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Through conversation with Kumar L Srikanth-B22348, I found that the function of getting memory usage (e.g., virsh dominfo) doesn't work for lxc with ns subsystem of cgroup enabled. This is because of features of ns and memory subsystems. Ns creates child cgroup on every process fork and as a result processes in a container are not assigned in a cgroup for domain (e.g., libvirt/lxc/test1/). For example, libvirt_lxc and init (or somewhat specified in XML) are assigned into libvirt/lxc/test1/8839/ and libvirt/lxc/test1/8839/8849/, respectively. On the other hand, memory subsystem accounts memory usage within a group of processes by default, i.e., it does not take any child (and descendant) groups into account. With the two features, virsh dominfo which just checks memory usage of a cgroup for domain always returns zero because the cgroup has no process. Setting memory.use_hierarchy of a group allows to account (and limit) memory usage of every descendant groups of the group. By setting it of a cgroup for domain, we can get proper memory usage of lxc with ns subsystem enabled. (To be exact, the setting is required only when memory and ns subsystems are enabled at the same time, e.g., mount -t cgroup none /cgroup.)
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>