Redis 6.0 源码阅读笔记(4)-String 数据类型源码分析

ゝ一世哀愁。 2023-10-05 09:18 12阅读 0赞

#### 文章目录 ####

*  1. 存储的结构
 *  2. 数据存储源码分析
 *   *   *  2.1 数据存储过程
         *  2.2 简单动态字符串 SDS
         *   *   *  2.2.1 SDS 结构体
                 *  2.2.2 SDS 容量调整

## 1. 存储的结构 ##

在 [redis 字符串对象 String][redis _ String] 的介绍中，我们知道 redis 对于字符串的存储共有 3 种存储形式，其存储的内存结构如以下图片示例:

*  `OBJ_ENCODING_INT`： 保存的字符串长度小于 20，并且是可以解析为 long 类型的整数值，那么存储方式就是直接将 redisObject 的 ptr 指针指向这个整数值  
    ![在这里插入图片描述][20200829113959232.jpg_pic_center]
 *  `OBJ_ENCODING_EMBSTR`: 长度小于 44 （OBJ\_ENCODING\_EMBSTR\_SIZE\_LIMIT）的字符串将以简单动态字符串（SDS） 的形式存储在 redisObject 中，但是redisObject 对象头会和 SDS 对象连续存在一起  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NTUwNTMxMw_size_16_color_FFFFFF_t_70_pic_center]
 *  `OBJ_ENCODING_RAW`: 字符串以简单动态字符串（SDS） 的形式存储，redisObject 对象头和 SDS 对象在内存地址上一般是不连续的两块内存  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NTUwNTMxMw_size_16_color_FFFFFF_t_70_pic_center 1]

## 2. 数据存储源码分析 ##

#### 2.1 数据存储过程 ####

1.  在 [Redis 6.0 源码阅读笔记(1)-Redis 服务端启动及命令执行][Redis 6.0 _1_-Redis] 中我们已经知道客户端保存字符串的 `set` 命令将会调用到 `t_string.c#setCommand()` 函数，其源码实现如下：
    
    > 该方法中有以下两个重点函数被调用，本节主要关注 tryObjectEncoding() 函数
    > 
    > 1.  tryObjectEncoding() 将客户端传输过来的需保存的字符串对象尝试进行编码，以节约内存
    > 2.  setGenericCommand() 将 key-value 保存到数据库中
    
        void setCommand(client *c) {
              
        
         ......
        
         c->argv[2] = tryObjectEncoding(c->argv[2]);
         setGenericCommand(c,flags,c->argv[1],c->argv[2],expire,unit,NULL,NULL);
        }
2.  `object.c#tryObjectEncoding()` 函数逻辑很清晰，可以看到主要进行了以下几个操作：
    
    > 1.  当字符串长度小于 20 并且可以被解析为 long 类型数据时，这个数据将以整数形式保存，并以 `robj->ptr = (void*) value` 这种直接赋值的形式存储
    > 2.  当字符串长度小于等于 OBJ\_ENCODING\_EMBSTR\_SIZE\_LIMIT 配置并且还是 raw 编码时，调用 createEmbeddedStringObject() 函数将其转化为 embstr 编码
    > 3.  这个字符串对象已经不能进行转码了，只好调用 trimStringObjectIfNeeded() 函数尝试从字符串对象中移除所有空余空间
    
        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;
        
         /* 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. */
         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;
                 } else if (o->encoding == OBJ_ENCODING_EMBSTR) {
              
                     decrRefCount(o);
                     return createStringObjectFromLongLongForValue(value);
                 }
             }
         }
        
         /* 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));
             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
          * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
         trimStringObjectIfNeeded(o);
        
         /* Return the original object. */
         return o;
        }
3.  `object.c#createEmbeddedStringObject()` 函数实现 embstr 编码也很简单，主要步骤如下：
    
    > 1.  首先调用 zmalloc() 函数申请内存，可以看到此处不仅申请了需要存储的字符串的内存及 redisObject 的内存，还申请了 SDS 实现结构体之一 sdshdr8 的内存，这也就是上文所说embstr 编码只申请一次内存，并且redisObject 对象头会和 SDS 对象连续存在一起的由来
    > 2.  将 redisObject 对象的 ptr 指针指向 sdshdr8 开始的内存地址
    > 3.  填充 sdshdr8 对象各个属性，包括 `len` 字符串长度，`alloc` 字符数组容量，实际存储字符串的 `buf` 字符数组
    
        robj *createEmbeddedStringObject(const char *ptr, size_t len) {
              
         robj *o = zmalloc(sizeof(robj)+sizeof(struct sdshdr8)+len+1);
         struct sdshdr8 *sh = (void*)(o+1);
        
         o->type = OBJ_STRING;
         o->encoding = OBJ_ENCODING_EMBSTR;
         o->ptr = sh+1;
         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;
        }
4.  raw 编码字符串的创建可参考 `object.c#createRawStringObject()` 函数，其涉及到两次内存申请，`sds.c#sdsnewlen()` 申请内存创建 SDS 对象，`object.c#createObject()` 申请内存创建 redisObject 对象
    
        robj *createRawStringObject(const char *ptr, size_t len) {
              
         return createObject(OBJ_STRING, sdsnewlen(ptr,len));
        }
5.  从检测容量大小的的函数`t_string.c#checkStringLength()`看，字符串最大长度为 512M，超出该数值将报错
    
        static int checkStringLength(client *c, long long size) {
              
         if (size > 512*1024*1024) {
              
             addReplyError(c,"string exceeds maximum allowed size (512MB)");
             return C_ERR;
         }
         return C_OK;
        }

#### 2.2 简单动态字符串 SDS ####

###### 2.2.1 SDS 结构体 ######

`SDS(简单动态字符串)` 在 Redis 中是实现字符串存储的工具，本质上依然是字符数组，但它不像C语言字符串那样以‘\\0’来标识字符串结束

> 传统C字符串符合ASCII编码，这种编码的操作的特点就是：遇零则止 。即当读一个字符串时，只要遇到’\\0’就认为到达末尾，忽略’\\0’以后的所有字符。另外其获得字符串长度的做法是遍历字符串，遇零则止，时间复杂度为O(n)，比较低效

SDS 的实现结构定义在 `sds.h` 中，其定义如下。因为 SDS 判断是否到达字符串末尾的依据是表头的 len 属性，所以能高效计算字符串长度并快速追加数据

> sds 结构一共有 5 种 Header 定义，目的是为不同长度的字符串提供不同大小的 Header，以节省内存。以 sdshdr8 为例，其 len 属性为 uint8\_t 类型，占用内存大小为 1 字节，则存储的字符串最大长度为256。Header 主要包含以下几个属性：
> 
> 1.  `len`: 字符串真正的长度，不包含空终止字符
> 2.  `alloc`: 除去表头和终止符的 buf 数组长度，也就是最大容量
> 3.  `flags`: 标志 header 的类型
> 4.  `buf`: 字符数组，实际存储字符

/* Note: sdshdr5 is never used, we just access the flags byte directly.
     * However is here to document the layout of type 5 SDS strings. */
    struct __attribute__ ((__packed__)) sdshdr5 {
        
        unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
        char buf[];
    };
    struct __attribute__ ((__packed__)) sdshdr8 {
        
        uint8_t len; /* used */
        uint8_t alloc; /* excluding the header and null terminator */
        unsigned char flags; /* 3 lsb of type, 5 unused bits */
        char buf[];
    };
    struct __attribute__ ((__packed__)) sdshdr16 {
        
        uint16_t len; /* used */
        uint16_t alloc; /* excluding the header and null terminator */
        unsigned char flags; /* 3 lsb of type, 5 unused bits */
        char buf[];
    };
    struct __attribute__ ((__packed__)) sdshdr32 {
        
        uint32_t len; /* used */
        uint32_t alloc; /* excluding the header and null terminator */
        unsigned char flags; /* 3 lsb of type, 5 unused bits */
        char buf[];
    };
    struct __attribute__ ((__packed__)) sdshdr64 {
        
        uint64_t len; /* used */
        uint64_t alloc; /* excluding the header and null terminator */
        unsigned char flags; /* 3 lsb of type, 5 unused bits */
        char buf[];
    };

###### 2.2.2 SDS 容量调整 ######

1.  SDS 扩容的函数是`sds.c#sdsMakeRoomFor()`，当字符串长度小于 1M 时，扩容都是加倍现有的空间，如果超过 1M，扩容时一次只会多扩 1M 的空间。以下为源码实现：
    
    > 字符串在长度小于 SDS\_MAX\_PREALLOC(1024\*1024，也就是1MB，定义在 sds.h) 之前，采用 2 倍扩容，也就是保留 100% 的冗余空间。当长度超过 SDS\_MAX\_PREALLOC 之后，每次扩容只会多分配 SDS\_MAX\_PREALLOC 大小的冗余空间，避免加倍扩容后的冗余空间过大导致浪费
    
        sds sdsMakeRoomFor(sds s, size_t addlen) {
              
         void *sh, *newsh;
         size_t avail = sdsavail(s);
         size_t len, newlen;
         char type, oldtype = s[-1] & SDS_TYPE_MASK;
         int hdrlen;
        
         /* Return ASAP if there is enough space left. */
         if (avail >= addlen) return s;
        
         len = sdslen(s);
         sh = (char*)s-sdsHdrSize(oldtype);
         newlen = (len+addlen);
         if (newlen < SDS_MAX_PREALLOC)
             newlen *= 2;
         else
             newlen += SDS_MAX_PREALLOC;
        
         type = sdsReqType(newlen);
        
         /* Don't use type 5: the user is appending to the string and type 5 is
          * not able to remember empty space, so sdsMakeRoomFor() must be called
          * at every appending operation. */
         if (type == SDS_TYPE_5) type = SDS_TYPE_8;
        
         hdrlen = sdsHdrSize(type);
         if (oldtype==type) {
              
             newsh = s_realloc(sh, hdrlen+newlen+1);
             if (newsh == NULL) return NULL;
             s = (char*)newsh+hdrlen;
         } else {
              
             /* Since the header size changes, need to move the string forward,
              * and can't use realloc */
             newsh = s_malloc(hdrlen+newlen+1);
             if (newsh == NULL) return NULL;
             memcpy((char*)newsh+hdrlen, s, len+1);
             s_free(sh);
             s = (char*)newsh+hdrlen;
             s[-1] = type;
             sdssetlen(s, len);
         }
         sdssetalloc(s, newlen);
         return s;
        }
2.  SDS 缩容函数为`sds.c#sdsclear()`，从源码实现来看，其主要有以下操作，也就是它并不释放实际占用的内存，体现出一种惰性策略
    
    > 1.  重置 SDS 表头的 len 属性值为 0
    > 2.  将结束符放到 buf 数组最前面，相当于惰性地删除 buf 中的内容
    
        /* Modify an sds string in-place to make it empty (zero length).
        * However all the existing buffer is not discarded but set as free space
        * so that next append operations will not require allocations up to the
        * number of bytes previously available. */
        void sdsclear(sds s) {
              
         sdssetlen(s, 0);
         s[0] = '\0';
        }

[redis _ String]: https://blog.csdn.net/weixin_45505313/article/details/108241541#221__String_175
[20200829113959232.jpg_pic_center]: https://img-blog.csdnimg.cn/20200829113959232.jpg#pic_center
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[Redis 6.0 _1_-Redis]: https://blog.csdn.net/weixin_45505313/article/details/107816424