memcached（八）-- set指令内部实现

　　在一个set指令传输到memcached后：

set abc 0 3600 5
hello

　　memcached是怎么样执行set/add等对象的咧？

　　首先从memcached启动后，要先监听端口，监听到有对应的动作后，执行指令。从源码中，堆栈是这样子：

main_base从启动到监听
main——>（初始化main_base了）server_sockets——>server_socket——>
conn_new——>event_handler——>drive_machine——>
try_read_command——>process_command——>（process_get_command/process_update_command/process_arithmetic_command/process_delete_command/process_touch_command/process_stat）;
(flush_all/version/quit/shutdown/slabs/lru_crawler/tocrawl/sleep/verbosity/最后ERROR)

自定义协议类型，在try_read_command中根据字符编码，判定是否二进制协议。
二进制协议的话执行dispatch_bin_command

　　drive_machine是memcached的主循环函数。根据connection的状态，决定接下来的动作。也会把当前动作执行后，改变为某个状态，从而执行下一个动作。

　　connection的状态有这些（见memcached.h）：

enum conn_states {
    conn_listening,  /**< the socket which listens for connections  */
    conn_new_cmd,    /**< Prepare connection for next command */
    conn_waiting,    /**< waiting for a readable socket */
    conn_read,       /**< reading in a command line */
    conn_parse_cmd,  /**< try to parse a command from the input buffer */
    conn_write,      /**< writing out a simple response */
    conn_nread,      /**< reading in a fixed number of bytes */
    conn_swallow,    /**< swallowing unnecessary bytes w/o storing */
    conn_closing,    /**< closing this connection */
    conn_mwrite,     /**< writing out many items sequentially */
    conn_closed,     /**< connection is closed */
    conn_max_state   /**< Max state value (used for assertion) */
};

　　nread还细分了几个宏，区分不同的指令：

#define NREAD_ADD 1
#define NREAD_SET 2
#define NREAD_REPLACE 3
#define NREAD_APPEND 4
#define NREAD_PREPEND 5
#define NREAD_CAS 6

　　set指令就是，NREAD_SET 2。

　　在set命令中，分为2个动作：

set abc 0 3600 5     /*conn_parse_cmd*/
hello    /*conn_nread*/

　　针对set命令来说，第一个动作是分配空间，第二个动作是关联hash桶。

分配空间

　　源码中，针对文本协议，process_update_command函数就是执行set/add等指令的。 

 else if ((ntokens == 6 || ntokens == 7) &&
               ((strcmp(tokens[COMMAND_TOKEN].value, "add") == 0 && (comm = NREAD_ADD)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "set") == 0 && (comm = NREAD_SET)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "replace") == 0 && (comm = NREAD_REPLACE)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "prepend") == 0 && (comm = NREAD_PREPEND)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "append") == 0 && (comm = NREAD_APPEND)) )) {
        process_update_command(c, tokens, ntokens, comm, false);
    } else if ((ntokens == 7 || ntokens == 8) && (strcmp(tokens[COMMAND_TOKEN].value, "cas") == 0 && (comm = NREAD_CAS))) {
        process_update_command(c, tokens, ntokens, comm, true);

    }

　　跟着堆栈走， 分配空间的代码是：

it = item_alloc(key, nkey, flags, realtime(exptime), vlen);

　　item_alloc在thread.c中（其实就是通过worker线程执行代码逻辑），方法的参数说明一下：

/*
 * Allocates a new item.
 * @param key 字符串
 * @param nkey 字符串长度
 * @param flags 命令中的第二个参数 很多情况下都系0
 * @param exptime 过期时间，精确到毫秒
 * @param nbytes 命令中第四个参数 value的长度
 *
 * @return 分配的空间（具体chunk）的指针
 */
item *item_alloc(char *key, size_t nkey, int flags, rel_time_t exptime, int nbytes) {
    item *it;
    /* do_item_alloc handles its own locks */
    it = do_item_alloc(key, nkey, flags, exptime, nbytes, 0);
    return it;
}

　　会调用到item.c中的“do_item_alloc”函数。这个函数是分配内存空间给这个对象的最关键部分：

/**
 * @param key 字符串
 * @param nkey 字符串长度
 * @param flags 命令中的第二个参数 很多情况下都系0
 * @param exptime 过期时间，精确到毫秒
 * @param nbytes 命令中的最后一个参数的长度
 * @param cur_hv 在thread.c的item_alloc方法中， 设置成0了
 */
item *do_item_alloc(char *key, const size_t nkey, const int flags,
                    const rel_time_t exptime, const int nbytes,
                    const uint32_t cur_hv)

　　在这个函数中， 关键步骤如下：

• 先要知道这个对象应该放到哪个slab中

    size_t ntotal = item_make_header(nkey + 1, flags, nbytes, suffix, &nsuffix);
    if (settings.use_cas) {
        ntotal += sizeof(uint64_t);
    }

    //计算长度，得知这个item应该在哪个slab
    unsigned int id = slabs_clsid(ntotal);

　　得知在哪个slab：

unsigned int slabs_clsid(const size_t size) {
    int res = POWER_SMALLEST;

    if (size == 0)
        return 0;
    while (size > slabclass[res].size)
        if (res++ == power_largest)     /* won't fit in the biggest slab */
            return 0;
    return res;
}

　　slabs_clsid返回slabclass下标，告知这个对象属于哪一个slab。

　　PS：mc创建的时候，会初始化slabclass[]这个数组。下一个slabclass元素的size是一定比现在这个slabclass元素更大的。

• LRU判定

　　可能指定的slabclass存满数据了，分配空间和LRU的逻辑：

    /**
     * 1、先执行一次lru。
     * 2、分配空间。分配得到空间就退出循环，否则重复5次。
     */
    for (i = 0; i < 5; i++) {
        /* Try to reclaim memory first   lru_maintainer_thread默认就是false。  */
        if (!settings.lru_maintainer_thread) {
            lru_pull_tail(id, COLD_LRU, 0, false, cur_hv);
        }
        it = slabs_alloc(ntotal, id, &total_chunks);
        if (settings.expirezero_does_not_evict)
            total_chunks -= noexp_lru_size(id);
        if (it == NULL) {
            if (settings.lru_maintainer_thread) {
                lru_pull_tail(id, HOT_LRU, total_chunks, false, cur_hv);
                lru_pull_tail(id, WARM_LRU, total_chunks, false, cur_hv);
                lru_pull_tail(id, COLD_LRU, total_chunks, true, cur_hv);
            } else {
                lru_pull_tail(id, COLD_LRU, 0, true, cur_hv);
            }
        } else {
            break;
        }
    }

• slabs.c中，分配空间

static void *do_slabs_alloc(const size_t size, unsigned int id, unsigned int *total_chunks) {
    slabclass_t *p;
    void *ret = NULL;
    item *it = NULL;

    if (id < POWER_SMALLEST || id > power_largest) {
        MEMCACHED_SLABS_ALLOCATE_FAILED(size, 0);
        return NULL;
    }
    p = &slabclass[id];
    assert(p->sl_curr == 0 || ((item *)p->slots)->slabs_clsid == 0);

    *total_chunks = p->slabs * p->perslab;
    /* fail unless we have space at the end of a recently allocated page,
       we have something on our freelist, or we could allocate a new page */
    if (! (p->sl_curr != 0 || do_slabs_newslab(id) != 0)) {
        /* We don't have more memory available */
        ret = NULL;
    } else if (p->sl_curr != 0) {
        /* return off our freelist */
        it = (item *)p->slots;
        p->slots = it->next;
        if (it->next) it->next->prev = 0;
        /* Kill flag and initialize refcount here for lock safety in slab
         * mover's freeness detection. */
        it->it_flags &= ~ITEM_SLABBED;
        it->refcount = 1;
        p->sl_curr--;
        ret = (void *)it;
    }

    if (ret) {
        p->requested += size;
        MEMCACHED_SLABS_ALLOCATE(size, id, p->size, ret);
    } else {
        MEMCACHED_SLABS_ALLOCATE_FAILED(size, id);
    }

    return ret;
}

　　sl_curr记录当前这个slabclass还有多少个可用的位置。slots是一个指针链表，记录每个'位置' 的指针。

　　最后回到process_update_command。

• ITEM_data这个宏就是存储数据。
• 把分配空间的指针放到当前connection，
• connection的状态修改为"conn_nread"状态。

　　driver_machine循环会把这个connection case到 conn_nread状态的代码中执行。

    c->item = it;
    c->ritem = ITEM_data(it);
    c->rlbytes = it->nbytes;    
    c->cmd = comm;
    conn_set_state(c, conn_nread);    //conn_nread 状态来处理接下来的东西

关联hashTable

 　　在driver_machine中，conn_nread状态会执行complete_nread方法——>complete_nread_ascii（不谈论二进制协议了）。store_item（worker线程实现）——>do_store_item。

　　在上一步的分配空间中，对象还没有跟hashtable进行关联的。而且要根据指令判定一下，是替换还是删除旧对象。比如说set指令，就是要删除旧的对象了。

enum store_item_type do_store_item(item *it, int comm, conn *c, const uint32_t hv) {
    char *key = ITEM_key(it);
    item *old_it = do_item_get(key, it->nkey, hv);    //通过哈希表找到这个对象
    enum store_item_type stored = NOT_STORED;

    item *new_it = NULL;
    int flags;

    if (old_it != NULL && comm == NREAD_ADD) {    //有旧的数据而且是add操作，add不成功
        /* add only adds a nonexistent item, but promote to head of LRU */
        do_item_update(old_it);
    } else if (!old_it && (comm == NREAD_REPLACE
        || comm == NREAD_APPEND || comm == NREAD_PREPEND))    //无旧的数据，replace append prepend 操作
    {
        /* replace only replaces an existing value; don't store */
    } else if (comm == NREAD_CAS) {        //cas read操作-----开始
        /* validate cas operation */
        if(old_it == NULL) {
            // LRU expired
            stored = NOT_FOUND;
            pthread_mutex_lock(&c->thread->stats.mutex);
            c->thread->stats.cas_misses++;
            pthread_mutex_unlock(&c->thread->stats.mutex);
        }
        else if (ITEM_get_cas(it) == ITEM_get_cas(old_it)) {
            // cas validates
            // it and old_it may belong to different classes.
            // I'm updating the stats for the one that's getting pushed out
            pthread_mutex_lock(&c->thread->stats.mutex);
            c->thread->stats.slab_stats[ITEM_clsid(old_it)].cas_hits++;
            pthread_mutex_unlock(&c->thread->stats.mutex);

            item_replace(old_it, it, hv);
            stored = STORED;
        } else {
            pthread_mutex_lock(&c->thread->stats.mutex);
            c->thread->stats.slab_stats[ITEM_clsid(old_it)].cas_badval++;
            pthread_mutex_unlock(&c->thread->stats.mutex);

            if(settings.verbose > 1) {
                fprintf(stderr, "CAS:  failure: expected %llu, got %llu
",
                        (unsigned long long)ITEM_get_cas(old_it),
                        (unsigned long long)ITEM_get_cas(it));
            }
            stored = EXISTS;
        }    //cas read操作-----结束
    } else {    //其他， 应该正确的情况
        /*
         * Append - combine new and old record into single one. Here it's
         * atomic and thread-safe.
         */
        if (comm == NREAD_APPEND || comm == NREAD_PREPEND) {    //append preappend开始
            /*
             * Validate CAS
             */
            if (ITEM_get_cas(it) != 0) {
                // CAS much be equal
                if (ITEM_get_cas(it) != ITEM_get_cas(old_it)) {
                    stored = EXISTS;
                }
            }

            if (stored == NOT_STORED) {        //新对象的操作
                /* we have it and old_it here - alloc memory to hold both */
                /* flags was already lost - so recover them from ITEM_suffix(it) */

                flags = (int) strtol(ITEM_suffix(old_it), (char **) NULL, 10);

                new_it = do_item_alloc(key, it->nkey, flags, old_it->exptime, it->nbytes + old_it->nbytes - 2 /* CRLF */, hv);

                if (new_it == NULL) {
                    /* SERVER_ERROR out of memory */
                    if (old_it != NULL)
                        do_item_remove(old_it);

                    return NOT_STORED;
                }

                /* copy data from it and old_it to new_it */

                if (comm == NREAD_APPEND) {
                    memcpy(ITEM_data(new_it), ITEM_data(old_it), old_it->nbytes);
                    memcpy(ITEM_data(new_it) + old_it->nbytes - 2 /* CRLF */, ITEM_data(it), it->nbytes);
                } else {
                    /* NREAD_PREPEND */
                    memcpy(ITEM_data(new_it), ITEM_data(it), it->nbytes);
                    memcpy(ITEM_data(new_it) + it->nbytes - 2 /* CRLF */, ITEM_data(old_it), old_it->nbytes);
                }

                it = new_it;
            }
        }            //append preappend结束

        if (stored == NOT_STORED) {
            if (old_it != NULL)
                item_replace(old_it, it, hv);
            else
                do_item_link(it, hv);

            c->cas = ITEM_get_cas(it);

            stored = STORED;
        }
    }

    if (old_it != NULL)
        do_item_remove(old_it);         
    if (new_it != NULL)
        do_item_remove(new_it);

    if (stored == STORED) {
        c->cas = ITEM_get_cas(it);
    }

    return stored;
}

参考

memcached connection的状态：http://www.cnblogs.com/ilfmonday/p/memcached_conn.html