Redis源码分析： String（SDS）容量调整分析

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  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