How to clear memory cache in Linux

Depending on what you want to do you can use 1,2 or 3

from https://www.kernel.org/doc/Documentation/sysctl/vm.txt

drop_caches

Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.

To free pagecache:

echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the user should run 'sync' first.

Try sync; echo 1 > /proc/sys/vm/drop_caches.