How to clear cached memory in Debian?

Try this as root (not sudo):

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

The problem with:

sudo echo 1 > /proc/sys/vm/drop_caches

is that the redirect happens in the initial shell - i.e. under your own account - before the "sudo echo 1" happens, which isn't the part that really needs root access. You need to get the opening of drop_caches by ">" to be inside of the sudo. One lazy way (lazy because it clones the 3 back to stdout, which you don't actually need) is:

echo 3 | sudo tee /proc/sys/vm/drop_caches

The options to write into drop_caches are:

1. Free pagecache
2. Free dentries and inodes
3. Free pagecache, dentries, and inodes.

And you should sync first, so all in all:

sync ; echo 3 | sudo tee /proc/sys/vm/drop_caches

or if you don't like the spurious "3" on stdout:

sudo sh -c 'sync ; echo 3 >/prod/sys/vm/drop_caches'

We can make it automatically using crontab as root.

~$ sudo crontab -e

You'll see the your/new crontab file like it:

# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command

And you'll add this one in new lines to flush every hour:

# Every hour flushes the memory cache on system
0 * * * * sync; echo 3 > /proc/sys/vm/drop_caches

I see in my syslog tailf output.

~$ tailf /var/log/syslog | grep 'cron'

And I see the output below:

May 31 14:07:16 debian crontab[17353]: (root) BEGIN EDIT (root)
May 31 14:07:20 debian crontab[17353]: (root) END EDIT (root)

...

May 31 15:00:02 debian CRON[22169]: (root) CMD (sync; echo 3 > /proc/sys/vm/drop_caches)
May 31 15:17:01 debian CRON[18828]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)