memcached 中有两张hash 表，一个是“主hash 表”（primary_hashtable），另外一个是“原hash 表”（old_hashtable）。一般情况下都在主表中接受操作，在插入新item时判断是否需要进行扩；每次操作的时候，先会检测表是否正处于扩展(expanding)状态，如果是，则原表中进行操作，当扩容完成在转移到主表中进行操作。 在扩容时，采取逐步迁移策略：即每次只从原表中迁移一个bucket节点的item到新主表中，进行逐步迁移。

总的来看，这与Redis中的hash操作几乎一致。因此不再做详细讲解，具体分析见代码注释。

//hash表的初始化，参数hashtable_init为所设置的hashpower大小（阶数），默认大小为16
void assoc_init(const int hashtable_init) {
    if (hashtable_init) {
        hashpower = hashtable_init;
    }
	//创建主表（hashsize(hashpower)：计算bucket节点数目=2的hashpower次方）
    primary_hashtable = calloc(hashsize(hashpower), sizeof(void *));
    if (! primary_hashtable) {
        fprintf(stderr, "Failed to init hashtable.\n");
        exit(EXIT_FAILURE);
    }
	//emcached内部有很多全局的统计信息，用于实时获取各个资源的使用情况，
	//对统计信息的更新都需要加锁
    STATS_LOCK();//对全局统计信息加锁，已更新信息
    stats.hash_power_level = hashpower;
    stats.hash_bytes = hashsize(hashpower) * sizeof(void *);
    STATS_UNLOCK();//解锁
}

//在哈希表中查找给定key的item：找到对应的哈希表,再找对应的桶节点，最后遍历链表找到目标key的item
item *assoc_find(const char *key, const size_t nkey, const uint32_t hv) {
    item *it;//桶节点
    unsigned int oldbucket;//在原表中的桶节点索引

	//正在扩容，且当前节点在愿表中，还未迁移到主表
	//注意：i&(2^n-1)结果即为i除以2^n的余数
    if (expanding &&
        (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
    {
        it = old_hashtable[oldbucket];
    } else {//没有扩容，或者已经迁移到主表中
        it = primary_hashtable[hv & hashmask(hashpower)];
    }

    item *ret = NULL;
    int depth = 0;//目标节点在桶中的深度
    while (it) {//遍历桶节点链表
        if ((nkey == it->nkey) && (memcmp(key, ITEM_key(it), nkey) == 0)) {
            ret = it;
            break;
        }
        it = it->h_next;
        ++depth;
    }
    MEMCACHED_ASSOC_FIND(key, nkey, depth);
    return ret;
}

</pre><pre name="code" class="cpp">/* returns the address of the item pointer before the key.  if *item == 0,
   the item wasn't found */
//内部函数：返回目标key item的前一个item的指针，这样在删除目标item时只需要将该返回item指针的next指针指向目标item的next item即可。
static item** _hashitem_before (const char *key, const size_t nkey, const uint32_t hv) {
    item **pos;
    unsigned int oldbucket;

    if (expanding &&
        (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
    {
        pos = &old_hashtable[oldbucket];
    } else {
        pos = &primary_hashtable[hv & hashmask(hashpower)];
    }

    while (*pos && ((nkey != (*pos)->nkey) || memcmp(key, ITEM_key(*pos), nkey))) {
        pos = &(*pos)->h_next;
    }
    return pos;
}

/* grows the hashtable to the next power of 2. */
//哈希表扩容为原来的2倍（将原来的主表拷贝到久表中，对主表扩容）
static void assoc_expand(void) {
    old_hashtable = primary_hashtable;

    primary_hashtable = calloc(hashsize(hashpower + 1), sizeof(void *));
    if (primary_hashtable) {
        if (settings.verbose > 1)
            fprintf(stderr, "Hash table expansion starting\n");
        hashpower++;
        expanding = true;
        expand_bucket = 0;
        STATS_LOCK();
        stats.hash_power_level = hashpower;
        stats.hash_bytes += hashsize(hashpower) * sizeof(void *);
        stats.hash_is_expanding = 1;
        STATS_UNLOCK();
    } else {
        primary_hashtable = old_hashtable;
        /* Bad news, but we can keep running. */
    }
}

static void assoc_start_expand(void) {
    if (started_expanding)
        return;
    started_expanding = true;
    pthread_cond_signal(&maintenance_cond);
}

/* Note: this isn't an assoc_update.  The key must not already exist to call this */
//将给定item插入到哈希表的桶的头部中  注意：该item不能已经存在于hash表中（hv：哈希值）
int assoc_insert(item *it, const uint32_t hv) {
    unsigned int oldbucket;

//    assert(assoc_find(ITEM_key(it), it->nkey) == 0);  /* shouldn't have duplicately named things defined */

	//正在扩容，还未完成，则将该item放到原hashtable的对应bucket的单链表的头部
    if (expanding &&  
        (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)//注意hashpower已经加倍，因此是hashpower-1
    {
        it->h_next = old_hashtable[oldbucket];
        old_hashtable[oldbucket] = it;
    } else {//没有正在扩容则放到主hashtable中
        it->h_next = primary_hashtable[hv & hashmask(hashpower)];
        primary_hashtable[hv & hashmask(hashpower)] = it;
    }

    hash_items++;
	//是否需要开始扩容
    if (! expanding && hash_items > (hashsize(hashpower) * 3) / 2) {
        assoc_start_expand();
    }

    MEMCACHED_ASSOC_INSERT(ITEM_key(it), it->nkey, hash_items);
    return 1;
}

//删除对应item（只是将item从桶链表中移除）
void assoc_delete(const char *key, const size_t nkey, const uint32_t hv) {
    item **before = _hashitem_before(key, nkey, hv);//查找该item的前一个item

    if (*before) {
        item *nxt;
        hash_items--;//hash表中的item总数
        /* The DTrace probe cannot be triggered as the last instruction
         * due to possible tail-optimization by the compiler
         */
        MEMCACHED_ASSOC_DELETE(key, nkey, hash_items);
        nxt = (*before)->h_next;
        (*before)->h_next = 0;   /* probably pointless, but whatever. */
        *before = nxt;
        return;
    }
    /* Note:  we never actually get here.  the callers don't delete things
       they can't find. */
    assert(*before != 0);
}

//迁移函数start_assoc_maintenance_thread()，创建迁移线程，调用函数assoc_maintenance_thread进行迁移
//线程函数：迁移bucket节点，默认一次迁移一个bucket
static void *assoc_maintenance_thread(void *arg) {

    while (do_run_maintenance_thread) {
        int ii = 0;

        /* Lock the cache, and bulk move multiple buckets to the new
         * hash table. */
        item_lock_global();
        mutex_lock(&cache_lock);

        for (ii = 0; ii < hash_bulk_move && expanding; ++ii) {
            item *it, *next;
            int bucket;

            for (it = old_hashtable[expand_bucket]; NULL != it; it = next) {
                next = it->h_next;

				//计算哈希值，并计算得桶节点索引值
                bucket = hash(ITEM_key(it), it->nkey) & hashmask(hashpower);
                it->h_next = primary_hashtable[bucket];
                primary_hashtable[bucket] = it;
            }

			//每迁移完一个bucket，就在久表中移除该bucket
            old_hashtable[expand_bucket] = NULL;

            expand_bucket++;
			//扩容结束
            if (expand_bucket == hashsize(hashpower - 1)) {
                expanding = false;
                free(old_hashtable);
                STATS_LOCK();
                stats.hash_bytes -= hashsize(hashpower - 1) * sizeof(void *);
                stats.hash_is_expanding = 0;
                STATS_UNLOCK();
                if (settings.verbose > 1)
                    fprintf(stderr, "Hash table expansion done\n");
            }
        }

        mutex_unlock(&cache_lock);
        item_unlock_global();

        if (!expanding) {
            /* finished expanding. tell all threads to use fine-grained locks */
            switch_item_lock_type(ITEM_LOCK_GRANULAR);
            slabs_rebalancer_resume();
            /* We are done expanding.. just wait for next invocation */
            mutex_lock(&cache_lock);
            started_expanding = false;
            pthread_cond_wait(&maintenance_cond, &cache_lock);
            /* Before doing anything, tell threads to use a global lock */
            mutex_unlock(&cache_lock);
            slabs_rebalancer_pause();
            switch_item_lock_type(ITEM_LOCK_GLOBAL);
            mutex_lock(&cache_lock);
            assoc_expand();
            mutex_unlock(&cache_lock);
        }
    }
    return NULL;
}