redis 源代码之数据结构(4)--zipmap实现

redis对于内存的使用精确到了bit了,redis引入了zipmap数据结构，可以在hash表的元素比较少的时候，使用zipmap来节约内存。下面就分析一下zipmap的源代码（zipmap.c)

作者在源文件开头讲述了zipmap的数据格式，假定有这么个映射关系“foo”=>"bar", "hello"=>"world",则内存布局为

<zmlen(2)><len(3)>"foo"<len(3)><free(0)>"bar"<len(5)>"hello"<len(6)><free(0)>"world"

zmlen是用一字节表示，代表zipmap结构中实际key的个数。当zipmap的长度大于等于254，zmlen字段将不被使用，要想知道key的数目，只能遍历zipmap，这样会导致zipmap的性能下降，实际上，zipmap可以使用ziplist来代替，性能不会打折扣~所以个人觉得zipmap有点鸡肋。

len代表key或者是value的字节长度，len本身可能占用1个字节，也可能占用5个字节。如果len编码的第一个字节为0~252，则len只占用1字节，如果len的第一个字节是数值是253，则其后的四字节表示key或value的长度（这样len总共就有5字节了，首字节只作为标记）。如果首字节是255，则表示zipmap的结尾，若为254，则代表hash表还有空间可以增加新的键值对。

free表示没有被使用的字节树，比如说，”foo"映射为“bar”，但是后来将“foo“映射为了”hi",则会在原处空处1个字节，zipmap并不是直接释放掉这个1字节，相反，会保留，下次再设定value的时候，可能就不需要分配内存了。

zipmap的查询是O(N),N是key的数目。

下面看看zipmap的常用的api。

unsigned char *zipmapNew(void);//此函数用于创建一个新的zipmap，实际上就分配了两个字节。一个字节表示zmlen，并初始化为0，另外一字节设为255，表示结尾

zipmapSet函数

/* Set key to value, creating the key if it does not already exist. * If 'update' is not NULL, *update is set to 1 if the key was * already preset, otherwise to 0. */unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {    unsigned int zmlen, offset;    unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);//zipmapRequiredLength返回key和value总共需要的字节数目    unsigned int empty, vempty;    unsigned char *p;       freelen = reqlen;    if (update) *update = 0;    p = zipmapLookupRaw(zm,key,klen,&zmlen);  //查询key是否在zm中,并得到zm的字节大小（zmlen）,如果存在，则返回key所在的指针偏移    if (p == NULL) {        /* Key not found: enlarge */        zm = zipmapResize(zm, zmlen+reqlen);  //调用的是realloc，将zm扩大        p = zm+zmlen-1;        zmlen = zmlen+reqlen;          //更新zm的总字节数        /* Increase zipmap length (this is an insert) */        if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;      //更新key的数目    } else {        /* Key found. Is there enough space for the new value? */        /* Compute the total length: */        if (update) *update = 1;              freelen = zipmapRawEntryLength(p);        if (freelen < reqlen) {            /* Store the offset of this key within the current zipmap, so             * it can be resized. Then, move the tail backwards so this             * pair fits at the current position. */            offset = p-zm;            zm = zipmapResize(zm, zmlen-freelen+reqlen);  //重新分配内存，以便扩大zm的容量            p = zm+offset;                                 //找到要更新key的位置            /* The +1 in the number of bytes to be moved is caused by the             * end-of-zipmap byte. Note: the *original* zmlen is used. */            memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));//将要修改的key/value之后的字符串向后顺移            zmlen = zmlen-freelen+reqlen;            freelen = reqlen;        }    }    /* We now have a suitable block where the key/value entry can     * be written. If there is too much free space, move the tail     * of the zipmap a few bytes to the front and shrink the zipmap,     * as we want zipmaps to be very space efficient. */    empty = freelen-reqlen;    if (empty >= ZIPMAP_VALUE_MAX_FREE) {//如果空的太多，大于4的话，就需要释放掉这些空出来的字节        /* First, move the tail <empty> bytes to the front, then resize         * the zipmap to be <empty> bytes smaller. */        offset = p-zm;        memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));//移除free空间        zmlen -= empty;        zm = zipmapResize(zm, zmlen);//重新设定zm的大小        p = zm+offset;        vempty = 0;    } else {        vempty = empty;    }    /* Just write the key + value and we are done. */    /* Key: */    p += zipmapEncodeLength(p,klen);    memcpy(p,key,klen);    p += klen;    /* Value: */    p += zipmapEncodeLength(p,vlen);    *p++ = vempty;//每个value 没用到的空间（字段free）的大小    memcpy(p,val,vlen);    return zm;}

zipmapDel是删除key/value的api

unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted) {     unsigned int zmlen, freelen;     unsigned char *p = zipmapLookupRaw(zm,key,klen,&zmlen); //首先去查找key的位置    if (p) { //找到key了        freelen = zipmapRawEntryLength(p);         memmove(p, p+freelen, zmlen-((p-zm)+freelen+1)); //移动整个zm，这样 就可以将key/value给覆盖掉，从而删除key/value        zm = zipmapResize(zm, zmlen-freelen);        //收回空间         /* Decrease zipmap length */         if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;    //key的数目自减一         if (deleted) *deleted = 1;     } else {         if (deleted) *deleted = 0;     }     return zm; }

淘宝核心系统团队博客的关于zipmap的内存布局http://rdc.taobao.com/blog/cs/?p=1314 ，里面有图有真相~