Tip submitted by my friend J.K when we were in need of a way to recover reserved cache space without booting.
PageCache only: sync ; echo 1 > /proc/sys/vm/drop_caches Dentries and inodes: sync ; echo 2 > /proc/sys/vm/drop_caches PageCache, Dentries and inodes: (not recommanded in production as it forces a full cache rebuilt. you may still need it): sync ; echo 3 > /proc/sys/vm/drop_caches
In case you need to do it in a sudo:
echo 3 | sudo tee /proc/sys/vm/drop_caches