Redis成本优化-版本升级-1.SDS优化历史

Redis实验版本(撰写文本的最新版本)：2.8.24、3.0.7、3.2.13、4.0.14、5.0.14、6.0.20、6.2.15、7.0.12，
容量MB：去掉了Redis自身的容量
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本部分只是阐述SDS是什么，所以用最简单的Redis 3版本进行阐述。
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struct sdshdr {
    unsigned int len //sds使用长度(字符串长度);
    unsigned int free //sds剩余长度;
    char buf[] //柔性字符数组;
};
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127.0.0.1:12307> set hello world
OK
127.0.0.1:12307> object encoding hello
"embstr"
127.0.0.1:12307> set bigstr helloaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
OK
127.0.0.1:12307> strlen bigstr
(integer) 41
127.0.0.1:12307> object encoding bigstr
"raw"
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#define REDIS_ENCODING_EMBSTR_SIZE_LIMIT 39
robj *createStringObject(char *ptr, size_t len) {
    if (len <= REDIS_ENCODING_EMBSTR_SIZE_LIMIT)
        return createEmbeddedStringObject(ptr,len);
    else
        return createRawStringObject(ptr,len);
}
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struct sdshdr {
    unsigned int len;
    unsigned int free;
    char buf[];
};
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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[];
};
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robj *tryObjectEncoding(robj *o) {
    long value;
    sds s = o->ptr;
    size_t len;
    //......一些验证.....
    len = sdslen(s);
    if (len <= 20 && string2l(s,len,&value)) {
        //针对shareobject
        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];
        } 
        //这里是优化重点
        else {
            if (o->encoding == OBJ_ENCODING_RAW) sdsfree(o->ptr);
            o->encoding = OBJ_ENCODING_INT;
            o->ptr = (void*) value;
            return o;
        }
    }
    ...
}
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robj *createStringObjectFromLongLongForValue(long long value) {
    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;
}
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