[lxc-users] How properly to find what consumes memory inside the container.

[Fajar A. Nugraha]

On Tue, Sep 19, 2017 at 11:20 AM, Ivan Kurnosov <zerkms at zerkms.ru> wrote:

>
> But if I clear the system caches on the host
>
> echo 3 > /proc/sys/vm/drop_caches
>
>
> the container memory consumption drops to the expected <100mb.
>
> So the question, how to monitor the memory consumption from the container
> reliably?
>

As in 'know what the kernel thinks the container is using? You're doing it
already.

If you use lxcfs (which should be the case on ubuntu), /proc entries inside
containers are fake, created using data from the relevant cgroups.



> And why does `free` count caches as used memory inside container?
>
>
>

>From https://www.kernel.org/doc/Documentation/cgroup-v2.txt:
"
Memory
------

The "memory" controller regulates distribution of memory.  Memory is
stateful and implements both limit and protection models.  Due to the
intertwining between memory usage and reclaim pressure and the
stateful nature of memory, the distribution model is relatively
complex.

While not completely water-tight, all major memory usages by a given
cgroup are tracked so that the total memory consumption can be
accounted and controlled to a reasonable extent.  Currently, the
following types of memory usages are tracked.

- Userland memory - page cache and anonymous memory.

- Kernel data structures such as dentries and inodes.

- TCP socket buffers.

The above list may expand in the future for better coverage.

...
Memory Ownership
~~~~~~~~~~~~~~~~

A memory area is charged to the cgroup which instantiated it and stays
charged to the cgroup until the area is released.  Migrating a process
to a different cgroup doesn't move the memory usages that it
instantiated while in the previous cgroup to the new cgroup.

A memory area may be used by processes belonging to different cgroups.
To which cgroup the area will be charged is in-deterministic; however,
over time, the memory area is likely to end up in a cgroup which has
enough memory allowance to avoid high reclaim pressure.

If a cgroup sweeps a considerable amount of memory which is expected
to be accessed repeatedly by other cgroups, it may make sense to use
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
belonging to the affected files to ensure correct memory ownership.
"



An interesting question would be "can you exclude page cache from cgroup
memory accounting". I don't think that's possible though.

-- 
Fajar
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