Redis的内存淘汰算法

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int freeMemoryIfNeeded(void){
    while (mem_freed < mem_tofree) {
        if (server.maxmemory_policy == REDIS_MAXMEMORY_NO_EVICTION)
        return REDIS_ERR; /* We need to free memory, but policy forbids. */

        if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
                server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM)
            {......}
        /* volatile-random and allkeys-random policy */
        if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM ||
                server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_RANDOM)
            {......}
        /* volatile-lru and allkeys-lru policy */
        else if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
            server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_LRU)
        {
            // 淘汰池函数
            evictionPoolPopulate(dict, db->dict, db->eviction_pool);
            while(bestkey == NULL) {
                evictionPoolPopulate(dict, db->dict, db->eviction_pool);
                // 从后向前逐一淘汰
                for (k = REDIS_EVICTION_POOL_SIZE-1; k >= 0; k--) {
                    if (pool[k].key == NULL) continue;
                    de = dictFind(dict,pool[k].key); // 定位目标

                    /* Remove the entry from the pool. */
                    sdsfree(pool[k].key);
                    /* Shift all elements on its right to left. */
                    memmove(pool+k,pool+k+1,
                        sizeof(pool[0])*(REDIS_EVICTION_POOL_SIZE-k-1));
                    /* Clear the element on the right which is empty
                     * since we shifted one position to the left.  */
                    pool[REDIS_EVICTION_POOL_SIZE-1].key = NULL;
                    pool[REDIS_EVICTION_POOL_SIZE-1].idle = 0;

                    /* If the key exists, is our pick. Otherwise it is
                     * a ghost and we need to try the next element. */
                    if (de) {
                        bestkey = dictGetKey(de); // 确定删除键
                        break;
                    } else {
                        /* Ghost... */
                        continue;
                    }
                }
            }
        }
        /* volatile-ttl */
        else if (server.maxmemory_policy == EDIS_MAXMEMORY_VOLATILE_TTL) {......}

        // 最终选定待删除键bestkey
        if (bestkey) {
            long long delta;
            robj *keyobj = createStringObject(bestkey,sdslenbestkey)); // 目标对象
            propagateExpire(db,keyobj);
            latencyStartMonitor(eviction_latency); // 延迟监控开始
            dbDelete(db,keyobj); // 从db删除对象
            latencyEndMonitor(eviction_latency);// 延迟监控结束
            latencyAddSampleIfNeeded("eviction-del",iction_latency); // 延迟采样
            latencyRemoveNestedEvent(latency,eviction_latency);
            delta -= (long long) zmalloc_used_memory();
            mem_freed += delta; // 释放内存计数
            server.stat_evictedkeys++; // 淘汰key计数，info中可见
            notifyKeyspaceEvent(REDIS_NOTIFY_EVICTED, "evicted", keyobj, db->id); // 事件通知
            decrRefCount(keyobj); // 引用计数更新
            keys_freed++;
            // 避免删除较多键导致的主从延迟，在循环内同步
            if (slaves) flushSlavesOutputBuffers();
        }
    }
}