一、SDS结构
老版本
// 3.0及以前
//查看到redis2.6版本 sds.h/sdshdr
struct sdshdr {
// 记录buf数组中已使用字节数量
unsigned int len;
// 记录buf数组中未使用的字节数量
unsigned int free;
// 字节数组，存储字符串
//最后一个字节保存为空字符 '\0'
char buf[];
};
sds优点
频繁操作数据会对性能存在很大影响
- 空间预分配
a、小于1MB SDS分配的len和free 是相同，如len=5 ，free=5，buf总长度为 5+5+1 ，其中1为结束符\0。
b、大于1MB SDS分配的空间 3MB+1MB+1btype 每次只扩充1MB。
在扩展SDS空间之前，SDS API会先检查未使用空间是否足够，如果足够的话，API就会直接使用未使用空间，而无须执行内存重分配。
通过这种预分配策略，SDS将连续增长N次字符串所需的内存重分配次数从必定N次降低为最多N次。
（空间就是这么节省的）
- 惰性空间释放
sds在对字符串做缩短操作时候 程序并不立即使用内存重新分配来回收多出来的字符，而是通过free属性来记录起来多出来的字符长度，等待回来使用
现在使用版本
typedef char *sds;
/* 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[];
};
1. len 记录当前字节数组的长度（不包括\0）
2. alloc记录了当前字节数组总共分配的内存大小（不包括\0）
3. flags记录了当前字节数组的属性、用来标识到底是sdshdr8还是sdshdr16等
4. buf保存了字符串真正的值以及末尾的一个\0
5种不同类型的数据结构分别对应不同长度的字符串需求
static inline char sdsReqType(size_t string_size) {
if (string_size < 1<<5)
return SDS_TYPE_5;
if (string_size < 1<<8)
return SDS_TYPE_8;
if (string_size < 1<<16)
return SDS_TYPE_16;
#if (LONG_MAX == LLONG_MAX)
if (string_size < 1ll<<32)
return SDS_TYPE_32;
#endif
return SDS_TYPE_64;
}
sdshdr5数据结构说明：
没有alloc 和len
原因：
#define SDS_TYPE_5 0
#define SDS_TYPE_8 1
#define SDS_TYPE_16 2
#define SDS_TYPE_32 3
#define SDS_TYPE_64 4
#define SDS_TYPE_MASK 7
可以看出SDS_TYPE只占用了0,1,2,3,4五个数字，正好占用三位。
由于sdshdr5的只用来存储长度为32字节以下的字符数，
因此flags的5个bit就能满足长度记录，加上type所需的3bit，刚好为8bit一个字节。
因此sdshdr5不需要单独的len记录长度，并且只有32个字节的存储空间，动态的变更内存余地较小，
所以 redis 直接不存储alloc，当sdshdr5需要扩展时会直接变更成更大的SDS数据结构。
除此之外，SDS都会多分配1个字节用来保存'\0'。
SDS 创建
/* Create a new sds string with the content specified by the 'init' pointer
* and 'initlen'.
* If NULL is used for 'init' the string is initialized with zero bytes.
*
* The string is always null-termined (all the sds strings are, always) so
* even if you create an sds string with:
*
* mystring = sdsnewlen("abc",3);
*
* You can print the string with printf() as there is an implicit \0 at the
* end of the string. However the string is binary safe and can contain
* \0 characters in the middle, as the length is stored in the sds header.
*/
sds sdsnewlen(const void *init, size_t initlen) {
void *sh;
sds s;
char type = sdsReqType(initlen);
/* Empty strings are usually created in order to append. Use type 8
* since type 5 is not good at this.
直接强制赋值为SDS_TYPE_8
*/
if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
int hdrlen = sdsHdrSize(type);
unsigned char *fp; /* flags pointer. */
sh = s_malloc(hdrlen+initlen+1);
if (!init)
memset(sh, 0, hdrlen+initlen+1);
if (sh == NULL) return NULL;
s = (char*)sh+hdrlen;
fp = ((unsigned char*)s)-1;
switch(type) {
case SDS_TYPE_5: {
*fp = type | (initlen << SDS_TYPE_BITS);
break;
}
case SDS_TYPE_8: {
SDS_HDR_VAR(8,s);
sh->len = initlen;
sh->alloc = initlen;
*fp = type;
break;
}
case SDS_TYPE_16: {
SDS_HDR_VAR(16,s);
sh->len = initlen;
sh->alloc = initlen;
*fp = type;
break;
}
case SDS_TYPE_32: {
SDS_HDR_VAR(32,s);
sh->len = initlen;
sh->alloc = initlen;
*fp = type;
break;
}
case SDS_TYPE_64: {
SDS_HDR_VAR(64,s);
sh->len = initlen;
sh->alloc = initlen;
*fp = type;
break;
}
}
if (initlen && init)
memcpy(s, init, initlen);
s[initlen] = '\0';
return s;
}
SDS扩容
#define SDS_MAX_PREALLOC (1024*1024) 为1M
/* Enlarge the free space at the end of the sds string so that the caller
* is sure that after calling this function can overwrite up to addlen
* bytes after the end of the string, plus one more byte for nul term.
*
* Note: this does not change the *length* of the sds string as returned
* by sdslen(), but only the free buffer space we have. */
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;
}
该函数便是扩大sds空间，但是感觉上还是想让sds中available空间的大小能够容纳addlen大小的字符串，并不是改变了sds中buf的长度，
而是改变了sds中available空间的大小，
如果当前available空间的大小大于addlen的大小，那么便不作修；
如果available空间的大小小于addlen的大小，那么就会重新分配sds中alloc的大小，
newlen并不是无脑直接让alloc加上addlen，而且使用sds的长度加上addlen的长度
作为newlen，但是经常重新分配内存会对效率有所影响，但是为了防止重新分配内存
对效率的影响而让newlen无脑翻倍的话，又会对内存造成影响，造成内存占用过高，
但是很大一部分内存并没有使用，所以取得了一个折中的办法，就是在newlen小于
SDS_MAX_PREALLOC(1M)，对newlen进行翻倍，
在newlen大于SDS_MAX_PREALLOC的情况下，让newlen加上SDS_MAX_PREALLOC。
符合前面提的空间预分配
SDS惰性空间释放
在SDS的字符串缩短操作中，多余出来的空间并不会直接释放，而是保留这部分空间，待以后再用
/* Remove the part of the string from left and from right composed just of
* contiguous characters found in 'cset', that is a null terminted C string.
*
* After the call, the modified sds string is no longer valid and all the
* references must be substituted with the new pointer returned by the call.
*
* Example:
*
* s = sdsnew("AA...AA.a.aa.aHelloWorld :::");
* s = sdstrim(s,"Aa. :");
* printf("%s\n", s);
*
* Output will be just "Hello World".
*/
sds sdstrim(sds s, const char *cset) {
char *start, *end, *sp, *ep;
size_t len;
sp = start = s;
ep = end = s+sdslen(s)-1;
while(sp <= end && strchr(cset, *sp)) sp++;
while(ep > sp && strchr(cset, *ep)) ep--;
len = (sp > ep) ? 0 : ((ep-sp)+1);
if (s != sp) memmove(s, sp, len);
s[len] = '\0';
sdssetlen(s,len);
return s;
}
真正将空间释放还是会根据实际字符串情况返回对应类型。
例如以前是一个sdshdr64的sds，在redis运行过程中，buf的内容被修改了，变短了，那么多出来的内容就需要释放掉，还给系统，并且，如果修改得比较多，现在一个sdshdr16的sds就能容纳下，那么当前sds的type还会被修改，因为不同的sds类型占用的空间也是不一样的。
/* Reallocate the sds string so that it has no free space at the end. The
* contained string remains not altered, but next concatenation operations
* will require a reallocation.
*
* After the call, the passed sds string is no longer valid and all the
* references must be substituted with the new pointer returned by the call. */
sds sdsRemoveFreeSpace(sds s) {
void *sh, *newsh;
char type, oldtype = s[-1] & SDS_TYPE_MASK;
int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
size_t len = sdslen(s);
sh = (char*)s-oldhdrlen;
/* Check what would be the minimum SDS header that is just good enough to
* fit this string. */
type = sdsReqType(len);
hdrlen = sdsHdrSize(type);
/* If the type is the same, or at least a large enough type is still
* required, we just realloc(), letting the allocator to do the copy
* only if really needed. Otherwise if the change is huge, we manually
* reallocate the string to use the different header type. */
if (oldtype==type || type > SDS_TYPE_8) {
newsh = s_realloc(sh, oldhdrlen+len+1);
if (newsh == NULL) return NULL;
s = (char*)newsh+oldhdrlen;
} else {
newsh = s_malloc(hdrlen+len+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, len);
return s;
总结：
类似java 的ArrayList
遗留问题
- Redis的embstr编码方式和raw编码方式分别是什么，区别是什么
- 为什么会sdstype5 不被使用
