I found the systemd-journald takes like 1GB+ of memory. After some searching I set the value of SystemMaxUse and RuntimeMaxUse both to 8M in "/etc/systemd/journald.conf". Then restart the journald.
After a while I found that limit doesn't work, check through 'top' and procstat.memory_rss, the systemd-journald still consumes 100+M after a while (Below is the visualization of journald memory catch through procstat.memory_rss). Any idea?
My systemd version is 219.