Suricata

when there is a lot http flows in suricata, the suricata use memery of linux from 200MB to 1.2GB whatched by "top" command of linux,and call HTPMemuseGlobalCounter can see http use about 0.9GB memery.
when those http flows above time out ,and call HTPMemuseGlobalCounter can see http using mem become 0 B, but "top" cimmand still show suricata use memery of 1.2GB instead of reduce to 200MB。
see the code of HTPFree， if htp_memuse become 0， the HTPFree must has been called the same times。 But why the "top" and "free -m" command show the mem still not be freed to linux (centos 7.5)???

void HTPFree(void *ptr, size_t size)
{
    SCFree(ptr);

    HTPDecrMemuse((uint64_t)size);
}