _dictRehashStep(d);//又是这一步 //下面的跟_dictKeyIndex内容相同 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; } /* 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)//直接add,没成功说明有相同的key 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. */ //后面的要注意,setval之后还有freeval,将老的entry释放,可见dictSetVal方法不是直接去修改entry的val,而是重新创建内存放新的val auxentry = *entry; dictSetVal(d, entry, val); dictFreeVal(d, &auxentry); return 0; }
/* dictReplaceRaw() is simply a version of dictAddRaw() that always * returns the hash entry of the specified key, even if the key already * exists and can't be added (in that case the entry of the already * existing key is returned.) * * See dictAddRaw() for more information. */ dictEntry *dictReplaceRaw(dict *d, void *key) { dictEntry *entry = dictFind(d,key); return entry ? entry : dictAddRaw(d,key); } //这个方法并不能真正的replace key值,如果key存在则返回key对应的table,不然就key添加 //方法有点多,最后讲下redis.c serverCron会调用的方法 long long timeInMilliseconds(void) { struct timeval tv; gettimeofday(&tv,NULL); return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000); } /* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */ 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; } //在1毫秒的时间里去rehash,每次while100次
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