Re: [PATCH] memcg: implement low limits

[Greg Thelen <gthelen@google.com>]

On Wed, Feb 27 2013, Roman Gushchin wrote:

> Hi, all!
>
> I've implemented low limits for memory cgroups. The primary goal was to add an ability 
> to protect some memory from reclaiming without using mlock(). A kind of "soft mlock()".
>
> I think this patch will be helpful when it's necessary to protect production processes from
> memory-wasting backup processes.
>
> --
>
> Low limits for memory cgroup can be used to limit memory pressure on it.
> If memory usage of a cgroup is under it's low limit, it will not be
> affected by global reclaim. If it reaches it's low limit from above,
> the reclaiming speed will be dropped exponentially.
>
> Low limits don't affect soft reclaim.
> Also, it's possible that a cgroup with memory usage under low limit
> will be reclaimed slowly on very low scanning priorities.

So the new low limit is not a rigid limit.  Global reclaim can reclaim
from a cgroup when its usage is below low_limit_in_bytes although such
reclaim is less aggressive than when usage is above low_limit_in_bytes.
Correct?

Why doesn't memcg reclaim (i.e. !global_reclaim) also consider
low_limit_in_bytes?

Do you have demonstration of how this improves system operation?

Why is soft_limit insufficient?

> Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
> ---
>  include/linux/memcontrol.h  |    7 +++++
>  include/linux/res_counter.h |   17 +++++++++++
>  kernel/res_counter.c        |    2 ++
>  mm/memcontrol.c             |   67 +++++++++++++++++++++++++++++++++++++++++++
>  mm/vmscan.c                 |    5 ++++
>  5 files changed, 98 insertions(+)

Need to update Documentation/cgroups/memory.txt explaining the external
behavior of this new know and how it interacts with soft_limit_in_bytes.

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