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
	...

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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

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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

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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"

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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
	...

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In the end I could set my cgroup memory limit

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

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Pxe with Dnsmasq

Just a couple of words..

For a much too long period of my life I have always manually changed the "pxelinux" entry in the "dnsmasq.d/domain.conf" file, to achieve the boot with this and with that image depending to the needed install distro

	# PXE boot
	dhcp-boot = net:domain, pangolin-amd64/pxelinux.0, lurch, 192.168.1.100

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I just have finally found a way to serve each pxe images in one shot and even with a confortable menu list

That is accomplished adding something like:

	# Multi PXE boot
	pxe-prompt="Welcome to Lurch's TFTP"
	pxe-service=x86PC, "Pangolin-amd64 on Lurch TFTP", "pangolin-amd64/pxelinux", 192.168.1.100
	pxe-service=x86PC, "Pangolin-i386 on Lurch TFTP", "pangolin-i386/pxelinux", 192.168.1.100
	pxe-service=x86PC, "Debian-amd64 on Lurch TFTP", "debian-amd64/pxelinux", 192.168.1.100
	pxe-service=x86PC, "Fedora17-amd64 on Lurch TFTP", "fedora-x86_64/pxelinux", 192.168.1.100

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Needless to say that you could even make point each pxe entry to a different tftp servers