redis源码浅析--七-对象(下)（内存回收、共享）

环境说明：redis源码版本 5.0.3；我在阅读源码过程做了注释，git地址：https://gitee.com/xiaoangg/redis_annotation
参考书籍：《redis的设计与实现》

一 内存回收

c语言并不具备自动内存回收的功能，所以redis在自己的对象系统构建了一个引用计数（reference counting）
通过这一机制，程序可以跟踪对象的引用计数信息，在适当的时候自动释放对象，并进行内存回收

每个对象的引用计数信息由redisObject中refcount属性记录：

• 创建一个对象时引用计数值会被初始化为1
• 当对象被一个新程序使用时，引用计数+1
• 当对象不在被一个程序使用时，引用计数-1
• 当对象引用计数值变为0时，对象占用的内存释放

tips：可以使用命令 OBJECT REFOUNR 命令来查看一个键的引用计数的值

二 内存共享

redis在初始化服务时，会创建一万个字符串对象，这些对象包含了0到9999的整数值；
当服务需要用到0到9999的字符串对象时，服务就会使用这些共享对象，而不是创建新的对象；

当redis多个键共享同一个值的对象时，会执行一下两步操作：

1. 将数据库的键指向一个现有的值对象
2. 被共享的对象引用计数+1

以tryObjectEncoding（对字符串尝试编码）函数作为入口，可以看到对象共享和引用计数的操作工程：

//尝试对字符串进行编码，以节省空间
/* 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. */
    serverAssertWithInfo(NULL,o,o->type == OBJ_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;

    //共享对象 不会进行编码
    /* 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 20 chars is not
     * representable as a 32 nor 64 bit integer. */
    //检查字符串时候可以转化成整数；（字符串如果大于20位，64位存不下，所以一定不能转换）
    len = sdslen(s);
    if (len <= 20 && 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 & MAXMEMORY_FLAG_NO_SHARED_INTEGERS)) &&
            value >= 0 &&
            value < OBJ_SHARED_INTEGERS)
        {
            decrRefCount(o);
            incrRefCount(shared.integers[value]);
            return shared.integers[value];
        } else {
            if (o->encoding == OBJ_ENCODING_RAW) sdsfree(o->ptr);
            o->encoding = OBJ_ENCODING_INT;
            o->ptr = (void*) value;
            return o;
        }
    }

    /*
    如果字符串很小 并且 是使用 RAW编码的
    尝试使用 更高效的 EMBSTR编码方式
    */
    /* 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 <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT) {
        robj *emb;

        if (o->encoding == OBJ_ENCODING_EMBSTR) return o;
        emb = createEmbeddedStringObject(s,sdslen(s)); //创建 EMB 编码的对象
        decrRefCount(o);
        return emb;
    }

    /*
    不能使用 long 和embstr编码方式
    
    */
    /* 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
     * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
    if (o->encoding == OBJ_ENCODING_RAW &&
        sdsavail(s) > len/10)
    {
        o->ptr = sdsRemoveFreeSpace(o->ptr);
    }

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

 

三 对象的空转时长

redisObject结构中还包含来一个lru属性，lru属性记录来对象最后一次被命令程序访问的时间；

当服务器打开来maxmemory选项，并且服务器用于回收内存的算法是volatile-lru或者是allkeys-lru，那么当服务器占用的内存数超过来maxmeory所设的上限，空转时常较高的那部分键将优先释放；

tips；可以使用OBJECT IDLETIME  命令来查看key的空转时常；