How to clear cache from memory in Linux?

Answer:

From time to time, you might observed memory in Linux aren't free'ed after used, and they can be shown in the cached column of the free command.

# free -m
             total       used       free     shared    buffers     cached
Mem:          1033        468        564          0          0        343
-/+ buffers/cache:        124        908
Swap:          255          1        254

To manually free it, try

# sync; echo 3 > /proc/sys/vm/drop_caches

And try the free command again.

# free -m
             total       used       free     shared    buffers     cached
Mem:          1033        135        897          0          0         10
-/+ buffers/cache:        124        908
Swap:          255          1        254