文章目錄
- Redis 6.0源碼學習 String類型
- 前置知識
- 内部編碼
- OBJ_ENCODING_INT
- OBJ_ENCODING_RAW
- OBJ_ENCODING_EMBSTR
- 編碼推斷
Redis 6.0源碼學習 String類型
String字元串類型是RedisObject中最基礎的一個類型,相對應的編碼格式有3種,分别是OBJ_ENCODING_RAW、OBJ_ENCODING_INT和OBJ_ENCODING_EMBSTR。
前置知識
Redis 6.0源碼學習 RedisObject
内部編碼
OBJ_ENCODING_INT
OBJ_ENCODING_INT算是所有編碼格式中最特殊的一個。OBJ_ENCODING_INT編碼中,redisObject中指針屬性ptr存儲的資料是字面量而不是記憶體位址。由于複用了ptr屬性,是以OBJ_ENCODING_INT隻能存儲8位位元組的長整型。
set key 8653
object encoding key
在記憶體優化方面,OBJ_ENCODING_INT編碼還做了享元優化。當伺服器初始化的時候,會調用service.c中的createSharedObjects函數。
代碼片段 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是一種非常中規中矩的實作,它的ptr指針指向了一個SDS執行個體。
set key "這是一個等于45個字元的字元串...."
object encoding key
OBJ_ENCODING_EMBSTR
OBJ_ENCODING_RAW中robj與sds記憶體空間不是連續的,基于局部性思想就誕生了robj與sds采用連續記憶體空間的OBJ_ENCODING_EMBSTR。當字元串長度不大于44個字元時,會采用OBJ_ENCODING_RAW編碼。
set key "hello,world"
object encoding key
編碼推斷
setCommand是set指令的實作函數。以"SET a 111"為例,解析完請求之後,111對應的redisObject是編碼為OBJ_ENCODING_RAW的字元串。tryObjectEncoding會進行一次編碼推斷,如果符合OBJ_ENCODING_INT的限制,還可能從享元池中複用對象。
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);
}
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;
}