Redis内存节省策略

     Redis作为一款缓存软件，在Redis的源码中，处处体现着节省内存的思想，下面先从3个方面对Redis的内存节省策略做一个分享。（以下代码为Redis 3.0.5版本）

      1.redisObject 是Redis中最常见的一个数据结构，Redis存储的所有的键和值都是一个redisObject, 因此必须尽肯能节省redisObject的大小。由于Redis 数据类型只有我们所常见的5种，而Redis的数据编码类型总共8种，所以Redis中分别采用4Bit来表示，两者相加正好1个字节。

                        

typedef struct redisObject {
    unsigned type:4;
    unsigned encoding:4;
    unsigned lru:REDIS_LRU_BITS; /* lru time (relative to server.lruclock) */
    int refcount;
    void *ptr;
} robj;

          2.Redis对于经常使用变量，将其设为全局变量，并采用引用计数的方法。本策略主要目的是为了避免重复的创建，提高性能。只有当足够数量的数据值在0-9999时才会发挥节省内存的作用。如代码所示，在结构体中有一个变量integers, 里面存储着数据0-9999的对象指针。

struct sharedObjectsStruct {
    robj *crlf, *ok, *err, *emptybulk, *czero, *cone, *cnegone, *pong, *space,
    *colon, *nullbulk, *nullmultibulk, *queued,
    *emptymultibulk, *wrongtypeerr, *nokeyerr, *syntaxerr, *sameobjecterr,
    *outofrangeerr, *noscripterr, *loadingerr, *slowscripterr, *bgsaveerr,
    *masterdownerr, *roslaveerr, *execaborterr, *noautherr, *noreplicaserr,
    *busykeyerr, *oomerr, *plus, *messagebulk, *pmessagebulk, *subscribebulk,
    *unsubscribebulk, *psubscribebulk, *punsubscribebulk, *del, *rpop, *lpop,
    *lpush, *emptyscan, *minstring, *maxstring,
    *select[REDIS_SHARED_SELECT_CMDS],
    *integers[REDIS_SHARED_INTEGERS],
    *mbulkhdr[REDIS_SHARED_BULKHDR_LEN], /* "*\r\n" */
    *bulkhdr[REDIS_SHARED_BULKHDR_LEN];  /* "$\r\n" */
};

      3.Redis的8种编码类型，REDIS_ENCODING_INT, REDIS_ENCODING_EMBSTR, REDIS_ENCODING_RAW是基础，对于其他几种编码类型，除REDIS_ENCODING_INTSET外，其所对应对象存储的信息对象均是以前三种编码类型为基础，生成一个个具体的对象。如Redis链表中，每个节点存储的对象，其编码类型可以为REDIS_ENCODING_INT, REDIS_ENCODING_EMBSTR, REDIS_ENCODING_RAW 中的任意一种。

/* Try to encode a string object in order to save space */
robj *tryObjectEncoding(robj *o) {
    long value;
    sds s = o->ptr;
    size_t len;

    /* Make sure this is a string object, the only type we encode
     * in this function. Other types use encoded memory efficient
     * representations but are handled by the commands implementing
     * the type. */
    redisAssertWithInfo(NULL,o,o->type == REDIS_STRING);

    /* We try some specialized encoding only for objects that are
     * RAW or EMBSTR encoded, in other words objects that are still
     * in represented by an actually array of chars. */
    if (!sdsEncodedObject(o)) return o;

    /* It's not safe to encode shared objects: shared objects can be shared
     * everywhere in the "object space" of Redis and may end in places where
     * they are not handled. We handle them only as values in the keyspace. */
     if (o->refcount > 1) return o;

    /* Check if we can represent this string as a long integer.
     * Note that we are sure that a string larger than 21 chars is not
     * representable as a 32 nor 64 bit integer. */
    len = sdslen(s);
    if (len <= 21 && string2l(s,len,&value)) {
        /* This object is encodable as a long. Try to use a shared object.
         * Note that we avoid using shared integers when maxmemory is used
         * because every object needs to have a private LRU field for the LRU
         * algorithm to work well. */
        if ((server.maxmemory == 0 ||
             (server.maxmemory_policy != REDIS_MAXMEMORY_VOLATILE_LRU &&
              server.maxmemory_policy != REDIS_MAXMEMORY_ALLKEYS_LRU)) &&
            value >= 0 &&
            value < REDIS_SHARED_INTEGERS)
        {
            decrRefCount(o);
            incrRefCount(shared.integers[value]);
            return shared.integers[value];
        } else {
            if (o->encoding == REDIS_ENCODING_RAW) sdsfree(o->ptr);
            o->encoding = REDIS_ENCODING_INT;
            o->ptr = (void*) value;
            return o;
        }
    }

    /* If the string is small and is still RAW encoded,
     * try the EMBSTR encoding which is more efficient.
     * In this representation the object and the SDS string are allocated
     * in the same chunk of memory to save space and cache misses. */
    if (len <= REDIS_ENCODING_EMBSTR_SIZE_LIMIT) {
        robj *emb;

        if (o->encoding == REDIS_ENCODING_EMBSTR) return o;
        emb = createEmbeddedStringObject(s,sdslen(s));
        decrRefCount(o);
        return emb;
    }

    /* We can't encode the object...
     *
     * Do the last try, and at least optimize the SDS string inside
     * the string object to require little space, in case there
     * is more than 10% of free space at the end of the SDS string.
     *
     * We do that only for relatively large strings as this branch
     * is only entered if the length of the string is greater than
     * REDIS_ENCODING_EMBSTR_SIZE_LIMIT. */
    if (o->encoding == REDIS_ENCODING_RAW &&
        sdsavail(s) > len/10)
    {
        o->ptr = sdsRemoveFreeSpace(o->ptr);
    }

    /* Return the original object. */
    return o;
}