Kernel Samepage Merging is a cool feature of the Linux kernel, which you would really want to have if you are running a virtualized environment, but you would also benefit from it on a regular system as well.

Having KVM hypervisors aroud I wanted to have KSM enabled, and you can easily do that by echo’ing 1 to /sys/kernel/mm/ksm/run, but the downside of this is that it does not work accross reboots.

The other thing is that there is no kernel parameter that you can drop in /etc/sysctl.conf in order to enable KSM accross reboots. Looking at the existing init.d scripts on my Debian Wheezy system I didn’t find any KSM related script neither.

So, what are our possibilities here to make this simply work? Looking around on the Net I’ve seen a lot of people recommending to use /etc/rc.local, but I’m not a big fan of /etc/rc.local. In CentOS there’s a little script there in /etc/init.d/ksm which does the job, so I’ve decided to write my own version of it for Debian GNU/Linux.

Below you can find the script that I’ve used in order to enable KSM on my Debian GNU/Linux systems upon reboots:

#!/bin/sh
#
# Author:           Marin Atanasov Nikolov <dnaeon@gmail.com>
#
### BEGIN INIT INFO
# Provides:          ksm
# Required-Start:    
# Required-Stop:
# X-Start-Before:    
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6 
# Short-Description: Enable and disable KVM KSM
# Description:       Enables and disables the KVM  Kernel Samepage Merging
#                    feature of the kernel
### END INIT INFO

set -e
. /lib/lsb/init-functions

do_start() {
	echo 1 > /sys/kernel/mm/ksm/run
	log_success_msg "Enabling Kernel Samepage Merging"
}
 
do_stop() {
	echo 0 > /sys/kernel/mm/ksm/run
	log_success_msg "Disabling Kernel Samepage Merging"
}
		
do_status() {
	local ksm_status
	ksm_status=$( cat /sys/kernel/mm/ksm/run )

	if [ ${ksm_status} -eq 1 ]; then
		log_success_msg "Kernel Samepage Merging is enabled"
	else
		log_success_msg "Kernel Samepage Merging is disabled"
	fi
}

case "${1}" in
	start)
		do_start
		;;
	reset|stop)
		do_stop
		;;
	status)
		do_status
		;;
	reload|restart|force-reload)
		do_stop
		do_start
		;;
	*)
		log_success_msg "usage: ${0} {start|stop|status|reload|restart|force-reload|reset}" >&2
		;;
esac

You can also find the latest version of this script at my Github account on the link below:

• https://github.com/dnaeon/ksm-init.d-debian

In order to use this script and have KSM enabled during boot-time, drop the ksm script into your /etc/init.d directory. Then enable KSM init.d script during boot-time, by executing the command below:

$ sudo update-rc.d ksm defaults
update-rc.d: using dependency based boot sequencing

Here’s how to enable KSM by using the init.d script:

$ sudo service ksm start
Enabling Kernel Samepage Merging.

Disabling KSM by using the init.d script:

$ sudo service ksm stop
Disabling Kernel Samepage Merging.

Getting status info by using the init.d script:

% sudo service ksm status
Kernel Samepage Merging is disabled.