redis zipmap
来源:互联网 发布:zozoc软件 编辑:程序博客网 时间:2024/06/05 21:32
原帖地址: http://lobert.iteye.com/blog/1858823
在看此文件源码之前,先看到此文件头部的英文注释,以下是本人理解翻译版:</p>
该文件实现了一个数据结构映射到其他字符串的字符串,实施一个O(n)查找数据结构的设计是非常记忆高效的。 Redis的hase类型就是使用这种由小数量元素组成的数据结构,转换为一个哈希表。鉴于很多次Redis hase是用来表示对象组成的一些字段,这是一种在内存使用上很大的成功。
它的zipmap的格式为:
<zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
<zmlen>是1字节长度,持有的当前zipmap的大小。当zipmap长度大于或等于254,这个值并不使用,zipmap需要遍历找出长度 。
<len>是下列字符串的长度(键或值)。<len>长度编码在一个单一的值或在一个5字节值。如果第一个字节值(作为一个unsigned 8位值)是介于0和252,这是一个单字节长度。如果它是253,接着后面会是一个四字节的无符号整数(在主机字节排序)。一个值255用于信号结束的散列。特殊值254是用来标记空空间,可用于添加新的键/值对。
<free>是修改key关联的value后string后的未使用的空闲的字节数。例如,如果“foo” 设置为“bar”,后“foo”将被设置为“hi”,它将有一个免费的字节,使用如果值将稍后再扩大,甚至添加一对适合的键/值。
<free>总是一个unsigned 8位,因为如果在一个更新操作有很多免费的字节,zipmap将重新分配,以确保它是尽可能小。
通过注释可以清楚此结构的最大优点就是内存使用。由此也基本知道了其结构的组成,下面分析源码也轻松很多。
- /* Create a new empty zipmap. */
- unsignedchar *zipmapNew(void) {
- unsignedchar *zm = zmalloc(2);
- zm[0] = 0;/* Length */
- zm[1] = ZIPMAP_END;
- return zm;
- }
新建一个空的zipmap,其结构如图:
- /* Decode the encoded length pointed by 'p' */
- static unsignedint zipmapDecodeLength(unsigned char *p) {
- unsignedint len = *p;
- if (len < ZIPMAP_BIGLEN) return len;
- memcpy(&len,p+1,sizeof(unsignedint));
- memrev32ifbe(&len);//大小端转换
- return len;
- }
- /* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
- * the amount of bytes required to encode such a length. */
- static unsignedint zipmapEncodeLength(unsigned char *p, unsigned int len) {
- if (p == NULL) {
- return ZIPMAP_LEN_BYTES(len);
- }else {
- if (len < ZIPMAP_BIGLEN) {
- p[0] = len;
- return 1;
- }else {
- p[0] = ZIPMAP_BIGLEN;
- memcpy(p+1,&len,sizeof(len));
- memrev32ifbe(p+1);
- return 1+sizeof(len);
- }
- }
- }
//上为解码,下为编码(将key的长度转为char,返回所占的字节数)。主要是当key/value的长度大于等于ZIPMAP_BIGLEN(254)时,<len>的头字符就为ZIPMAP_BIGLEN,后将len转换为char型,存入len。(为了节省这四个字节)
- /* Search for a matching key, returning a pointer to the entry inside the
- * zipmap. Returns NULL if the key is not found.
- *
- * If NULL is returned, and totlen is not NULL, it is set to the entire
- * size of the zimap, so that the calling function will be able to
- * reallocate the original zipmap to make room for more entries. */
- static unsignedchar *zipmapLookupRaw(unsigned char *zm, unsigned char *key, unsignedint klen, unsigned int *totlen) {
- unsignedchar *p = zm+1, *k = NULL;//开始+1,跳过length
- unsignedint l,llen;
- while(*p != ZIPMAP_END) {
- unsignedchar free;
- /* Match or skip the key */
- l = zipmapDecodeLength(p);//取得key的长度
- llen = zipmapEncodeLength(NULL,l);//取得key占用的字节数
- if (key != NULL && k == NULL && l == klen && !memcmp(p+llen,key,l)) {
- /* Only return when the user doesn't care
- * for the total length of the zipmap. */
- if (totlen != NULL) {
- k = p;
- }else {
- return p;
- }
- }
- p += llen+l;
- /* Skip the value as well */
- l = zipmapDecodeLength(p);//取得value的长度
- p += zipmapEncodeLength(NULL,l);//取得value占用的字节数
- free = p[0];
- p += l+1+free;/* +1 to skip the free byte */
- }
- if (totlen != NULL) *totlen = (unsigned int)(p-zm)+1;
- return k;
- }
//查找key,注意totlen的值,如果没到key ,totlen将等于p的总长度,如果找到了,totlen等于key的下标
- static unsignedlong zipmapRequiredLength(unsigned int klen, unsigned int vlen) {
- unsignedint l;
- l = klen+vlen+3;//注意此处为何要加3? (klen和vlen本身要占用1字节,还有1字节是留给free的)
- if (klen >= ZIPMAP_BIGLEN) l += 4;//这里加4,是因为上面编码方法中所明
- if (vlen >= ZIPMAP_BIGLEN) l += 4;
- return l;
- }
- /* Return the total amount used by a key (encoded length + payload) */
- static unsignedint zipmapRawKeyLength(unsigned char *p) {
- unsignedint l = zipmapDecodeLength(p);
- return zipmapEncodeLength(NULL,l) + l;
- }
- //返回key总字节数
- /* Return the total amount used by a value
- * (encoded length + single byte free count + payload) */
- static unsignedint zipmapRawValueLength(unsigned char *p) {
- unsignedint l = zipmapDecodeLength(p);
- unsignedint used;
- used = zipmapEncodeLength(NULL,l);
- used += p[used] + 1 + l;
- return used;
- }
- //返回value总字节数,包含free字节
- /* If 'p' points to a key, this function returns the total amount of
- * bytes used to store this entry (entry = key + associated value + trailing
- * free space if any). */
- static unsignedint zipmapRawEntryLength(unsigned char *p) {
- unsignedint l = zipmapRawKeyLength(p);
- return l + zipmapRawValueLength(p+l);
- }
- //返回key和value总共所占的字节
- staticinline unsigned char *zipmapResize(unsignedchar *zm, unsigned int len) {
- zm = zrealloc(zm, len);
- zm[len-1] = ZIPMAP_END;
- return zm;
- }
- //重置zm
- /* 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. */
- unsignedchar *zipmapSet(unsigned char *zm, unsignedchar *key, unsigned int klen, unsignedchar *val, unsigned int vlen,int *update) {
- unsignedint zmlen, offset;
- unsignedint freelen, reqlen = zipmapRequiredLength(klen,vlen);
- unsignedint empty, vempty;
- unsignedchar *p;
- freelen = reqlen;
- if (update) *update = 0;
- p = zipmapLookupRaw(zm,key,klen,&zmlen);
- if (p == NULL) {
- /* Key not found: enlarge */
- zm = zipmapResize(zm, zmlen+reqlen);
- p = zm+zmlen-1;
- zmlen = zmlen+reqlen;
- /* Increase zipmap length (this is an insert) */
- if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
- }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);
- p = zm+offset;
- /* 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));
- 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) {
- /* 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));
- zmlen -= empty;
- zm = zipmapResize(zm, zmlen);
- 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;
- memcpy(p,val,vlen);
- return zm;
- }
此方法图解如下:
文件中还有几个方法如zipmapGet,zipmapNext等,如果zipmapSet搞懂,其它方法便无障碍。
- /* Return the number of entries inside a zipmap */
- unsignedint zipmapLen(unsigned char *zm) {
- unsignedint len = 0;
- if (zm[0] < ZIPMAP_BIGLEN) {
- //早在注释时就说过,如果size大小超过了ZIPMAP_BIGLEN,那么zipmap的第一个字节将不会记录size,size需要遍历才能得出
- len = zm[0];
- }else {
- unsignedchar *p = zipmapRewind(zm);
- while((p = zipmapNext(p,NULL,NULL,NULL,NULL)) != NULL) len++;
- /* Re-store length if small enough */
- if (len < ZIPMAP_BIGLEN) zm[0] = len;
- }
- return len;
- }
//为什么在记录zipmap长度时不效仿记录key/value长度的方法,以至于如果取个数都需要遍历一遍?
不过根据我的实际应用经验,很少会直接去取hase的size.
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