整体思路:
1 惰性缩容。不释放空间,留给到期释放等机制释放。
2 加倍扩容。在需要空间达1M之前按新空间两倍分配空间,否则按新空间大小+1M分配。注意,1M=1024*1024*Char。Char可以是5bits/8bits/16bits/32bits/64bits
具体代码块:
惰性缩容:https://github.com/antirez/sds/blob/master/sds.c line374-390
/* Grow the sds to have the specified length. Bytes that were not part of
* the original length of the sds will be set to zero.
*
* if the specified length is smaller than the current length, no operation
* is performed. */
sds sdsgrowzero(sds s, size_t len) {
size_t curlen = sdslen(s);
if (len <= curlen) return s;
s = sdsMakeRoomFor(s,len-curlen);
if (s == NULL) return NULL;
/* Make sure added region doesn't contain garbage */
memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */
sdssetlen(s, len);
return s;
}
加倍扩容: https://github.com/antirez/sds/blob/master/sds.c line204-220
/* 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;
其中:https://github.com/antirez/sds/blob/master/sds.h line36
#define SDS_MAX_PREALLOC (1024*1024)
源码见:
https://github.com/antirez/sds