When redis string is shortened, when does it reclaim the excessive memory?

For example, when a 30M string is shortened to 3 bytes, when does redis free the excessive free memory? It would be too bad to leave the large unused space there.

Redis string (SDS) structure:

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[];
};

The code to trim a string. There is no reclaiming memory code.

/* Remove the part of the string from left and from right composed just of
 * contiguous characters found in 'cset', that is a null terminated C string.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call.
 *
 * Example:
 *
 * s = sdsnew("AA...AA.a.aa.aHelloWorld     :::");
 * s = sdstrim(s,"Aa. :");
 * printf("%s\n", s);
 *
 * Output will be just "HelloWorld".
 */
sds sdstrim(sds s, const char *cset) {
    char *end, *sp, *ep;
    size_t len;

    sp = s;
    ep = end = s+sdslen(s)-1;
    while(sp <= end && strchr(cset, *sp)) sp++;
    while(ep > sp && strchr(cset, *ep)) ep--;
    len = (ep-sp)+1;
    if (s != sp) memmove(s, sp, len);
    s[len] = '\0';
    sdssetlen(s,len);
    return s;
}