Drop_caches doesn't drop caches?

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http://unix.stackexchange.com – Sometimes I see my swap file is a bit bigger then usual. I tried sync && echo 3 > /proc/sys/vm/drop_caches to no avail. But when I tried swapoff -a it takes a bit longer but the swap file is smaller after. How can I clear my swap file without using swapoff? (HowTos)