Redis的多线程到底该怎么理解

void delCommand(client *c) {
    delGenericCommand(c,server.lazyfree_lazy_user_del);
}

/* This command implements DEL and LAZYDEL. */
void delGenericCommand(client *c, int lazy) {
    int numdel = 0, j;

    for (j = 1; j < c->argc; j++) {
        expireIfNeeded(c->db,c->argv[j]);
        // 根据配置确定DEL在执行时是否以lazy形式执行
        int deleted  = lazy ? dbAsyncDelete(c->db,c->argv[j]) :
                              dbSyncDelete(c->db,c->argv[j]);
        if (deleted) {
            signalModifiedKey(c,c->db,c->argv[j]);
            notifyKeyspaceEvent(NOTIFY_GENERIC,
                "del",c->argv[j],c->db->id);
            server.dirty++;
            numdel++;
        }
    }
    addReplyLongLong(c,numdel);

}

/* Delete a key, value, and associated expiration entry if any, from the DB.  * If there are enough allocations to free the value object may be put into  * a lazy free list instead of being freed synchronously. The lazy free list  * will be reclaimed in a different bio.c thread. */ #define LAZYFREE_THRESHOLD 64 int dbAsyncDelete(redisDb *db, robj *key) {     /* Deleting an entry from the expires dict will not free the sds of      * the key, because it is shared with the main dictionary. */     if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);     /* If the value is composed of a few allocations, to free in a lazy way      * is actually just slower... So under a certain limit we just free      * the object synchronously. */           dictEntry *de = dictUnlink(db->dict,key->ptr);     if (de) {         robj *val = dictGetVal(de);         // 计算value的回收收益         size_t free_effort = lazyfreeGetFreeEffort(val);         /* If releasing the object is too much work, do it in the background          * by adding the object to the lazy free list.          * Note that if the object is shared, to reclaim it now it is not          * possible. This rarely happens, however sometimes the implementation          * of parts of the Redis core may call incrRefCount() to protect          * objects, and then call dbDelete(). In this case we'll fall          * through and reach the dictFreeUnlinkedEntry() call, that will be          * equivalent to just calling decrRefCount(). */         // 只有回收收益超过一定值，才会执行异步删除，否则还是会退化到同步删除         if (free_effort > LAZYFREE_THRESHOLD && val->refcount == 1) {             atomicIncr(lazyfree_objects,1);             bioCreateBackgroundJob(BIO_LAZY_FREE,val,NULL,NULL);             dictSetVal(db->dict,de,NULL);         }     }     /* Release the key-val pair, or just the key if we set the val      * field to NULL in order to lazy free it later.    */     if (de) {         dictFreeUnlinkedEntry(db->dict,de);         if (server.cluster_enabled) slotToKeyDel(key->ptr);         return 1;     } else {         return 0;     } }

// 3.2.5版本ZSet节点实现，value定义robj *obj
/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    robj *obj;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned int span;
    } level[];
} zskiplistNode;

// 6.0.10版本ZSet节点实现，value定义为sds ele
/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    sds ele;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned long span;
    } level[];
} zskiplistNode;

Now that values of aggregated data types are fully unshared, and client output buffers don’t contain shared objects as well, there is a lot to exploit. For example it is finally possible to implement threaded I/O in Redis, so that different clients are served by different threads. This means that we’ll have a global lock only when accessing the database, but the clients read/write syscalls and even the parsing of the command the client is sending, can happen in different threads.

int handleClientsWithPendingReadsUsingThreads(void) {
    ...
    /* Distribute the clients across N different lists. */
    listIter li;
    listNode *ln;
    listRewind(server.clients_pending_read,&li);
    int item_id = 0;
    // 将等待处理的客户端分配给I/O线程
    while((ln = listNext(&li))) {
        client *c = listNodeValue(ln);
        int target_id = item_id % server.io_threads_num;
        listAddNodeTail(io_threads_list[target_id],c);
        item_id++;
    }
    ...
    /* Wait for all the other threads to end their work. */
    // 轮训等待所有I/O线程处理完
    while(1) {
        unsigned long pending = 0;
        for (int j = 1; j < server.io_threads_num; j++)
            pending += io_threads_pending[j];
        if (pending == 0) break;
    }
    ...
    return processed;
}