LXC and cgroup.memory on Debian

Two days ago on Lurch, I was trying to show/set a memory limit for a container (LXC), using "lxc-cgroup -n <container> memory.limit_in_bytes"

Unfortunately, I got the message "lxc-cgroup: missing cgroup subsystem", that I've firstly intended as "I couldn't mount this cgroup at this session"

Briefly, asking about memory cgroup to LXC, everything was ok

	lurch:~# lxc-checkconfig
	--- Control groups ---
	Cgroup: enabled
	Cgroup clone_children flag: enabled
	Cgroup device: enabled
	Cgroup sched: enabled
	Cgroup cpu account: enabled
	Cgroup memory controller: enabled
	Cgroup cpuset: enabled
	...

view raw gistfile1.txt hosted with ❤ by GitHub

while asking to linux not

	lurch:~# cat /proc/cgroups
	#subsys_name hierarchy num_cgroups enabled
	cpuset 1 9 1
	cpuacct 1 9 1
	memory 0 9 0

view raw gistfile1.txt hosted with ❤ by GitHub

Another confusing point to me, was the check of the dmesg output, that showed memory cgroup between the others

	lurch:~# dmesg | grep cgroup
	...
	[ 0.008000] Initializing cgroup subsys cpuacct
	[ 0.008000] Initializing cgroup subsys memory
	[ 0.008000] Initializing cgroup subsys devices
	[ 0.008000] Initializing cgroup subsys freezer
	[ 0.008000] Initializing cgroup subsys net_cls
	[ 0.008000] Initializing cgroup subsys blkio
	[ 0.008000] Initializing cgroup subsys perf_event

view raw gistfile1.txt hosted with ❤ by GitHub

So, after a little of googling, I have understood like, the memory cgroup  is just not enabled on Debian by default. That because having the cgroup.memory enabled, costs around 15Mb of ram, that is obviously a waste if you don't use that cgroup

In order to have the availability of said cgroup, you need to instruct the Grub by /etc/default/grub with the boot parameter cgroup_enable=memory

	GRUB_DEFAULT=0
	GRUB_TIMEOUT=5
	GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
	GRUB_CMDLINE_LINUX_DEFAULT="quiet cgroup_enable=memory"

view raw gistfile1.as hosted with ❤ by GitHub

The amount of memory reserved to the cgroup nos is printed out during the boot time

	lurch:~# dmesg | grep cgroup
	[ 0.000000] Initializing cgroup subsys cpuset
	[ 0.000000] Initializing cgroup subsys cpu
	...
	[ 0.000000] allocated 16777216 bytes of page_cgroup
	[ 0.000000] please try 'cgroup_disable=memory' option if you don't want memory cgroups
	...

view raw gistfile1.as hosted with ❤ by GitHub

In the end I could set my cgroup memory limit

	lurch:~# lxc-cgroup -n container memory.limit_in_bytes "1G"
	lurch:~#

view raw gistfile1.txt hosted with ❤ by GitHub