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Redis 6.0源码学习 String类型


文章目录

  • ​​Redis 6.0源码学习 String类型​​
  • ​​前置知识​​
  • ​​内部编码​​
  • ​​OBJ_ENCODING_INT​​
  • ​​OBJ_ENCODING_RAW​​
  • ​​OBJ_ENCODING_EMBSTR​​
  • ​​编码推断​​

Redis 6.0源码学习 String类型

  String字符串类型是RedisObject中最基础的一个类型,相对应的编码格式有3种,分别是OBJ_ENCODING_RAW、OBJ_ENCODING_INT和OBJ_ENCODING_EMBSTR。

前置知识

Redis 6.0源码学习 RedisObject

内部编码

OBJ_ENCODING_INT

  OBJ_ENCODING_INT算是所有编码格式中最特殊的一个。OBJ_ENCODING_INT编码中,redisObject中指针属性ptr存储的数据是字面量而不是内存地址。由于复用了ptr属性,所以OBJ_ENCODING_INT只能存储8位字节的长整型。

set key 8653
object encoding key

  在内存优化方面,OBJ_ENCODING_INT编码还做了享元优化。当服务器初始化的时候,会调用service.c中的createSharedObjects函数。

代码片段 初始化createSharedObjects.integers

for (j = 0; j < OBJ_SHARED_INTEGERS; j++) {
shared.integers[j] =
makeObjectShared(createObject(OBJ_STRING,(void*)(long)j));
shared.integers[j]->encoding = OBJ_ENCODING_INT;
}

OBJ_ENCODING_RAW

  OBJ_ENCODING_RAW是一种非常中规中矩的实现,它的ptr指针指向了一个SDS实例。

set key "这是一个等于45个字符的字符串...."
object encoding key

OBJ_ENCODING_EMBSTR

  OBJ_ENCODING_RAW中robj与sds内存空间不是连续的,基于局部性思想就诞生了robj与sds采用连续内存空间的OBJ_ENCODING_EMBSTR。当字符串长度不大于44个字符时,会采用OBJ_ENCODING_RAW编码。

set key "hello,world"
object encoding key

编码推断

  setCommand是set命令的实现函数。以"SET a 111"为例,解析完请求之后,111对应的redisObject是编码为OBJ_ENCODING_RAW的字符串。tryObjectEncoding会进行一次编码推断,如果符合OBJ_ENCODING_INT的约束,还可能从享元池中复用对象。

代码片段 t_string.c setCommand

void setCommand(client *c) {
int j;
robj *expire = NULL;
int unit = UNIT_SECONDS;
int flags = OBJ_SET_NO_FLAGS;

...
...
...

c->argv[2] = tryObjectEncoding(c->argv[2]);
setGenericCommand(c,flags,c->argv[1],c->argv[2],expire,unit,NULL,NULL);
}

代码片段 object.c tryObjectEncoding

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;
}


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