概述

Redis对象系统是在数据结构上又一层封装

源码

robj结构体

typedef struct redisObject {
    unsigned type:4;
    unsigned encoding:4;
    unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
                            * LFU data (least significant 8 bits frequency
                            * and most significant 16 bits access time). */
  //LRU: 最近最少使用，以最近一次访问时间为参考。LFU: 最近一次被访问次数最少的数据，以次数为参考。                          
    int refcount;
    void *ptr;
} robj;
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在robj中每个元素冒号后面的是位数，sizeof(robj)的结果是16个字节，内存使用非常紧凑。
关于该结构体的详细描述可参考Redis 数据结构——robj，写的非常详细。

创建

robj *createObject(int type, void *ptr) {
    robj *o = zmalloc(sizeof(*o));
    o->type = type;
    o->encoding = OBJ_ENCODING_RAW;
    o->ptr = ptr;
    o->refcount = 1;

    /* Set the LRU to the current lruclock (minutes resolution), or
     * alternatively the LFU counter. */
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        o->lru = (LFUGetTimeInMinutes()<<8) | LFU_INIT_VAL;
    } else {
        o->lru = LRU_CLOCK();
    }
    return o;
}
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获取LRU

unsigned int getLRUClock(void) {
    return (mstime()/LRU_CLOCK_RESOLUTION) & LRU_CLOCK_MAX;
}
/* This function is used to obtain the current LRU clock.
 * If the current resolution is lower than the frequency we refresh the
 * LRU clock (as it should be in production servers) we return the
 * precomputed value, otherwise we need to resort to a system call. */
unsigned int LRU_CLOCK(void) {
    unsigned int lruclock;
    //默认LRU_CLOCK_RESOLUTION是1000，如果server.hz大于1000的话要使用原子类型的加载了
    if (1000/server.hz <= LRU_CLOCK_RESOLUTION) {
        atomicGet(server.lruclock,lruclock);
    } else {
        lruclock = getLRUClock();
    }
    return lruclock;
}
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设置成共享的对象

引入引用计数的功能，类似于C++中的shared_ptr

/* Set a special refcount in the object to make it "shared":
 * incrRefCount and decrRefCount() will test for this special refcount
 * and will not touch the object. This way it is free to access shared
 * objects such as small integers from different threads without any
 * mutex.
 *
 * A common patter to create shared objects:
 *
 * robj *myobject = makeObjectShared(createObject(...));
 *
 */
robj *makeObjectShared(robj *o) {
    serverAssert(o->refcount == 1);
    o->refcount = OBJ_SHARED_REFCOUNT;//INT_MAX
    return o;
}
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创建不同String对象

redis内置的sds结构

/* Create a string object with encoding OBJ_ENCODING_RAW, that is a plain
 * string object where o->ptr points to a proper sds string. */
robj *createRawStringObject(const char *ptr, size_t len) {
    return createObject(OBJ_STRING, sdsnewlen(ptr,len));
}

//可以理解为紧凑型的robj，string直接在robj后面
robj *createEmbeddedStringObject(const char *ptr, size_t len) {
	//robj后紧接着的就是这个sdshdr8
    robj *o = zmalloc(sizeof(robj)+sizeof(struct sdshdr8)+len+1);
    struct sdshdr8 *sh = (void*)(o+1);//指向这个sdshdr8对象

    o->type = OBJ_STRING;
    o->encoding = OBJ_ENCODING_EMBSTR;
    o->ptr = sh+1;//这里跳到的是字符串那里
    o->refcount = 1;
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        o->lru = (LFUGetTimeInMinutes()<<8) | LFU_INIT_VAL;
    } else {
        o->lru = LRU_CLOCK();
    }

    sh->len = len;
    sh->alloc = len;
    sh->flags = SDS_TYPE_8;
    if (ptr == SDS_NOINIT)
        sh->buf[len] = '\0';
    else if (ptr) {
        memcpy(sh->buf,ptr,len);
        sh->buf[len] = '\0';
    } else {
        memset(sh->buf,0,len+1);
    }
    return o;
}

/* Create a string object with EMBSTR encoding if it is smaller than
 * OBJ_ENCODING_EMBSTR_SIZE_LIMIT, otherwise the RAW encoding is
 * used.
 *
 * The current limit of 44 is chosen so that the biggest string object
 * we allocate as EMBSTR will still fit into the 64 byte arena of jemalloc. */
//小于44的话就创建EmbeddedStringObject，不然就是RawStringObject
#define OBJ_ENCODING_EMBSTR_SIZE_LIMIT 44
robj *createStringObject(const char *ptr, size_t len) {
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT)
        return createEmbeddedStringObject(ptr,len);
    else
        return createRawStringObject(ptr,len);
}

/* Same as CreateRawStringObject, can return NULL if allocation fails */
robj *tryCreateRawStringObject(const char *ptr, size_t len) {
    sds str = sdstrynewlen(ptr,len);
    if (!str) return NULL;
    return createObject(OBJ_STRING, str);
}

/* Same as createStringObject, can return NULL if allocation fails */
robj *tryCreateStringObject(const char *ptr, size_t len) {
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT)
        return createEmbeddedStringObject(ptr,len);
    else
        return tryCreateRawStringObject(ptr,len);
}

/* Create a string object from a long long value. When possible returns a
 * shared integer object, or at least an integer encoded one.
 *
 * If valueobj is non zero, the function avoids returning a shared
 * integer, because the object is going to be used as value in the Redis key
 * space (for instance when the INCR command is used), so we want LFU/LRU
 * values specific for each key. */
robj *createStringObjectFromLongLongWithOptions(long long value, int valueobj) {
    robj *o;

    if (server.maxmemory == 0 ||
        !(server.maxmemory_policy & MAXMEMORY_FLAG_NO_SHARED_INTEGERS))
    {
        /* If the maxmemory policy permits, we can still return shared integers
         * even if valueobj is true. */
        valueobj = 0;
    }

    if (value >= 0 && value < OBJ_SHARED_INTEGERS && valueobj == 0) {
        incrRefCount(shared.integers[value]);
        o = shared.integers[value];
    } else {
        if (value >= LONG_MIN && value <= LONG_MAX) {
            o = createObject(OBJ_STRING, NULL);
            o->encoding = OBJ_ENCODING_INT;
            o->ptr = (void*)((long)value);
        } else {
            o = createObject(OBJ_STRING,sdsfromlonglong(value));
        }
    }
    return o;
}

/* Wrapper for createStringObjectFromLongLongWithOptions() always demanding
 * to create a shared object if possible. */
robj *createStringObjectFromLongLong(long long value) {
    return createStringObjectFromLongLongWithOptions(value,0);
}

/* Wrapper for createStringObjectFromLongLongWithOptions() avoiding a shared
 * object when LFU/LRU info are needed, that is, when the object is used
 * as a value in the key space, and Redis is configured to evict based on
 * LFU/LRU. */
robj *createStringObjectFromLongLongForValue(long long value) {
    return createStringObjectFromLongLongWithOptions(value,1);
}

/* Create a string object from a long double. If humanfriendly is non-zero
 * it does not use exponential format and trims trailing zeroes at the end,
 * however this results in loss of precision. Otherwise exp format is used
 * and the output of snprintf() is not modified.
 *
 * The 'humanfriendly' option is used for INCRBYFLOAT and HINCRBYFLOAT. */
robj *createStringObjectFromLongDouble(long double value, int humanfriendly) {
    char buf[MAX_LONG_DOUBLE_CHARS];
    int len = ld2string(buf,sizeof(buf),value,humanfriendly? LD_STR_HUMAN: LD_STR_AUTO);
    return createStringObject(buf,len);
}
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后面是一些其他类型对象的创建，这里不多叙述

对象的解除（dismiss）

使用fork创建一个子进程用于生成快照时，主进程和子进程共享相同的物理内存页面，如果父进程更改了一些键，就会引起写时复制（copy on write， CoW），增加了内存的消耗。在子进程中，序列化键值之后这些数据将不会被再次访问，因此为了避免CoW，需要把这部分内存归还为操作系统，见dismissMemory()。鉴于迭代复杂数据类型中所有的node/field/member/entry比较耗时，我们仅在估计单个分配的大小大于 OS 的页面大小的近似平均值时迭代并关闭它们。'size_hint’是序列化值的大小。 这种方法并不准确，但它可以减少可能不会释放任何内存的复杂数据类型的不必要迭代。

void dismissObject(robj *o, size_t size_hint) {
    /* madvise(MADV_DONTNEED) may not work if Transparent Huge Pages is enabled. */
    if (server.thp_enabled) return;
	//这里只有使用Linux下的jemalloc才进行这个操作
    /* Currently we use zmadvise_dontneed only when we use jemalloc with Linux.
     * so we avoid these pointless loops when they're not going to do anything. */
#if defined(USE_JEMALLOC) && defined(__linux__)
    if (o->refcount != 1) return;
    switch(o->type) {
        case OBJ_STRING: dismissStringObject(o); break;
        case OBJ_LIST: dismissListObject(o, size_hint); break;
        case OBJ_SET: dismissSetObject(o, size_hint); break;
        case OBJ_ZSET: dismissZsetObject(o, size_hint); break;
        case OBJ_HASH: dismissHashObject(o, size_hint); break;
        case OBJ_STREAM: dismissStreamObject(o, size_hint); break;
        default: break;
    }
#else
    UNUSED(o); UNUSED(size_hint);
#endif
}
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内存解除

以dismissSds为例，详解一下这部分的函数：

void dismissSds(sds s) {
    dismissMemory(sdsAllocPtr(s), sdsAllocSize(s));
}
/* Try to release pages back to the OS directly (bypassing the allocator),
 * in an effort to decrease CoW during fork. For small allocations, we can't
 * release any full page, so in an effort to avoid getting the size of the
 * allocation from the allocator (malloc_size) when we already know it's small,
 * we check the size_hint. If the size is not already known, passing a size_hint
 * of 0 will lead the checking the real size of the allocation.
 * Also please note that the size may be not accurate, so in order to make this
 * solution effective, the judgement for releasing memory pages should not be
 * too strict. */
//内存大小超过一个页面的大小才可以这样做
void dismissMemory(void* ptr, size_t size_hint) {
    if (ptr == NULL) return;

    /* madvise(MADV_DONTNEED) can not release pages if the size of memory
     * is too small, we try to release only for the memory which the size
     * is more than half of page size. */
    if (size_hint && size_hint <= server.page_size/2) return;

    zmadvise_dontneed(ptr);
}
/* Use 'MADV_DONTNEED' to release memory to operating system quickly.
 * We do that in a fork child process to avoid CoW when the parent modifies
 * these shared pages. */
void zmadvise_dontneed(void *ptr) {
#if defined(USE_JEMALLOC) && defined(__linux__)
    static size_t page_size = 0;
    if (page_size == 0) page_size = sysconf(_SC_PAGESIZE);
    size_t page_size_mask = page_size - 1;

    size_t real_size = zmalloc_size(ptr);
    if (real_size < page_size) return;

    /* We need to align the pointer upwards according to page size, because
     * the memory address is increased upwards and we only can free memory
     * based on page. */
     //必须以页面为单位释放内存，先进行内存对齐
    char *aligned_ptr = (char *)(((size_t)ptr+page_size_mask) & ~page_size_mask);
    real_size -= (aligned_ptr-(char*)ptr);
    if (real_size >= page_size) {
        madvise((void *)aligned_ptr, real_size&~page_size_mask, MADV_DONTNEED);
    }
#else
    (void)(ptr);
#endif
}
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浮点型数值的存储

在redis中，浮点型数值也是使用sds类型进行存储的，主要的函数为：

int getDoubleFromObject(const robj *o, double *target) {
    double value;

    if (o == NULL) {
        value = 0;
    } else {
        //类型一定要是OBJ_STRING
        serverAssertWithInfo(NULL,o,o->type == OBJ_STRING);
        //首先判断encoding是不是OBJ_ENCODING_RAW || OBJ_ENCODING_EMBSTR
        if (sdsEncodedObject(o)) {
        //是的话将字符串转换为double
            if (!string2d(o->ptr, sdslen(o->ptr), &value))
                return C_ERR;
        } else if (o->encoding == OBJ_ENCODING_INT) {
        //是OBJ_ENCODING_INT的话直接读取就行
            value = (long)o->ptr;
        } else {
            serverPanic("Unknown string encoding");
        }
    }
    *target = value;
    return C_OK;
}

int getDoubleFromObjectOrReply(client *c, robj *o, double *target, const char *msg) {
    double value;
    if (getDoubleFromObject(o, &value) != C_OK) {
        if (msg != NULL) {
            addReplyError(c,(char*)msg);
        } else {
            addReplyError(c,"value is not a valid float");
        }
        return C_ERR;
    }
    *target = value;
    return C_OK;
}
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