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SubjectRe: [RFC 0/3] Implementation of cgroup isolation
On Mon, 28 Mar 2011 11:39:57 +0200
Michal Hocko <> wrote:

> Hi all,
> Memory cgroups can be currently used to throttle memory usage of a group of
> processes. It, however, cannot be used for an isolation of processes from
> the rest of the system because all the pages that belong to the group are
> also placed on the global LRU lists and so they are eligible for the global
> memory reclaim.
> This patchset aims at providing an opt-in memory cgroup isolation. This
> means that a cgroup can be configured to be isolated from the rest of the
> system by means of cgroup virtual filesystem (/dev/memctl/group/memory.isolated).
> Isolated mem cgroup can be particularly helpful in deployments where we have
> a primary service which needs to have a certain guarantees for memory
> resources (e.g. a database server) and we want to shield it off the
> rest of the system (e.g. a burst memory activity in another group). This is
> currently possible only with mlocking memory that is essential for the
> application(s) or a rather hacky configuration where the primary app is in
> the root mem cgroup while all the other system activity happens in other
> groups.
> mlocking is not an ideal solution all the time because sometimes the working
> set is very large and it depends on the workload (e.g. number of incoming
> requests) so it can end up not fitting in into memory (leading to a OOM
> killer). If we use mem. cgroup isolation instead we are keeping memory resident
> and if the working set goes wild we can still do per-cgroup reclaim so the
> service is less prone to be OOM killed.
> The patch series is split into 3 patches. First one adds a new flag into
> mem_cgroup structure which controls whether the group is isolated (false by
> default) and a cgroup fs interface to set it.
> The second patch implements interaction with the global LRU. The current
> semantic is that we are putting a page into a global LRU only if mem cgroup
> LRU functions say they do not want the page for themselves.
> The last patch prevents from soft reclaim if the group is isolated.
> I have tested the patches with the simple memory consumer (allocating
> private and shared anon memory and SYSV SHM).
> One instance (call it big consumer) running in the group and paging in the
> memory (>90% of cgroup limit) and sleeping for the rest of its life. Then I
> had a pool of consumers running in the same cgroup which page in smaller
> amount of memory and paging them in the loop to simulate in group memory
> pressure (call them sharks).
> The sum of consumed memory is more than memory.limit_in_bytes so some
> portion of the memory is swapped out.
> There is one consumer running in the root cgroup running in parallel which
> makes a pressure on the memory (to trigger background reclaim).
> Rss+cache of the group drops down significantly (~66% of the limit) if the
> group is not isolated. On the other hand if we isolate the group we are
> still saturating the group (~97% of the limit). I can show more
> comprehensive results if somebody is interested.

Isn't it the same result with the case where no cgroup is used ?
What is the problem ?
Why it's not a problem of configuration ?
IIUC, you can put all logins to some cgroup by using cgroupd/libgcgroup.

> Thanks for comments.

Maybe you just want "guarantee".
At 1st thought, this approarch has 3 problems. And memcg is desgined
never to prevent global vm scans,

1. This cannot be used as "guarantee". Just a way for "don't steal from me!!!"
This just implements a "first come, first served" system.
I guess this can be used for server desgines.....only with very very careful play.
If an application exits and lose its memory, there is no guarantee anymore.

2. Even with isolation, a task in memcg can be killed by OOM-killer at
global memory shortage.

3. it seems this will add more page fragmentation if implemented poorly, IOW,
can this be work with compaction ?

I think of other approaches.

1. cpuset+nodehotplug enhances.
At boot, hide most of memory from the system by boot option.
You can rename node-id of "all unused memory" and create arbitrary nodes
if the kernel has an interface. You can add a virtual nodes and move
pages between nodes by renaming it.

This will allow you to create a safe box dynamically. If you move pages in
the order of MAX_ORDER, you don't add any fragmentation.
(But with this way, you need to avoid tasks in root cgrou, too.)

2. allow a mount option to link ROOT cgroup's LRU and add limit for
root cgroup. Then, softlimit will work well.
(If softlimit doesn't work, it's bug. That will be an enhancement point.)


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